Evaluating the commercial application potential of polyesters with 1,4:3,6-dianhydrohexitols (isosorbide, isomannide and isoidide) by reviewing the synthetic challenges in step growth polymerization

Plastic materials play a pivotal role in modern society. Finding sustainable alternatives to established fossil-based polymers is an important part of the effort to reduce the environmental impact of the materials sector. 1,4:3,6dianhydrohexitols (isosorbide, isomannide and isoidide) are a group of biobased diol monomers that are promising for the synthesis of sustainable polyesters. Their rigid molecular structure imparts favorable material properties to polymers. Isosorbide is especially interesting due to its current commercial availability and synthesis from glucose. This potential is reflected in the large number of publications on polyesters with isosorbide in the past decade. Despite this, high molecular weight polyesters with high molar percentages of 1,4:3,6-dianhydrohexitols remain elusive due to the low reactivity of 1,4:3,6-dianhydrohexitols in (trans)esterification reactions. This review compares the efforts on the synthesis of polyesters with 1,4:3,6-dianhydrohexitols from both the academic and patent literature and puts them into perspective regarding industrial viability. Interesting synthesis strategies and possible future developments are highlighted together with material properties.

1,4:3,6-Dianhydrohexitols are a class of rigid diol monomers derived from D-glucose, D-mannose or D-idose.The monomers can be obtained after reduction of the respective sugar and two subsequent intramolecular dehydration reactions (Fig. 1).
Isosorbide is the most widely available 1,4:3,6-dianhydrohexitol due to it being derived from sorbitol.It is synthesized by acid catalyzed dehydration of sorbitol and its total global market in 2018 was 396.4 million USD [7].The current main producer of isosorbide is Roquette Frères with an annual production capacity of 20 kt/a.A very recent review by Saxon et al. provides an overview of current trends in the production of isosorbide and developments around improving its reactivity in polymer synthesis [8].Examples of polyesters are limited in that review, despite the fact that they are currently the main outlet for isosorbide (measured by their market size), followed by polycarbonates and polyurethanes [7].These polymers do not require chemical modification of the isosorbide backbone, which is probably one of the reasons for their current industrial deployment.This review will focus on polyester synthesis with 1,4:3,6-dianhydrohexitols, as the recent advances in the synthesis of polycarbonates, polyurethanes and polyolefins based on isosorbide have already been reviewed by Saxon et al [8].
The corresponding sugars are relatively rare in nature, which hampers the large scale production of isomannide and especially isoidide.Recent advancements on the epimerization of isosorbide to isoidide [9] and on the hydrogenation/epimerization of D-glucose to D-iditol enriched hexose mixtures [10] could increase the availability of isoidide and lead to further applications in polymer synthesis in the future.
Due to the rigidity of their bicyclic structure and subsequent polymer properties, the potential of isosorbide (and other 1,4:3,6-dianhydrohexitols once they become more widely available) for polyester production is enormous, as a large variety of diacids, both fossil-and biobased, are readily available.As will be shown in this review, many works from academia and industry report the synthesis of polyesters containing isosorbide by melt polymerization with diacids or diesters.Most of them however fail to produce reasonably high molecular weight materials with high contents of isosorbide (>50 mol% isosorbide as percentage of total diol).This is mainly due to the unreactive secondary alcohol groups of isosorbide.Common polyester diols like ethylene glycol, 1,4-butanediol and 1,4-cyclohexanedimethanol are primary alcohols.Secondary diols have rarely been used in commercial polyester applications.The main representative of a commercial polyester with a secondary diol is Eastman's Tritan®, which contains up to 41 mol% of the fossil-based 2,2,4,4-tetramethyl-1,3-cyclobutanediol (Fig. 3).Further information on the differences between this monomer and 1,4:3,6-dianhydrohexitols will be given in chapter 2.2.1 [11].
Attempts to employ 1,4:3,6-dianhydrohexitols in melt polyesterification reactions with dicarboxylic acids or esters face several challenges connected to the low reactivity of the secondary alcohol groups.They are much less reactive in (trans)esterification reactions than the typically used primary diols.This leads to low incorporation rates of 1,4:3,6-dianhydrohexitols in copolyesters with primary alcohols, as the higher reactivity of the latter usually favors removal of excess diols in the form of 1,4:3,6-dianhydrohexitols. This often results in low 1,4:3,6-dianhydrohexitols incorporation ratios (compared to the feed ratios) and thus makes it difficult to predict diol ratios in the final product.The V-shape of the molecule additionally results in a difference in the steric and electronic environment between the two hydroxyl groups.In the case of isosorbide, the endo-OH group points inside the fused bicycle, which results in steric shielding and lowered reactivity, whereas the exo-OH is positioned outside the bicycle and is thus less influenced by steric hindrance (see Fig. 2).Although the nucleophilicity of the endo-OH is increased by intramolecular hydrogen bonding with the oxygen atom fusing the opposite ring, the steric effect dominates and overall decreases its reactivity in esterification compared to the exo-OH.The stereochemistry of the hydroxyl groups also influences the thermal stability of the bicyclic structure with exo-OH being more thermally stable than endo-OH [12,13].Isomannide has an endo-endo configuration of hydroxyl groups, which results in a generally lower reactivity in (trans) esterification reactions compared to isosorbide.Isoidide, with its exoexo-OH configuration usually shows the highest reactivity in (trans)esterifications of the three 1,4:3,6-dianhydrohexitols (see Fig. 2).Due to the limited availability of the latter two, most examples in this review will cover isosorbide-based polyesters.

Polyesters with 1,4:3,6-dianhydrohexitols as the main diol
Early research on the synthesis of polyesters with 1,4:3,6-dianhydrohexitols mostly relied on the use of aliphatic and aromatic dicarboxylic acid chlorides in solution polycondensations.The highly reactive nature of acid chlorides facilitates transesterification and thus enables the synthesis of high molecular weight polyesters.This method is however not very attractive for the industrial synthesis of polyesters, due to the use of organic solvents, the wasteful synthesis of the respective acid   chlorides and the need for HCl acceptors like pyridine [14].Next to the polymer, stoichiometric amounts of salts are produced as waste, which apart from the cost is also highly unfavorable from a sustainability perspective.For a review of many of these examples on the acid chloride synthesis of aliphatic and aromatic polyesters with 1,4:3,6-dianhydrohexitols, we refer to the excellent overview given by Fennouillot et al. [15].Since the publication of that review, only a few studies used acid chlorides for the synthesis of 1,4:3,6-dianhydrohexitol-based polyesters [16][17][18][19][20].Some recent examples that use solution polymerization as part of their strategy, for example in combination with pre-synthesized oligo (isosorbide lactate) units, will be covered in this review.In addition to polyesters, a few examples of poly(ester carbonates) will be presented, as incorporation of carbonate units was shown to improve molecular weights of poly(ester carbonates) compared to similar polyesters.

Aromatic polyesters with 1,4:3,6-dianhydrohexitols
Aromatic polyesters with 1,4:3,6-dianhydrohexitols, obtained by solution polymerization with diacid chlorides, such as poly(isosorbide terephthalate) and poly(isosorbide 2,5-furanoate) exhibit very high T g values of 205 • C and 194 • C respectively [21,22].To obtain the same polyesters by melt polymerization is no easy task as this chapter will show, because the melt viscosity of these polyesters at typical polyesterification temperatures (up to 330 • C) is very high.As a rule of thumb for high T g , amorphous materials, roughly twice the expected T g of a polyester is required as the reaction temperature to reach melt viscosities low enough to obtain a stirrable polymer melt during polycondensation.This is not compatible with the limited thermal stability of 1,4:3,6-dianhydrohexitols at temperatures higher than 270 • C [23].
Early attempts to synthesize poly(isosorbide terephthalate) (PIT, Fig. 4) by conventional bulk polymerization with dimethyl terephthalate, using a Ti(OiPr) 4 catalyst at 250 • C, were unsuccessful [24].The intrinsic viscosity after the reaction only reached 0.11 dL/g, which indicates a very low molecular weight polymer.The addition of varying amounts of succinic acid as a comonomer did not yield higher molecular weights [25].A patent by Toyobo Co. Ltd. describes the synthesis of PIT from terephthalic acid or dimethyl terephthalate and isosorbide using a Ti(OBu) 4 catalyst at elevated temperatures up to 300 • C [26].The reduced viscosity of the polyester was 0.4 dl/g with a T g of 186 • C, which was increased upon crosslinking of the free carboxylic acid groups with 5-10 wt% of a crosslinking agent to 0.67 dl/g and a T g of 197 • C. The authors did not mention the color of the final polymer, although other publications describe heavy coloration of isosorbide-containing polyesters obtained in high temperature processes.Sablong et al. investigated the melt polycondensation between dimethyl terephthalate and isosorbide [27].The authors conducted the transesterification reaction with Ti(OiPr) 4 at 180-220 • C under an argon atmosphere for 6 h, and the polycondensation for 16 h at 1 mbar.The prolonged reaction times produced PIT with a M n value of 11,600 g/mol, a polydispersity of 2.4 and a T g of 186 • C. The polymer exhibited a deep brown color after the reaction.
An innovative strategy to overcome the low reactivity of isosorbide by using ethylene carbonate units as temporary chain linkers between the monomers was explored by Feng et al (Scheme 1) [28].While not a polyester, the carbonate linkages in the final poly(ester carbonate) are structurally similar to ester linkages, which is why this example is relevant within the scope of this review.
The authors synthesized oligo(isosorbide carbonate) and oligo (ethylene terephthalate) in two separate steps.Upon polycondensation, labile ethylene carbonate units were formed in the polymer chain, which underwent cyclization at elevated temperatures.The volatile ethylene carbonate was removed under the vacuum conditions during polycondensation, leading to the final polymer poly(isosorbide carbonate-coisosorbide terephthalate).Although the polymer still contains carbonate linkages, its properties are close to pure PIT.As can be seen in Table 1, the obtained M n values ranged from 22,700 to 28,500 g/mol and decreased at higher TPA loadings, as the increasing melt viscosity during the polycondensation impeded the removal of condensation products.The higher concentration of primary-OH groups at high TPA content also facilitated the transesterification at oligo(isosorbide carbonate) repeat units.The incorporation of the monomers into the polymer largely coincided with the feed ratios.The T g values and decomposition temperatures increased at higher TPA-loadings.All polyesters were amorphous due to the high isosorbide content.Although the achieved molecular weights are very high compared to other publications on PIT, the formation of stoichiometric amounts of ethylene carbonate, a low value chemical that has to be recycled in a separate step is the main drawback of this methodology.The influence of residual carbonate repeat units on the mechanical properties and (bio)degradability is not clear, although neither terephthalic acid-based polyesters nor aliphatic polycarbonates [29] are usually (bio)degradable.
Terzopoulou et al. investigated the synthesis of poly(isosorbide-2,5furanoate) (PIF, Fig. 5) from isosorbide and dimethyl-2,5furandicarboxylate using Ti(OBu) 4 (400 ppm) as a catalyst [30].The transesterification was conducted with a 2:1 diol:diester ratio for 4 h at 150-170 • C under an argon atmosphere.For the polycondensation step the authors used a modified procedure to obtain high molecular weight polyesters with low volatility long chain diols [31].The method involved the addition of another 1.05 equivalents of dimethyl-2,5furandicarboxylate after the first reaction step is complete.The reactants were transesterified for 5 h at 150-170 • C under an argon atmosphere, while methanol was removed.The polycondensation was conducted at 210-230 • C for 3 h under vacuum (0.05 mbar).This procedure enabled the researchers to obtain high molecular weight polymers at moderate temperatures.The conditions necessary to remove long chain diols typically involve temperatures between 240 and 270 • C, which can cause decomposition.The obtained polymers showed intrinsic viscosity values of 0.39 dL/g.Unfortunately the authors did not determine the molecular weights.The polyesters were amorphous and had a T g of 157 • C, which is significantly lower than the values of the same polymer obtained by solution polymerization (194 • C with an M n of 25,000 g/mol [21]).For other examples on the synthesis of PIF copolyesters with primary diols using the synthetic procedure as described above, see paragraph 3.1.2.
Wang et al. synthesized poly(isosorbide 2,5-furanoate) by melt polymerization with dimethyl-2,5-furandicarboxylate (COOMe/OH 1:1.6) using a dibutyltin oxide catalyst (0.15 mol% relative to dimethyl-2,5-FDCA) [32].After complete esterification, the polycondensation was conducted for 6 h at 0.1 mbar.The polyester exhibited an intrinsic viscosity of 0.27 dl/g and a T g of 162 A patent by PepsiCo contains entries on the polymerization of isosorbide and 2,5-furandicarboxylic acid (FDCA) with an Sb 2 O 3 catalyst [33].The authors conducted the esterification at 220-260 • C for 20 h under atmospheric pressure.After the melt became viscous, vacuum was applied to remove water.The polymer was dissolved in DMSO and stirred for 3-4 h at    A recent patent filed by Wageningen University and Research describes the synthesis of poly(isoidide 2,5-furanoate) (PIiF, Fig. 6) by melt polymerization of isoidide and dimethyl 2,5-furandicarboxylate [34].The esterification was conducted at 165-240 • C for 20 h, the  polycondensation at 240 • C for 3 h at 0.1 mbar.The obtained polyester exhibited a M n value of 21,000 g/mol with a PDI of 1.7.The authors compared the number-average molecular weight (analyzed by GPC) of the polymer after precipitation in methanol and noticed an almost twofold increase, as low-molecular weight oligomers were dissolved.The T g and T m of the unprecipitated polyester were 113 • C and 235 • C respectively, although neither T g nor T m were observed after precipitation.While some papers and patents report purification of their polyesters by precipitation, it is not a technique that can be employed on an industrial scale due to the low solubility of most polyesters and the large volumes of toxic solvents (often a DCM/MeOH mixture is used to dissolve/precipitate the polymers) that would thus be required.A comparative example employing isosorbide yielded polymers with similar molecular weights, a slightly decreased T g (101 • C) and an amorphous structure.
The T g values of both isoidide and isosorbide based polyesters obtained via melt polymerization reported here are lower than values obtained via solution polymerization.Under those synthesis conditions, the T g of poly(isosorbide 2,5-furanoate) reached  C (M n = 13,750 g/ mol) [35] or 194 • C (M n = 25,000 g/mol) [21] and the T g of poly(isoidide 2,5-furanoate) reached 140 • C (5,670 g/mol) with no detectable T m [35].This large difference in reported T g values cannot be easily explained considering the similar M n values reported for the same polyesters synthesized by melt polymerization.
Okada et al. explored 1,1-Bis(5-(methoxycarbonyl)-2-furyl)methane (Fig. 7; R = H) and 1,1-Bis(5-(methoxycarbonyl)-2-furyl)ethane (Fig. 7; R = Me) as comonomers in polymerizations with isosorbide and isomannide (Fig. 7) [36].The reactions were conducted at 230 • C with a Ti(OiPr) 4 catalyst.The obtained number-average molecular weights were quite low (4,800 g/mol after 24 h overall reaction time), although prolonged reaction times of 48 h did result in a higher M n of 9,300 g/mol.This lower reactivity was attributed partially to the aromatic diester units employed.In a subsequent publication the researchers partially replaced 1,1-Bis(5-(methoxycarbonyl)-2-furyl) ethane units with a range of aliphatic diesters, which resulted in higher molecular weight polymers [37].Number average molecular weights of up to 16,100 g/mol were reached when 76% of the diacid moiety was dimethyl adipate.Interestingly, the use of the difuranic dichlorides under solution polymerization conditions did not result in higher molecular weights.The enzymatic degradability of the copolyesters containing difuranic moieties was evaluated.The degradability was determined by measuring the total organic carbon content (TOC) after 24 h of incubation with P. pancreas lipase in aqueous phosphate buffer solutions.It was found that polyesters with a higher content of aliphatic diacid units generally yielded higher TOC values, which indicates more enzymatic degradation.
To summarize, the T g values of the presented examples of aromatic polyesters with 1,4:3,6-dianhydrohexitols obtained by melt polymerization are significantly lower than those of the same polymers obtained by solution polymerization.Solution polymerization typically leads to higher molecular weights.The T g is generally known to increase with molecular weight, until a plateau value is reached [38], which explains the difference in the T g .Interestingly, more examples explore FDCAbased polyesters than TPA-based polyesters.This could be due to the lower melt viscosities of furanoate polyesters compared to terephthalate polyesters and the practically easier melt polymerization resulting from it.Another reason for this could be the increased interest of the research community in the fully biobased FDCA compared to fossil-based TPA.It is apparent that high molecular weight aromatic polyesters with 1,4:3,6dianhydrohexitols are difficult to synthesize by conventional melt polymerization methods.The low reactivity of the secondary alcohols and the high viscosity during melt polymerization are the main causes for this.The material properties of these polyesters were elucidated in preceding publications in which solution polymerizations were used to obtain polyesters with higher molecular weights [21,35].Their properties rival those of common engineering plastics.

Chemically catalyzed polymerizations
Most polyesters of aliphatic diacids with common diols like ethylene glycol and 1,4-butanediol exhibit undesirable thermal properties, often with T g values way below room temperature.This limits the applicability of these polyesters in areas like plastic packaging or injection molded parts due to the low structural integrity of polyester materials with T g values below room temperature.The combination of these aliphatic diacids with rigid 1,4:3,6-dianhydrohexitols can lead to rigid biobased materials.Especially succinic acid shows high potential as the shortest aliphatic diacid with good thermal stability (oxalic acid and malonic acid undergo decarboxylation at typical polyesterification temperatures).Poly(isosorbide succinate) synthesized by solution polycondensation with succinyl chloride exhibits a T g value of 78 • C [16], which is higher than most biobased materials on the market and in the same range as PET.
An overview of the results on polyesters synthesized with 1,4:3,6dianhydrohexitols and a range of aliphatic diacids/diesters is shown in Table 2. Noordover et al. presented one of the syntheses of poly(isosorbide succinate) by melt polymerization between succinic acid and isosorbide (Table 2, entry 7) [39].The esterification was conducted for 4 h at 180-250 • C using Ti(OiPr) 4 (0.02 mol% relative to succinic acid).The polycondensation was conducted for 4 h at 230-250 • C at 1-5 mbar.The obtained number average molecular weights M n ranged between 2,000 and 3,100 g/mol and depended strongly on the molar ratio of the reactants (SA/IS 1:1.1 yielded the highest molecular weight).The authors noted that coloration of the product could be avoided by using high purity isosorbide (≥98.5%) and an inert gas atmosphere.Monomer purity is an important factor in polyester synthesis in general.Low monomer purity can have several negative effects on polyester synthesis [40].Firstly an offset in diol to diacid ratio hampers polycondensation, especially with high boiling diols such as 1,4:3,6-dianhydrohexitols that cannot be easily removed under vacuum.If either monomer is only 98% pure, the resulting, theoretical degree of polymerization is limited to 33 (assuming the excess of the other monomer cannot be removed) [40].For a polymer such as PIS this corresponds to a M n around 8,000 g/mol.Apart from offsetting the diol to diacid ratio, undesired compounds can also be introduced into the reaction by using low purity monomers.Depending on the impurity, effects can range from a decrease in molecular weight (if the impurity is monofunctional and thus removes a reactive end group) to discoloration due to sidereactions or deactivation of the catalyst.Many publications discussed in this review therefore use monomers with a purity of ≥ 99%.In the case of isosorbide, this is achieved by either recrystallization from acetone prior to polyester synthesis, or the use of commercially available isosorbide with a purity of > 99.5% (in the form of Roquette's Polysorb Isosorbide).To achieve these very high purities, a series of purification steps must be undertaken as outlined in Roquette's patents on isosorbide purification [41].
End-group analysis by 1 H NMR revealed that a majority of end groups were hydroxyl groups with endo orientation, which is in line with the difference in reactivity between isosorbide's two hydroxyl groups.Titrations revealed that only 2-3% of end groups were carboxylic acid, an excess of succinic acid did not affect this ratio.MALDI-TOF confirmed the presence of cyclic oligomers.To obtain higher molecular weights, and therefore possibly higher T g values, the authors partly replaced up to 40 mol% isosorbide with aliphatic diols (2,3-butanediol, 1,3-propanediol and neopentyl glycol).The resulting copolyesters did exhibit higher molecular weights (M n = 2,700-4,600 g/mol), but also much lower glass transition temperatures (20.6-50.6 • C).The authors also described poly (isoidide succinate) in a patent utilizing the same synthetic procedure as for the isosorbide analogue.These polymers exhibited higher molecular weights and T g values, as well as partial crystallinity [42].Isoididebased copolyesters with aliphatic diols unsurprisingly showed higher M n values (2,200-6,700 g/mol) and glass transition temperatures (39.8-63.4• C) than their isosorbide counterparts (Table 2, entries 7-10) [43].
Despite the different reactivity between the primary diols and isosorbide, neither 1 H NMR nor 13 C NMR indicated a block-type distribution of the monomers in isosorbide-based polyesters.The authors

Table 2
Molecular weights and thermal properties of polyesters based on 1,4:3,6-dianhydrohexitols and several aliphatic diacids.For structures of the discussed polyesters, see Fig. 9.All examples were obtained by melt polymerization.a IS = isosorbide, II = isoidide.attributed this to the efficiency of Ti(OBu) 4 in transesterifications.In fact, most works that will be presented in this review show a random distribution of monomers in the polymer chain.This is not surprising, as isosorbide incorporation into polyesters usually requires long reaction times at elevated temperatures to achieve satisfactory molecular weights.Theoretically envisioned by Flory in 1940 [44], past studies on the occurrence of transesterification reactions in the middle of growing polyester chains during the synthesis of PET found that these transesterification reactions always occur during melt polymerization.Their rate depends on the reaction temperature [45], and these transesterification reactions occur in all polyester melts, sometimes well below the melting temperature [46].Given the long reaction times of isosorbide containing polyesters, a randomization of the monomer distribution during melt polymerization is therefore not surprising.
In order to increase the acid number of the final material, the isosorbide-based polyester synthesized by Noordover et al. was reacted with citric acid (1.05 equiv.)for 5 h in the melt after the polycondensation.Preliminary experiments indicated that 75-80% of citric acid molecules will only react once with the free OH groups, which was confirmed by only slight branching of citric acid (Fig. 8) modified polyesters.
These polymers exhibited a dramatic increase in free acid groups which enabled efficient curing with common commercial reagents.Thermal degradation of the modified polyesters started at around 210 • C, which still allows curing of the powder coatings.The mechanical and chemical properties, as well as the weathering resistance of these coatings were comparable to terephthalic acid-based formulations.A follow-up publication investigated the mechanism of the OH functionalization of the polyesters with citric acid [47].It was found that anhydrides formed from citric acid under thermal conditions were the active species in the functionalization of the OH groups.Unlike common trifunctional cross-linking agents like trimellitic anhydride, citric acid could not be employed during esterification/polycondensation of the initial polyester due to limited thermal stability at T > 180 • C.
The synthesis of poly(isosorbide succinate) (PIS) was investigated in another publication [25].The authors employed a similar reaction procedure (1 wt% Ti(OiPr) 4 ), although shorter reaction times (4 h) led to lower molecular weights (M n 522-1,200 g/mol).The addition of adipic acid or dimethyl terephthalate as a comonomer slightly increased the molecular weights and T g values (Table 2, entries 12 and 13).The latter is remarkable for the adipic acid copolyester, as longer chain diacids typically result in decreased glass-transition temperatures.The higher reactivity and therefore higher M n values explain this observation.
The authors argue that the lower thermal stability of shorter diacids under the employed reaction conditions caused this discrepancy.The surface properties of the polyesters were evaluated by contact angle measurements.They revealed that the surface of all three polymers was largely hydrophobic.TGA analysis revealed a higher thermal stability of long-chain acid polyesters, as 5% weight loss for PIS occurred at 193 • C, whereas the same weight loss occurred at 344 • C for PISe.The cytocompatibility for biomedical application of the synthesized polyesters was also investigated.It was concluded that PIS would be best suited for these applications due to its favorable mechanical strength and cytocompatibility.The biodegradability was influenced by a number of factors, including the surface properties, porosity and the chemical structure.In vitro degradation tests found that PIS degraded slower than the other materials, and the increased hydrophobicity was suggested as one reason.
Qi et al. also synthesized poly(isosorbide succinate) [54].The esterification was conducted with a 3 mol% excess of isosorbide for 8.5 h at 160 • C, the polycondensation for 7 h at 210 • C (see Table 2, entry 17).The authors obtained the material with a M n of 7,300 g/mol and a T g of 65 • C.They also utilized 1,4-butanediol as a comonomer, for further discussion of these copolymers, see chapter 3.2.
Smiga-Matuszowicz et al. reported the synthesis of PIS by reaction of isosorbide with an equimolar amount of succinic anhydride [56].The authors used the commercially available heterogeneous Brønsted acid Amberlyst 15 (5 wt%) and conducted the polymerization with a Dean Stark attachment in refluxing toluene for seven days (see Table 2, entry 19).After removal of the catalyst by filtration, the material was investigated for the application as in situ forming implants with slow release of an antibacterial drug.Despite the low molecular weight of 2,900 g/ mol, the T g of 73.8 • C is the highest reported for poly(isosorbide succinate) obtained by non-acid chloride routes (compare entries 7-19 in Table 2).This value is however still below the materials' real T g value, as glass transition temperatures of up to 78 • C were found for poly(isosorbide succinate) synthesized from succinyl chloride (M n = 8,600 g/ mol) [16].
The large variations of T g 's of PIS obtained by different synthetic routes shows that molecular weights obtained by melt polymerizations are usually too low to approach the maximum T g plateau value of high molecular weight PIS.It also shows that the molecular weight determination by GPC can be misleading for polyester materials, as the standards used for calibrations are either polystyrene or poly(methyl methacrylate) which are very different in terms of polarity.A copolyester containing isosorbide, succinic acid and azelaic acid was presented in a patent for printing toner applications [52].The incorporation of azelaic acid (up to 15 mol% of total monomers) resulted in an increase in molecular weights and a decrease in T g values (see Table 2, entries 14, 15).The resulting polymers were emulsified and showed a satisfactory performance in the respective application.
One solution to overcome the difficulty of obtaining high molecular weight poly(isosorbide succinate) within reasonable reaction times was suggested in a patent filed by China Petroleum & Chemical [53].The authors employed methyl isobutyl ketone or diisobutyl ketone as azeotropic water removal reagents in a weight ratio dialkylketone:total monomers of 1:1 -0.5:1.Using a Ti(OBu) 4 catalyst and total reaction times of up to 8 h at 180-200 • C, polyesters with a M n value of up to 7,500 g/mol were obtained.In the last 2-4 h of reaction, a mild vacuum (10-20 mbar) was applied.The comparatively low reaction temperatures and short reaction times prevent the decomposition and volatilization of isosorbide commonly encountered in these type of polymerizations.These results show the influence of the removal rate of the esterification byproducts in polymerizations, although the molecular weights were still quite low.
The low molecular weights obtained in some examples described above could be traced back to the sensitivity of Ti-alkoxides towards hydrolysis.Some authors use either Ti(OBu) 4 or Ti(OiPr) 4 in the esterification step without compensating for an eventual loss of active catalyst [25,39,48].The catalysts, usually present at 0.02-1 wt%, can form condensed species and agglomerates, which generally exhibit lower solubility and therefore reduced catalytic activity [61].Evtushenko et al. hypothesized that Ti-alkoxides form poorly soluble titanium carboxylates with dicarboxylic acids after observing the evolution of the IR spectra in time of a mixture of Ti(OBu) 4 and benzoic acid [62].
The effect of catalyst addition before and after the esterification step on the transesterification rate between succinic acid and 1,4-butanediol was investigated in detail by Jacquel et al. [63].The rate of transesterification was found to be severely accelerated when the catalyst was added after the esterification.Although the investigated polymer poly (butylene succinate) does not include 1,4:3.6-dianhydrohexitols, the phenomenon of catalyst deactivation by the condensation product H 2 O could be applied to isosorbide-containing polyesters synthesized from dicarboxylic acids.Yang et al. synthesized a Ti-catalyst bearing chelating ethanolamine ligands and compared its performance with common Tialkoxide catalysts in the polymerization of terephthalic acid with ethylene glycol [64].The intrinsic viscosity of PET reached higher values in shorter times with these chelated catalysts, which could be due to less pronounced deactivation by H 2 O formed during esterification and polycondensation.
Tan et al. used dimethyl succinate in the polycondensation reaction with isosorbide (see Table 2, entry 18) [55].The authors added Ti(OBu) 4 in two portions, before and after esterification step and conducted the reaction for 9.5 h at 180-230 • C.After polycondensation, the reaction products were precipitated from CHCl 3 /MeOH.The obtained M n was 13,400 g/mol, with a yield of only 56.3% and a T g value of 56.4 • C. The very low T d,5% of 129.8 • C indicates that the polymer product is not representative of an ideal high molecular weight sample of poly(isosorbide succinate).The same author also investigated copolyesters with 1,4-butanediol (see paragraph 3.2).
Consuming 12% of the overall isosorbide demand in 2015, poly (isosorbide succinate) currently mainly finds application as a thermosetting powder coating [65].This application does not require high molecular weights as a curing step (e.g. with hexamethylene diisocyanate) is usually performed after applying the coating [42].
A recent study by Zakharova et al. used dimethylesters of the aliphatic diacids adipic acid, suberic acid and sebacic acid with isosorbide using dibutyltin oxide as a catalyst (Table 2, entries 21, 26, 29) [57].The transesterification was conducted at 180-200 • C for 3-5 h at 1 bar N 2 , while the polycondensation was carried out at 200-220 • C at 0.03-0.06mbar for 6-8 h.The number-average molecular weights reached 10,100 to 16,900 g/mol with polydispersities between 2.3 and 2.7.The glass-transition temperatures of the obtained polyesters ranged between 5 and 20 • C and solely the sebacic acid based polyester exhibited crystallinity and a measurable melting temperature of 52 • C.
The same study replaced isosorbide with bicyclic diacetalized hexitols derived from D-mannitol and D-glucitol (Fig. 10).These primary diols have been shown to increase the thermal and mechanical properties of polyesters [66].The obtained molecular weights were slightly higher (M n between 10,400 g/mol and 20,900 g/ mol) than with isosorbide, while the T g values were significantly higher (22-72 • C), showcasing the effectiveness of these diols to restrict chain mobility.The main drawback of these monomers is their current commercial unavailability and their costly purification by chromatography [67].
In a recent publication, Liu et al. investigated several polyesters of isosorbide or isomannide with aliphatic diacids for potential applications as food packaging materials [20].The authors synthesized the materials from the diacid chlorides of the respective acids without the use of a solvent.Despite long polycondensation times (0.1 mbar for 15 h at 160 • C), polyesters with succinic and adipic acid were brittle due to too low molecular weights (M n between 4,300 g/mol and 15,500 g/mol, not shown in Table 2).This further showcases the difficulty of obtaining non-brittle materials of 1,4:3,6-dianhydrohexitols and aliphatic diacids, even when employing highly reactive diacid chlorides.For that reason, the authors only reported the barrier and mechanical properties of sebacic acid-based polyesters.It was found that poly(isosorbide sebacate) was the more interesting polyester composition due to a more favorable chain alignment compared to the isomannide-based polyester.The  barrier properties with respect to CO 2 , O 2 and H 2 O of that polyester were better than those of PLA and PE but not as good as those of PET and PEF.The main problem with poly(isosorbide sebacate) as a packaging material is the materials low glass transition temperature (T g = 2 • C) and low melting range (T m = 35-72 • C).The uneconomical synthesis from diacid chlorides is another unfavorable factor.
Okada et al. investigated the biodegradability of aliphatic polyesters based on 1,4:3,6-dianhydrohexitols in a series of publications [68][69][70].The polyesters were synthesized from the respective diacid chlorides (C 4 -C 12 ) and biodegradability was determined via enzymatic degradation in aqueous solutions, soil burial degradation and degradation in activated sludge.It was found that polyesters PIS, PIA and PISe described in this chapter are biodegradable in activated sludge and after prolonged soil burial [68,69].The lowest degradability was found for poly(isomannide succinate) (not described in this chapter due to no reported synthesis from diacids or diesters), which is likely due to its semicrystalline structure.Studies on the degradability in different aqueous enzymatic solutions revealed a correlation between diacid length and degradability [70].An intermediate diacid chain length (PISe) favored enzymatic degradability, whereas short diacids (PIS) or crystallinity hindered it.
One example of polycondensation reactions of isosorbide with large diacids (Fig. 11) was investigated by Vendamme et al., who employed dimerized fatty acids containing a double bond, to obtain materials for pressure sensitive adhesion applications (see Table 2, entry 30) [60,71].
The esterification was carried out at 180 • C for 12 h under a N 2 -atmosphere, afterwards, the temperature was increased to 200 • C and a vacuum of 2-4 mbar was applied.The catalyst Ti(OBu) 4 (0.04 mol% of carboxylic acid functions) was introduced dissolved in xylene, and the reaction was continued at 220-250 • C for 6 h.The obtained molecular weights depended heavily on the ratio of diol to diacid, as the ratios diol: diacid of 0.95 and 0.91 yielded M n values 10,505 g/mol and 5,440 g/ mol respectively, with polydispersities of 2.57 and 2.61.In the same study isosorbide was replaced with 1,4-butanediol, the molecular weights however were in the same range.As the polyesters were modified further through epoxidation and subsequent gelation with free -COOH groups, the end-group concentration was an important parameter.Titrations revealed that isosorbide-based polyesters possessed a significant amount of free -OH groups compared to 1,4-butanediolbased polymers.The authors report that even prolonged reaction times at 250 • C did not increase the carboxylic acid end-group concentration.The T g values of polyesters based on 1,4-butanediol and isosorbide were − 46.5 and − 18.2 respectively.The researchers concluded, that the influence of isosorbide on the pressure sensitive adhesion properties of the novel materials is very complex and needs further elucidation.
An interesting approach to increase the reactivity of isosorbide for the synthesis of PICC was taken by Yoon et al. who modified the isosorbide structure during the esterification step by in-situ acetylation with acetic anhydride (Ac 2 O) (Scheme 2) [58].The authors employed 1,4cyclohexandicarboxylic acid as the diacid-monomer with a GeO 2 / dibutyltin oxide (200 ppm/150 ppm) catalytic system.Ac 2 O was added in amounts of 0-75 mol% relative to isosorbide.The esterification was conducted at 120-250 • C for 3 h under a nitrogen atmosphere.The polycondensation was conducted at 265 • C at 0.4 mbar.
The obtained results clearly show increased reactivity upon addition of Ac 2 O, although the high acidity catalyzed the ring-opening of isosorbide, forming up to 28 mol% of 1,4-sorbitan relative to isosorbide.The polyesters showed M n values between 11,600 and 18,300 g/mol with high polydispersities (PDI ≥ 4.6) at Ac 2 O concentrations > 1 mol% (Table 2, entry 25).This is a result from the incorporation of 1,4-sorbitan, which led to uncontrolled cross-linking in the polyester.The T g values ranged between 116 and 131 • C, again decreasing at Ac 2 O loadings > 15 mol% due to incorporation of the less rigid 1,4-sorbitan.The authors conclude that the presented method could be viable in industrial applications, as small amounts of Ac 2 O can have significant effects on the polymerization efficiency.A follow-up publication showed the effects of isosorbide ring opening on the deformation behavior of PICC in water at 100 • C [72].It was found that PICC with 1,4-sorbitan in the polymer chain deformed to an unexpectedly large extent through a mechanism similar to solvent-induced crystallization.
The described synthesis strategy was applied to the commercial copolyester poly(ethylene-co-isosorbide-co-1,4-cyclohexanedimethyl ene terephthalate) (marketed as Ecozen® by SK chemicals) in a subsequent study.Addition of Ac 2 O to the polymerization did not improve the polymerization outcome for this copolyester (see paragraph 3.1.1for more information on the synthesis of Ecozen®) [73].
Another rigid, biobased polyester based on isosorbide was reported in a recent work by Nsengiyumva et al [74].The authors synthesized copolyesters of various aliphatic diols with camphoric acid using a melt polymerization protocol with the strong Brønsted acid p-toluenesulfonic acid as a catalyst.
After prolonged reaction times (12 h under vacuum at up to 230 • C), poly(isosorbide camphorate) (Fig. 12) was synthesized with an M n = 6,900 g/mol and a PDI = 6.1 (Fig. 12).This high polydispersity was likely caused by the ring opening reaction of isosorbide to 1,4-sorbitan under Brønsted acidic conditions and subsequent cross-linking reactions.A T g of 125 • C and a high thermal stability (T 5%d = 355 • C) was reported for this material, although values for the same material with a higher molecular weight and lower polydispersity could be different.
The effect of the ester leaving group on the molecular weights of polyesters is showcased in a patent by Ube Industries on the synthesis of poly(isosorbide oxalate) [49].The authors employed diphenyloxalate in their procedure and obtained polyesters with intrinsic viscosities of up to 0.87 dl/g (see Table 2, entry 1).This value indicates a high molecular weight material.The excellent leaving group properties of phenol apparently outweigh the low reactivity of isosorbide.The polyester had a T g of 172 Two recent patents by Avantium explore the synthesis of copolyesters of poly(isosorbide oxalate) using the same synthesis strategy (Fig. 13).In the first patent, the authors replaced up to 50 mol% of isosorbide with linear diols (such as 1,4-butanediol and 1,6-hexanediol), which significantly reduces the T g compared to poly(isosorbide oxalate) (see Table 2, entries 2-4) [50].M n values of up to 20,287 g/mol were reported, depending on the linear diol and its amount in the copolyester.In the second patent, up to 50 mol% of diphenyl oxalate were replaced by reactive esters of adipic acid, namely di(4-methylphenyl) adipate or divinyl adipate [51].Incorporation of adipate units into poly(isosorbide oxalate) resulted in more flexible materials with lower T g values (see Table 2, entries 5-6) and lower molecular weights of up to 9,900 g/mol (with 19 mol% adipate units respective total diacid).Reactive esters of adipic acid exhibited a lower reactivity than diphenyl oxalate, although diphenyl adipate was not tested for a direct comparison.Poly(isosorbide adipate) synthesized with either di(4-methylphenyl) adipate or divinyl adipate yielded low molecular weight polyesters (see Table 2, entries 22-23), confirming their low reactivity.
Comparison experiments reported in both patents on the synthesis of poly(isosorbide oxalate) replacing diphenyl oxalate with either diethyl oxalate or oxalic acid resulted in very low molecular weight products (M n up to 1,700 g/mol starting from diethyl oxalate, even lower starting from oxalic acid, not shown in Table 2).This further shows the potential of using good leaving groups for the synthesis of high molecular weight polyesters, although it must be noted that oxalic acid has a low thermal stability and its short chain alkyl esters have relatively low boiling points, which limit their applicability in polyester synthesis in the first place.Other oxalate-based copolyesters of 1,4:3,6 dianhydrohexitols with linear diols (1,6-hexanediol and 1,8-octanediol) have been reported by Rajput et al. using a synthetic strategy that involves purification of pre-synthesized macromonomers by column chromatography (not pictured) [75].
The authors synthesized diphenyl-1,4-cyclohexanedicarboxylate by transphenylation of 1,4-cyclohexanedicarboxylic acid with an excess of diphenylcarbonate (5.2 equivalents versus 1,4-cyclohexanedicarboxylic acid) using various Brønsted bases as catalysts.The product was purified by distilling off excess diphenylcarbonate.Subsequently, diphenyl-1,4cyclohexanedicarboxylate was combined with excess diphenylcarbonate (61 mol% of Diphenyl-1,4-cyclohexanedicarboxylate), isosorbide and the Brønsted bases Cs 2 CO 3 and NaOH (50 ppm each).A slight excess of phenylesters of 1.02 equivalents relative to isosorbide was used.The authors obtained products with a M n of up to 11,000 g/ mol (after precipitation in MeOH) within 80 min of total reaction time at 210 • C, during which the pressure was slowly reduced to 2 mbar.The T g of the products was as high as 141 • C.
The use of phenylesters for the synthesis of high molecular weight polyesters with the unreactive secondary diol 2,2,4,4-tetramethyl-1,3cyclobutanediol (see Fig. 3) has been reported as early as 1967 by Eastman [77].
Today, the commercial synthesis of high molecular weight polyesters Scheme 2. Activation of isosorbide by in-situ acetylation with acetic anhydride during the polymerization with 1,4-cyclohexanedicarboxylic acid.The acid catalyzed ring opening hydration of isosorbide to 1,4-sorbitan is also depicted.While being more reactive in (trans)esterifications than aliphatic diesters or diacids, diphenylesters are typically synthesized from acid chlorides.Some works, as seen in the patent described above on poly (isosorbide-1,4-cyclohexanedicarboxylate-co-carbonate), utilize the transphenylation between dicarboxylic acids or esters and diphenylcarbonate [80,81] or phenylacetate [82].Both synthesis routes produce stoichiometric amounts of byproducts, which is not desirable from a sustainability point of view.There have been some recent efforts to synthesize phenylesters of carboxylic acids or esters by direct esterification [83,84].In this case, phenol could be reused after polymerization of the diphenylesters with diols.This is a promising strategy to obtain high molecular weight polyesters with 1,4:3,6-dianhydrohexitols, as it has already been shown to yield good results with diphenyl oxalate and diphenyl-1,4-cyclohexanedicarboxylate/diphenylcarbonate.Phenol is a significantly better leaving group than H 2 O and MeOH.Condensation reactions with diphenylesters do not require solvents or stoichiometric amounts of base as do reactions with acid chlorides.This way they combine the high reactivity of acid chlorides with the simple reaction procedures of diacids and diesters.The potential of using the phenol leaving group in polycondensations can also be seen in the synthesis of poly(isosorbide carbonate), a sustainable and health-concern-free alternative to poly(bisphenol A carbonate) [85,86].A number of recent examples investigated the melt polymerization of isosorbide with diphenylcarbonate and M n values of 61,700 g/mol can currently be reached using imidazolium-based ionic liquids [87].Poly(isosorbide carbonate) has already found commercial application as a replacement for poly(bisphenol A carbonate).It is marketed as DURABIO TM by the Mitsubishi Chemical Corporation.[https://www.m-chemical.co.jp/en/products/departments/mcc/pc/product/1201026_9368.html]The wide range of possible applications due to the materials' favorable mechanical properties combined with its resistance to chemicals and UV light resulted in patent applications from other major plastic producers such as Teijin Limited and Sabic Innovative Plastics [88,89].
To summarize, polyesters with 1,4:3,6-dianhydrohexitols synthesized by melt polymerization with aliphatic diacids or diesters are usually obtained with low molecular weights.The impact of this on the thermal properties can be seen by the large variations of T g values for the same material (see Table 2).Mechanical properties of low molecular weight materials are usually inferior compared to high molecular weight counterparts of the same chemical composition [38], which means that the potential of some aliphatic polyesters from this chapter is not yet fully clear.Especially the rigid poly(isosorbide succinate) with a T g of around 78 • C (determined after synthesis with succinyl chloride by solution polymerization [16]) bears a large unexplored potential, as amorphous biobased materials with a T g > 60 • C are highly sought after for commercial applications such as plastic packaging [90].
Most works focus on the use of isosorbide, which is currently the most widely available 1,4:3,6-dianhydrohexitol.While additives like Ac 2 O were shown to increase the chain lengths of the final polymers, side reactions and subsequent crosslinking exacerbate their application on larger scales.The use of phenylesters, which has not been reported for polyester synthesis in the academic literature until the time this review was written, could prove to be useful to synthesize high molecular weight polyesters with unreactive secondary diols by melt polymerization.

Enzymatically catalyzed polymerizations
A few recent studies used the enzyme Lipase B from Candida Antarctica immobilized on a macroporous resin to catalyze the polycondensation between 1,4:3,6-dianhydrohexitols and several aliphatic diacids.
Initial investigations employed isosorbide with diethyl esters of several aliphatic diacids in solvent systems allowing for the azeotropic distillation of EtOH [91].The best results were obtained with a cyclohexane:benzene 6:1 mixture, a catalyst loading of 10 wt% with respect to the diethylester monomer and reaction times of 168 h at 85 • C. The esterification was conducted at atmospheric pressure, whereas polycondensation was conducted at 0.6 mbar.The obtained M n values ranged from 4,300 to 47,000 g/mol (see Table 3).The low solubility of polyesters based on short chain diacids in the reaction solvent limited their molecular weights, as substrate diffusion and contact with the heterogeneous catalyst was limited.Accordingly, the efficiency of the polymerization increased with higher dilutions.
The authors noted temperatures higher than 90 • C should be avoided to prevent denaturation of the enzymes.A comparison of T g values of polyesters produced by enzymatic or chemical catalysis reveals similar glass transition temperatures (compare Table 2 and Table 3).Poly(isosorbide adipate) for example exhibits very similar T g values of 34.5 • C (M n = 8,764 g/mol) and 35.0 • C (M n = 16,818 g/mol) synthesized by chemical [48] or enzymatic catalysis [91] respectively.
A subsequent publication compared the reactivity of 1,4:3,6-dianhydrohexitols with succinic acid in biocatalyzed polycondensations [95].The order of reactivity observed was isomannide > isosorbide ≫ isoidide, which is due to preferential docking of the enzyme's active site to endo-OH groups.This was confirmed by substrate-imprinted dockinganalysis.Product analysis by MALDI TOF revealed that initial polycondensates were predominantly cyclic (60% after 8 h).Prolonged reaction times favored the formation of linear polyesters, with a product distribution of linear:cyclic of 65:35 after 24 h.Previous works on chemically catalyzed melt polymerizations do not explicitly mention the formation of cyclic reaction products.
Naves et al. investigated the biocatalytic synthesis of partly unsaturated polyesters based on isosorbide and diethyl adipate [93].Complete replacement of diethyl adipate with unsaturated diesters resulted in very low molecular weights (M w < 1,100 g/mol).The authors were able to introduce 5 mol% of unsaturated monomers like diethyl itaconate or diethyl fumarate into the polyester without a significant loss of reactivity, although cross-linking during polymerization decreased the molecular weights noticeably (Table 3).The authors concluded that the incorporation of up to 5 mol% of unsaturated diester monomers into poly(isosorbide adipate) can be viable to introduce double bonds into 1,4:3,6-dianhydrohexitol-based polyesters.Other monomers, such as isomannide (replacing isosorbide) or the free acids itaconic and fumaric acid (replacing diethyl fumarate) did not lead to sufficiently high molecular weight products.
One potential application of poly(isosorbide sebacate) obtained by enzymatic polymerization was presented by Smiga-Matuszowicz et al. [94].They applied conditions reported by Juais et al. and obtained slightly lower molecular weights (see Table 3).The authors investigated potential applications of the polyester and its copolymers with 1,2-propylene glycol as a parental (non-oral) drug delivery system.It was found that these polymers can function both as antibiotic carriers and as promotors of new bone tissue formation.Enzymatically catalyzed polycondensations show some promising results with regard to molecular weights and substrate range.The heterogeneous catalytic system allows for easy separation from the reaction medium.One advantage over traditional polycondensation methods is the absence of residual Lewis acid catalysts in the final polymer product.The reaction conditions necessary for biocatalytic polyesterifications however are still far from industrially applicable.This is due to use of organic solvents, high catalyst loadings (10 wt% with respect to monomer) and long reaction times of up to 165 h.

Polyesters with 1,4:3,6-dianhydrohexitols as a comonomer
The commercial interest to incorporate isosorbide as an comonomer (≤50 mol%) into both aliphatic and aromatic polyesters is considerable, as isosorbide content as low as 2 mol% can enhance the thermal properties of polyesters.This is shown by the fact that in 2018, 35% of the total isosorbide market (worth 396.4 million $) was used for the production of poly(ethylene-co-isosorbide terephthalate) or PEIT [7].Combined with the previously described inherent difficulty to incorporate isosorbide into polyesters in larger molar percentages, the use as a property-enhancing comonomer is currently more manageable from a synthetic point of view.One important difference to examples with high isosorbide content from the previous chapter is that incorporation of isosorbide as a minor constituent of diols usually results in materials with a semicrystalline structure.These polyesters can be subjected to solid state polymerization (SSP) to further increase their molecular weight, whereas previous examples are mostly amorphous polymers.The molecular weights of amorphous polyesters can't be increased by SSP, which means that molecular weights high enough for the desired application must be reached during melt polycondensation or via reactive extrusion with so called chain extenders.

Aromatic polyesters with 1,4:3,6-dianhydrohexitol comonomers 3.1.1. Polyterephthalates
A large portion of the efforts to copolymerize isosorbide with aromatic monomers revolves around the production of poly(ethylene-coisosorbide terephthalate) (PEIT, Fig. 15).Depending on the molar percentage of isosorbide in the copolymer, the properties can allow a variety of possible applications of the resulting polyesters.While smaller amounts of isosorbide (2-6 mol% of total diol) enable hot-filling of bottles due to a T g of 83-86 • C, larger amounts of > 20 mol% result in hard, amorphous copolymers with T g values higher than 120 • C (not shown in Table 4) [96].
Table 4 summarizes the synthetic conditions and product characteristics of PEIT polyesters obtained in several works (author names from the text are referred to in the table).A patent by Du Pont from 2003 describes early, large scale efforts (39 kg, 240 mol of terephthalic acid) on the synthesis of PEIT copolyesters containing up to 5.4 mol% of isosorbide using a GeO 2 catalyst [97].The authors conducted the experiments in steel reactors which allowed them to pressurize the reaction mixture during esterification.
In a later publication the mechanical and thermal properties of PEIT synthesized in the previously described patent were investigated [98].The material exhibited desirable thermal properties for hot-bottle filling with a T g of 86 • C and a semicrystalline structure.The mechanical properties were however not satisfactory for solid state film drawing or stretch blowing, as strain softening occurred upon isosorbide incorporation.This phenomenon typically leads to difficulties to obtain end-use materials with high mechanical and dimensional stability.The authors did to some extent solve this issue by adding a nucleating agent, the low molecular weight ionomer Sodium Aclyn 285®, which improved the mechanical properties under stress.
Although the aforementioned patent did obtain satisfactory isosorbide incorporation and yields, purely Ge-based catalysts are not optimal for large scale industrial processes due to the high price of pure GeO 2 [105].Attempts to incorporate larger amounts of isosorbide in PEIT suffer from two main problems.Firstly the low reactivity of isosorbide requires higher temperatures and longer residence times, leading to thermo-oxidation which results in undesirable colored byproducts.Secondly, common Sb-based catalysts for PET production are deactivated at higher isosorbide loadings [96].
One attempt by Bersot et al. to increase the catalytic efficiency of the common polycondensation catalyst Sb 2 O 3 for PEIT synthesis involves the addition of a second metal species known to form bimetallic complexes (M = Li + , Mg 2+ , Al 3+ ) [96].Addition of these metal-ions as organic salts (Li(OAc)x2H 2 O, Mg(OAc) 2 x4H 2 O, Al(OEt) 3 ) at molar ratios of 1:0.25 to 1:1 Sb:M, did indeed facilitate the transesterification during PEIT (20 mol% isosorbide of total diols) synthesis, as indicated by measurements of the torque-variation during the reaction.In the case of Al, the authors were able to reduce Sb-loadings to 50% of the initial concentration (280 ppm Sb to 140 ppm Sb) and obtained slightly increased molecular weights.The final product did show less coloring for some added metals.
A more recent study by Stanley et al. also investigated bimetallic catalytic systems for PEIT polymerization [99].The authors tested combinations of the common commercial PET polymerization catalysts Sb 2 O 3 , GeO 2 and Ti(OiPr) 4 for the synthesis of PEIT containing around 9 mol% isosorbide.The reaction rate and coloration of the resulting polymer was found to depend heavily on the catalytic system.While purely Sb-based catalysts did lead to a higher isosorbide incorporation (around 1% higher than Ti-and Ge-catalysts), with acceptable coloration mainly due to Sb-metal formation, the overall reactivity was lower than in other systems.Ti-based systems showed the highest reaction rates with the highest coloration, while GeO 2 -catalysis produced the lowest coloration with a slightly better reaction rate than Sb 2 O 3 .The authors concluded that depending on the desired properties and target applications, an ideal catalytic system can be found.No catalyst combination achieved high isosorbide incorporation, high reaction rates and low coloration.
A patent by Roquette Frères from 2016 focused on the impact of GeO 2 /Al(OEt) 3 catalysis on the coloration properties of PEIT [100].It was found that an increased catalyst concentration did improve the pale yellow coloration slightly, without impacting the molecular weight of the final polyester (the reduced viscosities did not decrease significantly).Addition of Co(OAc) × 4H 2 O (30 ppm Co) to the original catalytic system eliminated the yellow coloration and yielded a grey polymer.
A recent study by Zhang et al. investigating the optical properties of PEIT utilized an ethylene glycol antimony/γ-AlOOH catalytic system (Sb:Al mass ratio 6:4) [101].The resulting copolyesters showed improved optical properties compared to PET at 10 mol% isosorbide incorporation, allowing for applications that require high-transmittance materials.Similar copolyesters synthesized from isomannide exhibited  superior properties with regard to changes in color and rate of crystallization, which turned out to be slower, thus resulting in more favorable optical properties.
An environmentally more benign Ti-Mg bimetallic catalyst was explored by Li et al. [102].Loadings of only 5 ppm of this sustainable catalyst yielded similar polyester products as 230 ppm of Sb 2 O 3 .Thermal and mechanical properties of the polyester products are in line with previous reports.The coloration of the polymers was not mentioned except for the fact that loadings of 30 ppm of the Ti-Mg catalyst led to brown polyester products.
In a very recent study, Descamps et al. investigated the crystallization kinetics of PEIT containing up to 20.8 mol% of isosorbide [104].The catalyst used for the reaction was not described, but fairly high inherent viscosities (up to 0.56 dl/g with 20.8 mol% isosorbide) and a favorable coloration of the polymer products was reported.The findings on the effect of isosorbide on the crystallinity of the polyesters coincide with previous reports on different polyester compositions, as isosorbide incorporation increased the crystallization halftime while decreasing overall crystallinity.XRD studies revealed that crystallization occurs exclusively at ethylene terephthalate moieties.With respect to potential applications the authors concluded that PEIT with an isosorbide content of > 15 mol% are challenging and time consuming to crystallize.Polyesters with a lower isosorbide content can therefore be used for applications that require fast crystallization like injection stretch blow molding (hot bottle filling).Materials with a higher isosorbide content are interesting for applications that require high T g , transparent materials in optical or food contact applications.
While organic catalysts are fairly common in ring-opening polymerizations, their use in step growth polyesterifications is rarely reported.One recent example by Stanley et al. compares the performance of several organic catalysts in the preparation of PEIT with 13 mol% isosorbide [103].PEIT oligomers, synthesized without a catalyst in a separate step, were used to study the performance of several metal-free catalysts during polycondensation (40 g of oligomers from esterification used).While the highest molecular weights were obtained with Triazabicyclodecene (M n = 23,000 g/mol) and p-toluenesulfonic acid (24,300 g/mol), they resulted in strongly colored products and the latter in a high DEG content (10%).The best overall results were obtained with 1,8-Diazabicyclo(5.4.0)undec-7-ene (DBU), although it is inferior to Sb 2 O 3 (M n (DBU) = 17,700 g/mol; M n (Sb 2 O 3 ) = 34,400 g/mol).The authors conducted mechanistic studies on some of the organic catalysts and showed that the mechanism of action is different from traditional Lewis acid catalysts.
It is important to note that the performance of a catalytic system in a given polyesterification reaction is known to depend on many factors.This was shown in a series of publications by Otton et al. on the intricacies of PET polymerization [106][107][108][109].Among others, these include the specific monomers employed, their ratios, the reaction temperatures and end-group concentration.This is especially true when strongly coordinating bidentate diols, like ethylene glycol, are used.A direct comparison of the performance of different catalytic systems for different polyesterification systems is therefore only possible to a limited extent.
The majority of the efforts to introduce other 1,4:3,6-dianhydrohexitols into PET by melt polymerization are reported in the patent literature.A direct comparison of the incorporation ratios of isoidide (98% incorporated compared to feed) and isosorbide (83% incorporated compared to feed) during PET copolymer synthesis in a recent patent by Roquette Frères showed that the former reacts more readily during polyesterification [110].This is in line with previous reports that show that isoidide is the most reactive 1,4:3,6-dianhydrohexitol in (trans) esterification reactions.
The authors of a recent patent by Ester Industries Limited attempted the challenging incorporation of isosorbide into PEIT by performing the copolymerization of TPA and EG with preformed poly(isosorbide isophthalate) (not shown in Table 4) [111].The poly(isosorbide isophthalate) prepolymer was obtained by melt esterification of the respective monomers using butylstannoic acid catalysis at 240-250 • C. Terpolyesters with isosorbide contents between 1 and 20 wt% were synthesized by melt polycondensation of terephthalic acid, ethylene glycol and the poly(isosorbide isophthalate) prepolymer using a GeO 2 / Sb 2 O 3 /Co(OAc) 3 catalyst mixture.After extrusion and chipping, the reaction product exhibited an amorphous structure as both isophthalate and isosorbide disrupt the alignment of polymer chains.The polymer was dried and pre-crystallized and solid-state postcondensation was employed to improve the final molecular weight of the polymer (intrinsic viscosity from 0.62 dl/g before SSP to 0.84 dl/g after SSP).
For applications that require both structural toughness and high T g values, the polyester poly(ethylene-co-isosorbide-co-1,4-cyclohexanedimethylene terephthalate) (PEICT) is an interesting option (Fig. 16).It is commercially produced and distributed by SK Chemicals as Ecozen® and finds application in food and beverage containers, construction materials and more.
Yoon et al. employed a standard two-step synthetic protocol using GeO 2 /dibutyltin oxide catalysis to synthesize PEICT with an isosorbide content of up to 55 mol% (see Table 5) [73].
Increasing isosorbide content resulted in lower molecular weights, although the decrease was limited compared to previous studies with similarly high isosorbide loadings.A detailed experimental and theoretical investigation on the reactivity of the different diols was conducted.The results indicate that ethylene glycol plays a crucial role in achieving high molecular weights and a random monomer distribution.Firstly, the high reactivity of EG's primary OH groups and low steric hindrance compared to the other diols favored the transesterification at both chain ends and middle segments.The former were constituted largely by isosorbide's less reactive endo-OH groups after esterification, which shows the tendency of a block-type monomer distribution in this terpolyester.It was also found that chain scission in the polymer's middle segments occurred more favorably at carbonyl groups adjacent to ethylene glycol linkers.This resulted in additional scrambling of the monomer distribution.The high volatility of EG compared to the other diols enabled its removal under vacuum if transesterification occurred at the chain ends.Calculations revealed the minimum amount of ethylene glycol necessary to accelerate the reaction and randomize the sequential distribution to be around 5 mol% with respect to total diol content.
The PEICT terpolyesters exhibited T g values as high as 143 • C at 55 mol% isosorbide content.This value exceeds the T g of poly(ethylene naphthalate) or PEN (T g = 120 • C), a polyester used for high-demand applications for which PET is unsuitable.Subsequent publications on PEICT explored additional thermal properties [112] and possible nanofiber applications [113].
The incorporation of isosorbide into polyesters usually results in a decrease of the melting temperature compared to the parent polymer, as isosorbide's bent structure weakens interactions between polymer chains.The polyester poly(1,4-cyclohexanedimethylene terephthalate) or PCT, for example, already possesses excellent mechanical properties due to its rigid structure.The main drawback, however, is the high melting temperature range of 278-318 • C, which is close to the polymer's decomposition temperature.This results in difficult processing of the polyester, as thermal degradation can result in brittleness of the final material.One way to decrease the T m of PCT is the incorporation of isosorbide.Two recent publications investigate the synthesis of poly (1,4-cyclohexanedimethylene-co-isosorbide terephthalate) (PCIT, see Fig. 17 and Table 6) [114,115].
One striking difference between the two publications is the different incorporation ratios of isosorbide.Legrand   semicrystalline polyesters, which was also found for PEIT [104].The monomer distribution was found to be random for both PEIT and PCIT.This seems counterintuitive, but it was shown in previous studies that the dominant monomer in a statistically random copolyester can either in-or exclude other comonomers present in lower quantities during crystallization [116].Increasing isosorbide content decreased the crystallization rate of the polyesters.The authors conclude that incorporation of isosorbide into PCIT is only viable at low isosorbide content due to its large influence on the overall polymer structure and its low reactivity resulting in prolonged reaction times and lower molecular weights.It was also found that an isosorbide content above 40 mol% led to the formation of significant amounts of 1,4-sorbitan by acidolysis during esterification.Despite higher molecular weights reported by Koo et al, T g values of similar polyester compositions are between 7 and 10 semicrystalline structure up to an isosorbide content of 25 mol%.The latter authors successfully increased the molecular weight of these materials by SSP and their mechanical properties were compared before and after SSP.Some properties like Notched impact strength, elongation at break and tensile stress at break were enhanced while others like flexural modulus and tensile modulus slightly decreased.This shows that the relatively low molecular weights obtained negatively influence the properties of these polyesters.Despite this, the mechanical properties of PCIT polyesters were found to be comparable to commercial poly(bisphenol A carbonate) replacements such as Eastman's Tritan© TX 1001.
A similarly rigid material was reported in a recent publication by Legrand et al. [117].The authors synthesized a copolyester containing terephthalic acid, isosorbide and tricyclodecanedimethanol (TCDDM), a rigid, primary diol monomer already used for commercial polyester synthesis (Fig. 18).
The molecular weights of the polyester compositions are shown in Table 7.Interestingly, larger amounts of isosorbide resulted in polyesters with higher molecular weights, a trend which is counterintuitive when considering the lower reactivity of isosorbide's secondary alcohol groups.It was found that higher isosorbide contents lead to a slower esterification, an overall lower conversion of the monomers (62% with 67 mol% isosorbide) and shorter oligomers before polycondensation.Nonetheless, the authors reached higher final molecular weights for polyester compositions with high isosorbide content, which was hypothesized to be due to the bulky and hydrophobic nature of tricyclodecanedimethanol.This could favor the chain-growth reactions of isosorbide chain ends over tricyclodecanedimethanol chain ends, especially when considering the hydrophilic nature of ester linkages.The materials exhibited high T g values.The mechanical properties were investigated and proved to be promising, but larger amounts of isosorbide (>40 mol%) led to brittle fracture of samples, indicating that     molecular weights were too low to reach a fully ductile material.Ahn et al. investigated poly(ethylene-co-isosorbide succinate-coterephthalate) (PEIST) as a terpolyester containing biobased succinic acid (Fig. 19, Table 8) [118].
The terpolyesters contained 15 or 30 mol% isosorbide with regard to the total diol content and a varying ratio of succinic acid and dimethylterephthalate A different approach towards a similar polymer composition was taken in a study where recycled PET-flakes were first depolymerized with isosorbide and a metal catalyst and subsequently repolymerized with succinic acid and glycerol to obtain crosslinked terpolyesters for powder coating applications [119].The ideal metal catalyst for the depolymerization step was obtained by screening common polyesterification catalysts in the transesterification between dimethyl terephthalate and isosorbide.Out of those, Ti(OBu) 4 and butyltin oxide hydroxide achieved satisfactory conversions (62-63%) and only the latter yielded a product with low coloration.In order to obtain a sufficient amount of free COOH groups for the hardening process, the authors used 65 mol% isosorbide and 80 mol% succinic acid with respect to the PET repeating unit.The depolymerization was conducted at 230 • C for 1 h under nitrogen, after which no more solid PET flakes were visible.The oligomers showed a M n value of 428 g/mol with a PDI of 2.1 and an exo/endo ratio of the free isosorbide OH groups of 37/63, confirming the higher exo-OH reactivity.After addition of succinic acid and glycerol, the transesterification was performed under N 2 at 230 • C for 3.5 h.The polycondensation was conducted at the same temperature at 70 mbar for 6.5 h to reach M w and T g values of 6,300 g/mol (no M n value given) and 59.5 • C, respectively.The final properties of the uncured polyester were similar to a commercially available non-biobased powder coating polyester (see Table 9).The cured coating showed superior properties with regard to blooming and ageing resistance.
A different terephthalic acid based polyester, the properties of which can be significantly improved upon isosorbide incorporation, is poly (butylene terephthalate) (PBT).It has a less rigid structure than PET and can therefore additionally benefit from the isosorbide-induced rigidity.
The past years saw a few publications on the melt polymerization of poly (1,4-butylene-co-isosorbide terephthalate) (PBIT).One important difference to PEIT, from a synthetic point of view, is the use of dimethyl terephthalate instead of terephthalic acid.The following studies do not mention the reasoning behind this, which is remarkable since terephthalic acid should be a more reactive and cheaper monomer to use.It is possible that the acidity of terephthalic acid leads to a more significant dehydration reaction of 1,4-butanediol to tetrahydrofuran or to ring opening of isosorbide to sorbitans at the elevated reaction temperatures.Two recent studies report the synthesis of poly(1,4-butylene-co-isosorbide terephthalate) (PBIT, Fig. 20) with very different results (see Table 10) [120,121].

Table 7
Characteristics of poly(isosorbide-co-tricyclodecanedimethylene terephthalate).Dimethyltinoxide catalyst (200-400 ppm).COOH/OH ratio 1:1.1.Esterification conducted for 270 • C until the amount of theoretical distillate was collected.Polycondensation conducted at 280 • C until a desired torque value was reached.The reactions were conducted on a 9.64 mol TPA scale in a 7.5 l autoclave.The molecular weights reported by Chen et al. are significantly higher than those reported by Lavilla et al. for similar polyester compositions.The synthetic conditions of both works mainly differ with regards to the diester/diol ratio and the catalyst used (see caption of Table 10).Despite using a large diol excess of up to 2.2 equivalents relative to the diester (compared to 1.05 equivalents used by Lavilla et al.), the total loss of isosorbide is lower for Chen et al.The T g and T m values followed the typical trend in both cases.Combined with higher decomposition temperatures at higher isosorbide content, melt processing of PBIT becomes easier compared to PBT, as the window between T m and decomposition is increasing with higher isosorbide content.The copolyesters were semicrystalline until an isosorbide content of 36 mol% was reached.Degradation studies conducted by Chen et al. revealed that hydrolytic degradation under acidic conditions proceeds rapidly in the amorphous regions of the polyester.This degradation was found to slow down when it reached semicrystalline polyester layers.No significant degradation was observed under neutral hydrolytic or enzymatic conditions, regardless of isosorbide incorporation.The mechanism of degradation is mainly ester bond hydrolysis, the ether linkages in the isosorbide repeat units were found to stay intact.
Lavilla et al. compared polymerization results with isosorbide to two other rigid, primary diols, the diacetalized hexitols Manx and Galx (for examples on the polymerization of these monomers with aliphatic diacids, see chapter 2.2.1).
Under the same reaction conditions as reported for PBIT, double the molecular weight was obtained (see Fig. 21) and no rigid diol was lost with these two polyesters.These results confirm the significantly lower reactivity of the secondary OH groups of isosorbide, especially when compared to the primary hydroxyl groups of diacetalized hexitols.The influences of Galx and Manx on the polyester properties were comparable to isosorbide.The chemical stability of these (monomeric) diacetalized hexitols under polyesterification conditions was not discussed by the authors.The high molecular weights obtained suggest sufficiently high stability under the experimental conditions.Once Galx and Manx are bound in the polymer, the thermal stability of polyesters containing these diacetalize hexitols is comparable to those containing isosorbide.
An earlier publication on the synthesis of PBIT used a Ti(OBu) 4 catalyst and slightly elevated temperatures (250 • C) but the results largely coincide with Lavilla et al. [24].The molecular weights obtained in the melt polymerization were so low that no thermal characterization was conducted.Even a large excess of diols (DMT:diols 1:2, molar ratio of isosorbide:1,4-butanediol 1:1) did not improve the obtained molecular weight as it did for Chen et al.The incorporation of isosorbide in that case was only 50% compared to the feed ratio.The authors continued their investigation with terephthaloyl chloride.A patent published in 2000 by HNA Holdings presented similar results about the difficulty to incorporate > 10 mol% of isosorbide into PBIT and other aromatic polyesters [122].The authors also employed Ti(OBu) 4 as a catalyst.Most of the unreacted isosorbide, which, together with the second diol moiety was used in at least 1.5-fold excess, was collected in the distillate.Even examples where the esterification was conducted in steel reactors at a pressure of 5 bar resulted in an isosorbide incorporation of less than 50%.These results suggest that even complete incorporation of isosorbide into oligomers after transesterification does not prevent volatilization of isosorbide due to the equilibrium nature of these reactions.Chain-scission and subsequent re-release of free isosorbide or short chain oligomers into the reaction mixture during polycondensation could therefore be one reason for the challenging incorporation of isosorbide into aromatic polyesters.The present examples on PBIT synthesis also showcase the importance of catalyst choice, as examples that use Ti(OBu) 4 exhibit lower molecular weights than those that use dibutyltin oxide.It is important to note that the present examples proceed via a transesterification mechanism.Unlike examples from chapter 2.2.1, where Ti(OBu) 4 showed a decreased activity, possibly after reaction with H 2 O, no deactivation of Ti(OBu) 4 by the condensation product MeOH is known.
A different approach to the synthesis of PBIT was taken by Sablong et al. [27].The authors used solid-state polymerization in an attempt to achieve higher molecular weights and isosorbide incorporation.Initial trials to directly incorporate 15 wt% of isosorbide into presynthesized PBT were not successful as isosorbide was too volatile under SSP conditions.The authors subsequently synthesized a less volatile macromonomer (see Fig. 22) by stepwise addition of isosorbide and 1,4butanediol to terephthaloyl chloride followed by purification by recrystallization.
Before starting SSP, the macromonomer and PBT were mixed by dissolution in 1,1,1,3,3,3-hexafluoro-2-isopropanol and subsequent evaporation of the solvent.Solid-state polymerization was conducted at 180-195 • C under argon for 48-422 h (see Table 11).Monitoring of M n throughout the reaction revealed that the molecular weight of the PBT decreased significantly in the beginning of SSP.The amorphous phase of PBT was initially subjected to transesterification initiated by free hydroxyl groups of the macromonomer. 1H NMR analysis during the reaction also revealed free endo-and exo-OH groups of isosorbide, a result of transesterification of the reaction primary alcohols of 1,4-butanediol with isosorbide-terephthalate esters.After the initial chain-scissionphase, the molecular weight of the polyester compositions increased.High isosorbide-loadings required extended reaction times to obtain satisfactory molecular weights (Table 11).
The obtained M n values ranged between 85,200 and 142,600 g/mol with little discoloration of the products.The polyesters exhibited crystallinity up to an isosorbide content of 25 mol%.This is a result of the block-like structure of the final polymer, as the macromonomer was only incorporated into the amorphous regions of PBT.This was indicated by T m peak that occurred in the first heating runs of DSC analysis, regardless of isosorbide content.To draw a comparison to classical polymerization methodology, the authors additionally synthesized PBIT copolyesters with varying amounts of isosorbide by melt polymerization.Low M n values (8,400-11,600 g/mol) and strong coloration of the products after 24 h reaction time confirmed the superiority of SSP with regard to molecular weights and discoloration.A downside of the SSP approach is the use of terephthaloyl chloride for the synthesis of the PBIT macromonomer and the required purification of the same, which makes this synthesis method less interesting from an industrial point of view.
A patent by Du Pont describes a similar approach [123].Two      separately synthesized prepolymers, poly(butylene terephthalate) (PBT, inherent viscosity (IV) = 1.3 dL/g) and poly(isosorbide terephthalate) (PIT, IV = 0.26 dL/g), were submitted to transesterification and subsequent polycondensation in a molten blend.The resulting polyester showed an inherent viscosity of 0.53 dL/g with a T g of 108.7 • C. The incorporation of isosorbide was satisfactory (55.2 mol% of total diols) as 92% were incorporated relative to the feed ratio (60 mol% of total diols).One example in the same patent on the synthesis of PEIT reported the same synthetic procedure with similar results.The polyester had an IV of 0.57 dL/g and a T g of 139 • C, with very similar isosorbide incorporation compared to the feed ratio.An entirely different approach was taken by Wang et al. [124].The authors investigated the polycondensation-coupling ring opening polymerization of cyclic PBT oligomers with isosorbide (Scheme 3).
With isosorbide loadings between 5 and 20 mol%, M n values of 9,000-10,000 g/mol were obtained within three hours of polymerization at 250 • C using a Ti(OBu) 4 catalyst (0.2 wt%).Considering the low molecular weights and the expensive cyclic PBT oligomers required, this polymerization method is inferior to regular melt polyesterifications. Results from the thermal and structural analysis of the polyesters largely coincide with previous reports.
Several examples of copolyesters with isophthalic acid derivatives, isosorbide and lactic acid are covered in chapter 3.4.
One recent patent by Avantium and Roquette Frères describes the synthesis of PEIF with an isosorbide content up to 19.8 mol% [126].The authors conducted their experiments in both a 200 ml glass reactor and a 2 l stainless steel reactor (see Table 12).
Under similar reaction conditions, the latter yielded polymers with molecular weights more than three times as high, with M n values up to 37,300 g/mol and much lower PDI values.One reason might be the ability to pressurize the reaction mixture in the steel reactor, thus resulting in less volatilization of the monomers during initial transesterification.It was found that a large excess of diols increases both molecular weights and isosorbide incorporation ratios.Some experiments with up to 55 mol% of isosorbide were conducted on small scale in a film reactor with a butyltin(IV)tris octoanoate catalyst (for description of film reactor, see [127]) The M n values obtained were low with 6,750 g/mol and a T g of 126 • C at 55 mol% isosorbide.A patent by Dow Global Technologies describes the high gas barrier properties of PEIF films, which allows for transparent packaging as no metal foil or metalized polymer film layers are necessary to decrease gas-and moisture permeability to an acceptable level [128].
Large amounts of FDCA (>70%) resulted in unprocessable polyesters at isosorbide content higher than 50% of the total diols.A decrease in molecular weight was observed with increasing isosorbide content, independent of the other comonomers.The ideal isosorbide content to reach M w values suitable for coating applications (2,000-6,000 g/mol) was found to be between 10 and 30 mol% of diols.Thermal decomposition of the polyesters was found to increase at higher isosorbide content for 1,5-pentanediol based polyesters, while T d was largely independent of the isosorbide content for 1,3-propanediol polyesters.The evaluation of coating properties revealed the highest performing polyester to contain 30% isosorbide (see above, otherwise limitation by low molecular weights) and 85% FDCA, as lower FDCA content resulted in brittle coatings.
As seen in previous works, the authors reported decreasing molecular weights at increasing isosorbide contents.Copolyesters with isosorbide contents between 20 and 50 mol% were found to have good mechanical properties with a tensile modulus of about 1,400 MPa, tensile strength at break > 53 MPa and elongation at break > 46%.Materials with higher isosorbide contents were synthesized in too low molecular weights for representative measurements of their mechanical properties.Chen et al. also describe the synthesis of PBIF in a more recent publication (Table 14) [130].Molecular weights and polydispersity of the products are slightly higher, while T g values are lower.Scheme 3. PBIT synthesis by ring opening polymerization of cyclic oligo(1,4-butylene terephthalate) units.

Table 12
Characteristics of poly(ethylene-co-isosorbide-2,5-furanoate) synthesized by melt polymerization with different diester to diol ratios and reactor systems.Ti (OBu) 4 was used as a catalyst.The authors additionally investigated the hydrolytic and enzymatic degradability of PBIF.Similar to the results on PBIT degradability, it was found that isosorbide incorporation enhances the hydrolytic degradability under acidic (pH 2.4, 80 • C) conditions.The biodegradability (under neutral aqueous conditions at 37 • C) of PBIF copolyesters was found to be dependent on the enzyme used for the experiment.Experiments conducted with Porcine pancreatic lipase enzymes showed an improvement in biodegradation upon isosorbide incorporation.Degradation experiments performed with cutinase enzymes (Novozym 51032) revealed the opposite effect, as only PBF degraded to a significant extent under these conditions.The authors attribute this to the vastly different crystallinities and T g values of PBIF copolyesters and different biodegradation mechanisms of the respective enzymes.

D.H. Weinland et al.
At similar isosorbide content as reported by Wang et al., the resulting poly(ester carbonates) exhibited higher molecular weights with similar isosorbide losses (compare Table 14 and Table 15).Higher molecular weights were obtained when succinic acid was added in the form of bis (4-hydroxy-butyl)-succinate prior to polycondensation.This was attributed to the lower melt viscosity which allowed for easier removal of condensation byproducts, although much lower isosorbide incorporation was achieved with succinate poly(ester carbonates).Gas barrier measurements of the polymer films showed that the barrier properties of the reported poly(ester carbonates) were inferior to PBF but better than other biobased polyesters like PLA.The biodegradation was found to be enhanced at higher isosorbide contents.No cyclic carbonates were found to leave the reaction, unlike the example shown in chapter 2.1 on the synthesis of high molecular weight poly(isosorbide terephthalate-cocarbonate) where ethylene carbonate was distilled out during polycondensation.This is likely due to the unfavorable eight-membered ring structure of 1,4-butylene carbonate.
While the molecular weights obtained are higher with the method presented above, one should consider the difficulty of chemically recycling polyesters with five different monomers.It is important from a sustainability point of view that future materials are designed in a way that allows for efficient recycling [132].
The esterification was conducted in a non-pressurized system at 230 • C, which resulted in volatilization of EG and thus a decreased availability compared to the other diols.This led to the preferential placement of EG at the chain ends of the oligomers, which in turn facilitated polycondensation due to the easier displacement of ethylene glycol.At first glance, these results contradict the previous observations on PEICT by Yoon et al. (see chapter 3.1.1),where the oligomer chainends mainly constituted isosorbide.In that case, however, esterification was conducted at 255 • C, under pressure, preventing volatilization of EG and thus increasing its availability and placing it preferentially in the middle of the oligomers.This showcases the impact of reaction design on the overall polymerization process.The observation that the EG block length decreased with increasing isosorbide content is found in both publications.This is an indicator for the aforementioned higher volatility of EG under polyesterification conditions, as a shorter block length means less incorporation into the polymer chain despite a constant EG feed ratio.This effect becomes obvious when comparing the incorporation ratio of EG with the feed ratio at increasing isosorbide content (see Table 16).Polycondensation times to reach similar molecular weights are significantly longer for PEICF than for PEICT, despite the end groups of the former being predominantly ethylene glycol.This could be due to the deactivation of Ti(OBu) 4 by hydrolysis as the authors added the catalyst before esterification and Ti(OBu) 4 is known to be susceptible to hydrolysis [63], unlike dibutyltin oxide/GeO 2 used in the PEICT synthesis.The viscoelasticity of PCEIF polymers increased linearly with higher isosorbide loadings, although polyesters without isosorbide exhibited Newtonian behavior, while higher isosorbide incorporation resulted in increased shear-thinning behavior.
Similar molecular weights and incorporation ratios were reported in a recent patent by Avantium and Roquette Frères on PEICF polyesters where both dimethyl-2,5-furandicarboxylate and FDCA were used as monomers [134].
It is becoming apparent that a large excess of diols is necessary for a successful melt polymerization of polyester compositions with isosorbide starting from dimethyl-2,5-furandicarboxylate.The reason for this large excess, which hampers large scale applicability of the polyester synthesis due to the recovery of a mixture of diols after polycondensation, is not clear.A large excess of diols furthermore leads to mass transfer limitations on larger reaction scales, as the removal of bulky, high-boiling diols like 1,6-hexanediol or isosorbide from a viscous polymer melt is unfavorable.
This way no excess of diols is present in the reaction mixture and the volatilization of isosorbide was partially prevented, as shown by the incorporation ratios (see Table 17) The molecular weights were only indirectly determined in the form of intrinsic viscosities.There was however no decrease in molecular weights at higher isosorbide content observable.The analysis of the sequence distribution of the monomers revealed that the degree of randomness increased with increasing isosorbide content.Accordingly, the homopolyester poly(isosorbide-2,5-furandicarboxylate) (PIF) undergoes transesterification much more easily than poly(1,4cyclohexanedimethanol-2,5-furandicarboxylate) (PCF).This result confirms the observation made by Rajput et al. that isosorbide is rapidly liberated from preformed macromonomers under transesterification conditions [75].
Investigation of the thermal properties of the polyesters revealed that increasing isosorbide content increased the T g values and decreased T m values (see Table 17).The crystallization of polyesters was inhibited by increasing isosorbide content, as spherulite growth rates were shown to be retarded upon isosorbide incorporation.
The same trends regarding lower intrinsic viscosities and higher degrees of randomness with increasing isosorbide content were observed (Table 18).T g values are significantly lower due to the increased flexibility of 1,6-hexanediol compared to 1,4-cyclohexanedimethanol. The crystallinity of PHIF is decreased compared to PCIF, as samples with > 10 mol% isosorbide were amorphous.Important information missing from both works discussed above is the scale at which the reactions were conducted.
The authors reported relatively high PDI values, especially at higher isosorbide content (Table 19).The polyesters were random at higher isosorbide content.As in the previous examples using the same macromonomer approach, -COOH end groups were found in the final polymer

Polyester Feed ratio
Composition ratio  .The polyesters were semicrystalline, which is in accordance with results obtained by solution polymerization of isosorbide with long aliphatic diacids [16].Degradation studies in soil revealed that the enzymatic degradation rate decreased with increasing isosorbide content, which is contrary to most previous results.This could be due to the already facile biodegradation of 1,10-decanediol-based polyesters due to the large space between ester groups, which allows for easy access of

Table 17
Properties of selected PCIF samples after melt polymerization using a macromonomer approach described in Scheme 5 using Ti(OiPr) 4 catalyst (400 ppm, added during macromonomer synthesis).Transesterification was conducted for 4.5 h at 190 the active sites of enzymes.
Both the yields and molecular weights of the resulting polyesters decreased at higher isomannide-loadings: overall they were higher than PBIT polyester obtained by ring opening polymerizations of cyclic PBT oligomers with isosorbide (see chapter 3.1.1). 13C NMR studies revealed that larger amounts of isomannide (≥40 mol% respective total diols) additionally resulted in a block-type sequence distribution.Only polyesters with an isomannide content below 10 mol% were semicrystalline.Their crystal structure and crystallization mechanism were similar to PBF, although the kinetics of crystallization was inhibited.Incorporation of isomannide coincided largely with the feed ratio, which can be attributed to the different mechanism of ROP, as isomannide usually exhibits the lowest reactivity of 1,4-3,6-dianhydrohexitols in polycondensations.Degradation tests in pH 7.4 phosphate buffer solutions revealed significant hydrolysis at increasing isomannide content compared to the largely water resistant poly(1,4-butylene-2,5furanoate).The polyester poly(isomannide-2,5-furanoate) exhibited 20% degradation after 30 days in the buffer solution, and 50% degradation in the presence of porcine pancreas lipase.These results showcase the strong influence of 1,4:3,6-dianhydrohexitols on polymer degradation characteristics.According to the authors, cyclic oligomers from isosorbide were not obtainable by the described method.Molecular simulations confirmed the lower stability of those species compared to isomannide-based cyclic oligomers.
The presented examples on melt polymerization of aromatic diacids or diesters with 1,4:3,6-dianhydrohexitols show that the main monomer used in most current examples is isosorbide.Especially PEIT with up to 20 mol% of isosorbide received a lot of attention from both the academic and industrial research community.Extensive catalyst screening revealed some active catalytic systems, which can be more sustainable than traditional Sb

Aliphatic saturated polyesters with 1,4:3,6-dianhydrohexitol comonomers
Poly(butylene succinate) is a promising candidate for fully bioderived and biodegradable polyesters, as the monomer succinic acid can be produced from lignocellulosic biomass [139] or glucose [140] and 1,4-butanediol can be obtained via the reduction of succinic acid [141].The properties of PBS are unsuitable for many applications, as the highly crystalline polymer exhibits a T g of -32 • C and a T m of 114 • C [142].The high crystallinity, resulting from the linear chains, renders PBS inapplicable in film-drawing applications, as rapid crystallization results in opaque materials [143].The incorporation of isosorbide into PBS can lead to fully biobased copolyesters with tunable, application-oriented properties.
Table 21 compares the results of several publications on the synthesis of poly(1,4-butylene-co-isosorbide succinate) (PBIS, Fig. 29).The synthetic conditions are given together with the molecular weights obtained.The obtained M n values of the polyesters with the same isosorbide feed ratios varies vastly between publications.This can be explained by several factors that determine the outcome of a polymerization, especially when an excess of low-reactivity, high boiling diols like isosorbide are used.The scale at which the reaction is conducted, together with parameters like stirrer geometry and the general reaction setup, can have a large influence on the reaction times necessary for polycondensation, as mass transfer will depend on the factors stated above [144].If mass transfer is unfavorable in a reaction setup, high molecular weights will be difficult to obtain.
In general, the crystallinity and crystallization rate decreased with higher isosorbide content, as the bent structure of isosorbide disrupts the linear alignment of the polymer chains, while the glass-transition temperatures increased as expected.The elongation at break improved with increasing isosorbide contents, which is favorable for film applications.
Duan et al. obtained slightly higher molecular weights compared to Jacquel et al. [146], which is likely due to longer polycondensation times [145].Interestingly, the polydispersity of the polymer products decreased noticeably upon isosorbide incorporation from 2.93 for PBS to 1.35-2.03for PBIS (both after precipitation in CHCl 3 /MeOH).The number-average molecular weights upon isosorbide incorporation did not follow the typical near-linear trend, but showed an intriguing spike at an isosorbide content of 10 mol% regarding diols, with a fast decline at higher isosorbide loadings.The incorporation into the polymer was more efficient than in the other studies, as feed and incorporation ratio were mostly identical.The PBIS polyesters were shown to have a completely random monomer sequence, as indicated by 13 C NMR spectroscopy.The findings on thermal and mechanical properties are similar to the study by Jacquel et al., while additional experiments on the oxygen barrier properties of the copolyesters were conducted.The oxygen permeability increased up to a content of 10 mol% isosorbide with respect to the total diol content, then sharply decreased at higher isosorbide content.The authors argue that the increased chain stiffness initially leads to weaker relaxation and shifts of the polymer chains, thus decreasing the ability of oxygen to permeate.At higher isosorbide content, the increasingly amorphous structure dominates the overall polymer morphology and allows for higher O 2 -diffusion.A similar trend was observed for the elongation at break and tensile strength, which increased significantly at isosorbide-loadings of 5 mol% and 11 mol%, but decreased at loadings ≥ 21 mol%.The lower molecular weights and

Table 21
Characteristics of selected PBIS polyesters with varying amounts of isosorbide.

Conditions Polyester Feed ratio
Composition ratio Ref.

1,4-BDO/IS 1,4-BDO/IS
Esterification: 1 bar N 2 , 180   reduced crystallinity can explain this observation.Incorporation of isosorbide also increased the hydrophilicity of PBIS compared to PBS.Qi et al. modified their synthetic procedures with increasing isosorbide feed ratios.The molar excess of diols was decreased for higher isosorbide contents due to the difficult removal of excess isosorbide during polycondensation [54].Additionally, the reaction temperatures were decreased due to more significantly colored polymer products at high isosorbide contents. 13C NMR showed that all polymers exhibited a random monomer sequence.Thermal decomposition studies revealed that polyesters with higher isosorbide contents have an increased resistance towards thermal degradation (29 • C higher T d,5% for > 50 mol % isosorbide contents).This is contrary to Duan et al.'s findings, where isosorbide incorporation was found to have no significant effect on T d,5% [145].Products with an isosorbide content ≥ 50 mol% were amorphous.Interestingly, wide-angle X-ray diffraction studies revealed that materials with an isosorbide content between 30 and 50 mol% showed the same diffraction patterns as semicrystalline samples with a lower isosorbide content.These materials did not show any crystallization peaks during DSC, which indicates that materials with 30 and 50 mol% isosorbide are indeed semicrystalline, albeit with a very slow crystallization rate.Water contact angle measurements showed that PBIS polyesters become more hydrophilic with higher isosorbide content, which positively influences hydrolytic and enzymatic degradation.It was shown that increasing isosorbide content led to faster degradation in both cases.The rate-enhancing effect of isosorbide was explained by two factors: Firstly isosorbide decreases the crystallinity in polyesters, thus enhancing the chain mobility during degradation; secondly isosorbide-succinate esters were found to be much more prone to degradation than butylene-succinate units.
Qu et al. investigated PBIS with a wide range of isosorbide content for potential applications as hot-melt adhesives [147].There are several striking differences with the work by Qi et al.The molecular weights of the polyesters appear to be much higher than those obtained by Qi et al. for similar polymer compositions, while PDI values are lower for polymers with high isosorbide content.The T g values obtained by Qu et al. are also higher due to higher molecular weights, especially for polyester compositions with large molar percentages of isosorbide.One important difference between the two publications is the choice of polycondensation catalyst.Upon comparison of three common polycondensation catalysts (Ti(OBu) 4 , Sb 2 O 3 , Sb(OCH 2 CH 2 O) 3 ) Qu et al. found Sb 2 O 3 to be the most active.The authors argue, while considering some molecular simulations (gauss 09) of the polymer chain fragments, this is mainly due to the low steric hindrance of that catalyst, as the bent structure of isosorbide creates sterically crowded carbonyl regions in the growing polymer chains.Coordination to these carbonyl groups is essential for the catalyst's activity.Thermal analysis of the copolyesters indicates results similar to those observed by Qi et al.Some shear strength measurements were performed to evaluate possible applications as hot-melt adhesives.It was found that the shear strength reaches a maximum at an isosorbide content of 80 mol%.The biodegradability of an adhesive film with an isosorbide content of 10 mol% was tested: it was found to be biodegradable in a natural environment.
A recent patent by China Petroleum & Chemical Corporation describes the efficient incorporation of ≤ 10 mol% isosorbide with respect to the total diol content into PBIS, by employing two different catalysts for esterification and polycondensation (only one example given in table ; for examples with lower isosorbide content please refer to the source) [148].The thermal and mechanical properties of the copolyesters largely coincided with the previously cited studies on PBIT.
Tan et al. replaced succinic acid with dimethyl succinate in the synthesis of PBIS [55].Despite similar synthetic conditions and addition of the catalyst in two portions, the resulting M n value of 18,500 g/mol at 14 mol% isosorbide with respect to total diol content is significantly lower than in the previous examples.On the other hand, the reported T g value of 37.5 • C at 14 mol% isosorbide is much higher than the previously reported T g values of PBIS at 11 mol% isosorbide (between − 24 and − 22 • C) [145,146].This large discrepancy, regardless of the much higher molecular weights previously reported, cannot be explained, as no structural analysis of the polyesters are given.The use of diester or diacid should not cause this difference, as all mentioned publications started the polycondensation after complete distillation of the (trans) esterification byproduct.
Wu et al. described the use of enzymatic catalysis in a recent publication on the synthesis of PBIS [92].Similar to previous reports on the synthesis of aliphatic polyesters with isosorbide by Juais et al. (see chapter 2.2.2), the authors used lipase enzymes from Candida Antarctica immobilized on a polymer network.Molecular weights of the PBIS copolymers were relatively low despite long reaction times of 80 h at 80 • C (see Table 21).It was found that PBIS copolymer synthesized with enzymatic catalysis showed an increased thermal stability compared to the same compositions synthesized with common Lewis acid catalysts.T 5%d values were found to be between 20 • C and 40 • C higher for the former polymers.The authors also described the synthesis of the previously unreported copolyester poly(1,4-butylene-co-isomannide succinate) (PBImS) via the same synthesis method.It was found that slightly higher molecular weights could be obtained when isomannide was used (M n (PBIm 68 S) = 9,700 g/mol compared to M n (PBI 66 S) = 7,400 g/mol), which is due to the more favorable hydrogen bonding with isomannide's two endo-OH groups within the active sites of enzyme catalyst.
A copolyester that received much less attention than the previously mentioned PBIT is poly(ethylene-co-isosorbide succinate) (PEIS, Fig. 30).It was recently synthesized and characterized by Dezhi et al. [149].
A characteristic decrease in molecular weights was observed with higher isosorbide loadings (Table 22).The elongation at break of the polyesters peaked at 674% at an isosorbide content of 12 mol%.It was observed that the hydrolysis rate (at pH 14) increases with increasing isosorbide content, which is in line with previous results with PBIT.The hydrolytic degradation was found to follow a surface erosion mechanism.
The cross-linked polyester poly(isosorbide-co-glycerol succinate) (Fig. 31) was explored by Wilbon et al. [150].The authors synthesized the diacid-derivative of isosorbide by microwave assisted ring opening of succinic anhydride with isosorbide using Sn(Oct) 2 (1 mol%) as a catalyst.The reaction was conducted at 100-140 • C for 5 min under N 2 .Previous attempts to achieve this transformation under thermal conditions suffered from long reaction times of 24 h at 120 • C, which led to colored products [151].The monomer was polymerized with glycerol (67 mol% respective the isosorbide-succinate monomer synthesized in the first step) at 100-150 • C for 5 h under a nitrogen atmosphere to obtain cross-linked thermosets, which were annealed for an additional 12 h at 160 • C. The authors utilized the remaining Sn(Oct) 2 catalyst from the previous reaction step.Note that the only indication of the molecular weights of the polymers was given as the molar mass between crosslinks, a value derived from the storage modulus E'.The obtained polyesters were very rigid and densely cross-linked, which led the researchers to introduce oligomeric poly(ethylene oxide) (PEO) in the polycondensation step, as to obtain more desirable material properties.This led to an increase in the molar mass between crosslinks from 1,590 g/ mol without PEO to up to 3,240 g/mol with 23 wt% PEO.The T g value decreased from 55 • C to 7 • C upon incorporation of 23 wt% PEO.The obtained copolymers showed good mechanical and thermal properties and were proven to undergo hydrolysis under acidic, basic (hydrolysis in under 5 h) and neutral (hydrolysis within a month) conditions.The  hydrolysis products were shown to undergo subsequent repolymerization, although with noticeable changes in color and tensile properties.
The polymerization results of isosorbide with sebacic acid and two different linear diols, 1,3-propanediol [152] and 1,10-decanediol [59], are shown in Table 23 (see Fig. 32 for structures of the polyester products).Both publications use similar reaction conditions and obtain similar molecular weights.
The molecular weights obtained in both studies are relatively low compared to M n values obtained in the PBIS synthesis with similar isosorbide contents (see Table 21).The main difference with the works on PBIS is the use of Sn(Oct) 2 catalyst, which is not a very common step growth polyesterification catalyst.It is mainly used for the ring opening polymerization of cyclic monomers like lactide and ε-caprolactone.A recent study comparing different transesterification catalysts for the synthesis of poly(ethylene-2,5-furanoate) found that Sn(Oct) 2 is less active than other common catalysts such as Ti(OBu) 4 or dibutyltin oxide [153].
Both works reported detailed studies on the crystallization behavior of PPISe and PDISe copolymers.Similar to previous reports on the effect of isosorbide on crystallization in polyesters, it was found that isosorbide incorporation retards crystallization rates without affecting the crystallization mechanism.
Wei et al. state that the sequence distribution of the monomers in PDISe could not be resolved by NMR spectroscopy, as no additional signals for the copolyester, compared to both parent polyesters, arose.A single T g value for the copolyesters suggests a random sequence distribution (see Table 23).DSC studies on the crystallization behavior of the PDISe copolyesters revealed that even large amounts of isosorbide (66.9% of total diols) did not suppress crystallization.It did, however, shift the crystallization peak temperature to a lower value, decreased crystallization rate, overall crystallinity and the corresponding ΔH c .The homopolymer poly(isosorbide sebacate) did not show a crystallization exotherm.A singular melting peak, which indicates perfect crystals, occurred even at an isosorbide content of 25.4 mol%.This was not the case for PIPSe polymers, where isosorbide incorporation resulted in the appearance of several melting peaks [152].Wide-angle X-ray diffraction measurements showed that the crystal structure of the PDISe did not change upon isosorbide incorporation compared to pure poly(isosorbide sebacate).The glass-transition temperatures of PDISe were not detectable at isosorbide loadings below 30%.This led the authors to the hypothesis that the chain irregularity caused by isosorbide could act as a nucleus for crystallite growth.From the two publications discussed above it is not clear whether the observed crystallization properties would change if molecular weights were increased.It would be interesting to see if different results would be obtained with a more active esterification catalyst, like Ti(OBu) 4 , or higher molecular weight samples, for example after exposing them to solid state polycondensation.The examples in this section exclusively use isosorbide as the 1,4:3,6dianhydrohexitol comonomer.It is apparent from the presented examples that higher molecular weights can be reached with aliphatic diacids compared to aromatic diacids covered in chapter 3.1.This is probably due to the lower melt viscosities during polycondensation with more flexible aliphatic diacids, which facilitates the removal of excess isosorbide.On the other hand, larger amounts of isosorbide are required to elevate the T g values of copolyesters with aliphatic diacids to values that exceed room temperature.Unusual synthetic strategies as seen in chapter 3.1 are not reported, as all presented works on aliphatic polyesters with isosorbide employ standard polyester synthesis protocols and

Table 22
Characteristics of poly(ethylene-co-isosorbide succinate).Ti(OBu) 4 catalyst 0.05 wt% of total reactants, (added at the end of esterification).Diacid:diol ratio 1:1.2.Esterification was conducted for 4 h at 190    catalysts.The copolyesters show similar trends with regard to increased degradability upon incorporation of isosorbide.Due to the higher chain flexibility with aliphatic diacids and long chain diol comonomers, aliphatic polyesters with isosorbide exhibit semicrystallinity with isosorbide loadings up to 66 mol%.

Aliphatic unsaturated polyesters with isosorbide
All work on unsaturated polyesters with 1,4:3,6-dianhydrohexitols exclusively involves isosorbide.Target applications usually are thermosets and powder coatings, as the double bonds in the polymer can be crosslinked in a second reaction step.A popular strategy to reach high molecular weight crosslinked materials is the synthesis of isosorbidecontaining macromonomers with double bonds and their subsequent chain growth polymerization [154][155][156].These examples will not be covered in this review, as many of them were covered in the review by Saxon et al. [8].
Please note, that all presented polymerizations with unsaturated substrates involve the addition of small amounts of antioxidants, such as hydroquinone.These additives will not be mentioned for every example.One common monomer to synthesize these unsaturated polyesters is maleic anhydride, which could be obtained by oxidation of bioderived 5-hydroxymethylfurfural (currently not produced on large scale) [157].
One study by Sadler et al. describes the synthesis of terpolyesters containing isosorbide, maleic anhydride, ethylene glycol and aliphatic diacids (adipic, suberic and sebacic acid, Fig. 33), which were specifically included to increase the solubility of the otherwise too polar polymer in common apolar solvents used for the curing process (crosslinking) [158].The reactants were suspended in xylenes and refluxed using p-toluenesulfonic acid (5 wt%) as the catalyst, until the desired acid number was reached.Subsequently, the reaction pressure was reduced for another 2.5 h until the polymerization was complete.The M n values ranged between 1,500 and 3,000 g/mol before the curing process.Even though the obtained properties of the cured resins were promising, it was not possible to reach the necessary properties for highperformance resin applications, as the prepolymers with a high isosorbide content were too insoluble in the reactive solvents needed for curing.
These challenges were addressed in a follow-up publication by the same group [159].Based on their previous observations, they employed isosorbide, diethylene glycol, maleic anhydride and phthalic anhydride (Fig. 34), with the same synthetic procedure as described above.The molecular weights are similar to those obtained previously, with decreasing M n values with increasing isosorbide content, even at reaction times of 96 h for 25 mol% isosorbide regarding the total reactants.These high isosorbide loadings were necessary to reach the thermal and rheological properties of commercial resins, which in turn renders the reaction conditions unsuitable for larger scale operations.
A different approach was taken by Jasinska et al., as they used isosorbide as the sole diol moiety in a polycondensation with maleic anhydride and succinic acid under Ti(BuO) 4 catalysis (Fig. 35) [160].The esterification was conducted at 190-230 • C for 3.5 h under an argon atmosphere.The polycondensation proceeded at 230 • C at 3-5 mbar for an additional 3.5 h.The obtained M n values ranged between 2,400 and 3,370 g/mol with high PDI values between 2.1 and 3.7, due to crosslinking of the unsaturated moieties.As can be expected, larger amounts of maleic anhydride led to a decrease in molecular weights due to more side reactions, and it was shown that around 20% of the unsaturated C=C bonds disappeared in the polyesters before curing.2D NMR experiments revealed that the end groups of the polymers were predominantly hydroxyl groups from isosorbide.The T g values of the pre-curing polyesters ranged between 50 and 75 • C with decomposition temperatures above 270 • C.
Other work by the same authors was aimed at synthesizing watersoluble polyesters for coating applications containing isosorbide, poly (ethylene glycol) (PEG 600 and PEG 200 were used) and maleic anhydride (Fig. 36) [161].The reaction was conducted under similar conditions (slightly lower T at 210 • C) and led to slightly cross-linked products through the Michael addition of double bonds and PEG-OH groups, as observed by 2D NMR experiments.The polyethyleneglycol was incorporated without bond cleavage, creating a block type polyester with high water solubility.An increase in polyethyleneglycol content did result in more cross-linking during polycondensation, as the primary -OH groups are more reactive.The T g values ranged between -22 and 45 • C, increasing with higher isosorbide loadings.The decomposition temperatures were above 260 • C. The authors concluded that this type of bioderived polyester can be applied industrially.
A patent by the Nanjing University of Technology directly utilized fumaric acid in the polymerization with isosorbide [162].The authors used a p-TsOH catalyst at temperatures below 150 • C, which yielded a slightly yellow product (Fig. 37).After precipitation in DCM/methanol, the polyester exhibited a M n value of 3,750 g/mol with a PDI of 2.42.The same reaction with a Ti(OBu) 4 catalyst yielded slightly lower molecular weights.Biodegradation tests in an enzyme/phosphate buffer solution revealed a good degradability under physiological conditions, which allows for possible applications in drug encapsulation.Crosslinking with benzoyl peroxide yielded a tougher material for tissue engineering applications.
Itaconic acid is another bifunctional unsaturated monomer with promising potential applications.It is already manufactured from biomass at an industrial scale of 80 kt/a by fermentation of carbohydrate biomass using filamentous fungus strains such as Aspergillus terreus [163].The highest yields are obtained when glucose or sucrose is used in the fermentation, however non-food-competitive materials like wood can also be used [164].
Nakayama et al. investigated the polycondensation between itaconic acid and isosorbide with different catalysts (Fig. 38) [165].The authors conducted the esterification and polycondensation for 24 h each, at 140 • C and 120 • C respectively.The common polyesterification catalysts Ti(OiPr) 4 and Sb 2 O 3 did not yield a polymer product.TsOH and HfCl 4 yielded polymers with M n values of 3,400 g/mol (PDI 3.8) and 1,600 g/ mol (PDI 1.7), respectively.The higher polydispersity with TsOH is likely caused by increased cross-linking.Different radical inhibitors present at 0.1 wt%, were also investigated, and it was shown that the free galvinoxyl radical was the most effective at preventing cross-linking during the polymerization.It was the only inhibitor that produced fully THF-soluble polymers.The T g values reached 83.9 • C and heavily depended on the reaction temperatures, while the T m values were not detectable by DSC.Higher reaction temperatures led to improved molecular weights but also a higher cross-linking.
Goerz et al. combined three bioderived monomers (isosorbide, succinic acid, itaconic acid) in an unsaturated polyester (Fig. 39) [166].The Fig. 33.Structure of terpolyester based on isosorbide, maleic anhydride, ethylene glycol and aliphatic diacids [157].authors used a one-step, H 2 SO 4 -catalyzed (0.005 mol%) esterification/ polycondensation protocol under microwave irradiation in toluene.The reported yields increased drastically with increasing succinic acid content and the M w values ranged between 1,200 and 3,500 g/mol.The amorphous copolyesters were successfully cross-linked with dimethyl itaconate and the free radical initiator V-65, and showed a shapememory effect at itaconic acid content higher than 50 mol% of total diacid content.
Another study by Kang et al. employed five different bio-based monomers (1,3-propanediol, 1,4-butanediol, isosorbide, itaconic acid, sebacic acid) in the synthesis of bio-elastomers (Fig. 40) [167].Isosorbide was incorporated to reduce the crystallinity of the polymer chains, as it is known to induce amorphism at high content.The esterification was conducted without a catalyst at 180 • C for 2 h, after which Ti(OBu) 4 (0.05 mol% of total reactants) was added.The polycondensation was conducted at 220 • C for 3-6 h until the product began to exhibit a phenomenon called the "Weissenberg effect" (reaction mixture starts climbing up the stirrer due to increasing viscosity).The obtained copolyesters exhibited M n values between 21,350 and 32,840 g/mol with PDIs between 4.2 and 4.8.As reported in previous examples, the content of incorporated isosorbide units in the polyester was significantly lower than the feed ratio and the T g values increased with increasing isosorbide content.Isosorbide was incorporated up to a maximum of 30 mol% of the total diol content, as the obtained molecular weights were too low at higher amounts.Longer reaction times did not increase the reactivity, but led to more side reactions.The authors additionally investigated composites of the synthesized bio-elastomer with nanosilica (SiO 2 ), which significantly improved the mechanical properties compared to the unmodified polyester.
A subsequent publication by the same authors explored similar polyesters containing four bio-derived monomers (1,4-butanediol, isosorbide, itaconic acid, sebacic acid, see Fig. 40), utilizing a similar synthetic procedure with longer reaction times in the polycondensation stage (4-10 h) [168].The findings concerning molecular weights, polydispersity and isosorbide incorporation were comparable with the Fig. 34.Structure of terpolyester based on isosorbide, maleic anhydride, diethylene glycol and phthalic anhydride [158].author's previous work, as were thermal properties and crystallinity.The cross-linked polyesters exhibited excellent shape recovery, with variable switching temperatures depending on the isosorbide content and curing times.The biocompatibility of the cured polyesters combined with the possibility to obtain switching temperatures close to the human body temperature, allows for biomedical applicaA patent using various combinations of the aforementioned unsaturated monomers (itaconic acid, maleic anhydride, phthalic anhydride) with 1,2-propylene glycol and isosorbide (Fig. 41) was filed by DSM [169].M n values of the polyesters ranged between 700 and 5,000 g/mol and were obtained without the use of a catalyst for the esterification/polycondensation step.T g values ranged between − 12 • C and 8 • C and all resins were cured using styrene as a reactive diluent.The authors noticed an unexpected synergistic effect that occurred when incorporating itaconic acid and isosorbide in the resins.Both the thermal stability and mechanical properties were improved when the two monomers were used together, an effect that did not occur when the monomers were incorporated separately.The mechanical properties of the cured resins depended on the monomer ratios, with values for the tensile modulus ranging between 3,500 MPa and 4,200 MPa (at maximum isosorbide content, see Table 24 Resins A and B for examples with their exact composition), tensile strengths between 72 MPa and 75 MPa and elongation at break values up to 3%, which indicate strong, stiff materials.
The presented examples show that the polycondensation of unsaturated aliphatic monomers with isosorbide and other comonomers has to be tuned carefully, as side reactions can occur easily.A dilemma arises when incorporating low-reactivity monomers like isosorbide into unsaturated polyesters, as higher temperatures and more active Lewis and Brønsted acid catalysts also facilitate side reactions such as the Michael addition, which in turn leads to unwanted crosslinking during polycondensation.Catalysts that operate under milder conditions, such as Candida Antarctica Lipase B could enable the synthesis of higher molecular weight polymers without preemptive crosslinking.This catalyst was already shown to be as efficient as Ti(OBu) 4 in the synthesis of unsaturated polyesters with itaconic acid and 1,4-butanediol [170], although work presented in chapter 2.2.2 indicated difficulties in synthesizing unsaturated polyesters containing 1,4:3,6-dianhydrohexitols with Candida Antarctica Lipase B. On the other hand, high molecular weights are not as important as for the examples in previous chapters, as the subsequent chain growth polymerizations yield cross-linked polymer networks.The properties of the final materials are promising, although the low solubility of polyesters containing larger amounts of isosorbide in typical reactive solvents is problematic.

Polyesters with hydroxy acids and isosorbide
Lactic acid is undoubtedly one of the most investigated bioderived monomers for the production of sustainable polyesters.The lactic acid dimer lactide is usually employed in Lewis or Brønsted acid catalyzed ring-opening polymerizations.There have been several attempts to incorporate lactic acid into polyesters with isosorbide.
Ristic et al. investigated the isosorbide-initiated chain-growth polymerization of lactide (Scheme 7) using different catalytic systems (Sn (Oct) 2 , trifluoromethanesulfonic acid and a catalyst-free microwaveassisted approach [171].The reaction times necessary to obtain certain molecular weights were measured to evaluate the efficiency of the different catalytic systems.The results show that said efficiency varied widely, from up to 96 h (Sn(Oct) 2 for 60 kg/mol) to 0.5 h with microwave heating for the same molecular weight.Isosorbide was employed in very low quantities of 0.23-0.71mol%, as only one molecule was incorporated in the middle of each poly(L-lactic acid) chain.This was confirmed by 1 H NMR studies and thermal analysis of the resulting polyesters revealed an increase of T g values of 2-3 • C. In some cases, the crystallinity of isosorbide-containing polyesters increased compared to pure PLA.The authors conclude that isosorbide participation in the polymerization of PLA can allow for the fine-tuning of properties.
A patent filed by the Council of Scientific and Industrial Research (India) followed a similar approach but managed to incorporate up to 15 wt% of isosorbide into the polymer [172].The authors charged a glass ampoule with the reactants and Sn(Oct) 2 catalyst in toluene and dried the mixture at 0.01 mbar for 2 h.After, they flushed the reaction vessel with N 2 , sealed it and conducted the polymerization at 200 • C for 1 h.The M n values reached 37,200-45,400 g/mol, with an increase in molecular weight at higher isosorbide loadings (5-15 wt%), and a polydispersity of around 1.55.This enhanced reactivity at higher isosorbide concentrations showcases the fundamental difference of the chain-growth mechanism that underlies this ring-opening polymerization compared to the polycondensations in most other presented works of this review.The T g and T m values increased by 13 • C and 43 • C, respectively, compared to unmodified PLA.The large deviation from the previously reported results with regard to obtained molecular weights in a shorter timeframe, could be due to the higher reaction temperature and pressure.The provided 1 H NMR spectra do hint towards a complete incorporation of isosorbide into the polyester, although only one molecule per polymer chain can theoretically be inserted.The authors did not work up the reaction after the polycondensation, which could lead to misinterpretation of unreacted isosorbide in the 1 H NMR spectra.This however does not explain the significantly enhanced T g values obtained upon isosorbide incorporation.Another method to synthesize copolyesters of isosorbide and lactic acid was presented in the same patent.A prepolymer of PLA (IV = 1.6 dl/g) was copolymerized with isosorbide in a melt phase-and solution-disproportionation reaction.Copolyester based on isosorbide, lactic acid and succinic acid reported by [172,173].
The reagents and catalyst Ti(OiPr) 4 were subjected to the same synthetic procedure as described before.After, the resulting copolymer was subjected to solid-state postcondensation at 100-155 • C for 10 h, which produced polyesters with similar properties to those mentioned before.These products additionally showed high degrees of crystallinity (the authors do not mention crystallinity for the example before).Both methods achieved isosorbide incorporation in the final polyester greater than 90% compared to the feed value and colorless final polymers.
Inkinen et al. reported the direct polycondensation of lactic acid, isosorbide and succinic acid (Fig. 42) using Ti(OBu) 4 as an esterification catalyst [173].The obtained M n values of 600-1,800 g/mol were very low, even after prolonged reaction times of 54 h at 1-5 mbar.
A different approach was taken by Casarano et al., who used either succinic anhydride or separately synthesized prepolymers to synthesize copolyesters from lactide, isosorbide and succinic anhydride (Fig. 42) [175].The authors used Sn(Oct) 2 as a catalyst for the copolymerization, as it is a catalyst capable of both ring-opening polymerization and polycondensation.The direct polymerization of the three monomers resulted in a random polyester, whereas the transesterification of preformed poly(L-lactic acid) with isosorbide and succinic anhydride resulted in the formation of block copolymers.Similarly, employing preformed poly(isosorbide succinate) with lactide (without catalyst) and copolymerizing poly(isosorbide succinate) with poly(L-lactic acid) resulted in block polymers.The reaction conditions for the first example involved esterification for 10 h at 1 atm and polycondensation for 24 h at 1.3 mbar, both at 150 • C. The latter examples were conducted at atmospheric pressure at 120-150 • C for 16.5-36 h.The obtained M n values ranged between 1,800 and 2,700 g/mol with polydispersities between 1.2 and 1.9, and yields between 40 and 58%.Interestingly, the random copolyester was completely amorphous, whereas the polymers with a block-type monomer distribution showed various degrees of crystallinity.The molecular weights of the polymer products were not much higher than examples reported by Inkinen et al.
Another study by the same authors applied the most promising reaction conditions, that is the polymerization of preformed poly(isosorbide succinate) (M n 2,700 g/mol) with lactide and Sn(Oct) 2 as the catalyst, in a more application-oriented publication [174].The reactants were weighed in an ampoule, which was sealed under a vacuum of 1.3 mbar, and stirred at 120-130 • C for 165 h.The incorporation ratios in the final polymer were closely related to the feed composition.The obtained M n values ranged between 8,200 and 28,000 g/mol, with a considerable decrease in molecular weight at higher isosorbide content.This hints at an initiation of the ring-opening polymerization by the hydroxyl end-groups of the poly(isosorbide succinate) prepolymer.Further investigations on crystallinity, thermal properties and electrospinning of fibers were conducted after chain extension with hexamethylene diisocyanate, which produced molecular weights 2-6 times greater.The resulting poly(ester urethanes), however, are out of the scope of this review.The chain-extended copolymers were recently studied as bioactive membranes for periodontal regeneration [176].
Another study by Casarano et al. shows the copolymerization of presynthesized poly((R)-3-hydroxybutyrate-co-isosorbide) (M n 7,100 g/ mol) with lactide using Sn(Oct) 2 as the catalyst (Scheme 8) [177].The prepolymer end-groups consisted of hydroxyl groups of the hydroxybutyrate moiety and contained only one isosorbide molecule per polymer chain.The final polyester showed the same structural features, only with the end-groups being constituted of lactic acid chains, plus some chains without isosorbide, resulting from transesterification.The thermal analysis of the reaction product was inconclusive as to whether isosorbide incorporation enhanced the thermal properties of the material.
The direct polycondensation of lactide, isosorbide and isophthalic acid was described in a recent patent by the Toray Fiber Institute (Fig. 43) [178].A variety of both Lewis and Brønsted acid catalysts was used under vacuum conditions with reaction times varying between 6 and 72 h.The obtained M w values ranged between 52,000 and 205,000 g/mol, depending on the polymerization time.No PDI values were given.
A series of publications describes the formation of bifunctional macromonomers containing isosorbide and lactic acid prior to polycondensation, a strategy first described by Vogt et al. [179].The ringopening oligomerization between isosorbide and lactide was conducted with Lewis acid catalysts (SnCl 2 , ZnCl 2 ) in a closed system at 130-150 • C for 1-2 h in bulk or in an organic solvent (chlorobenzene, xylene).Reaction monitoring showed an equilibrium conversion of 96% after 4 h and around 80-90% after 2 h.The resulting mixture of oligomers with varying hydroxyl end groups from both lactic acid and isosorbide was then subjected to polycondensation with various diacid chlorides (see Fig. 44).These include terephthaloyl chloride [180,181], isophthaloyl chloride [181,182], 5-tert-buty-isophthaloyl chloride [181] and fumaroyl chloride [183].This type of solution polymerization is, as previously mentioned, not suitable for industrial applications, but provides useful insights in the possible material properties.The resulting copolyesters were predominantly cyclic, as indicated by MALDI-TOF experiments.Isophthalic acid-based polyesters were more prone towards cyclization and therefore yielded lower molecular weights.All polymers showed a random sequence of the monomers with a complete incorporation, indicated by matching molar ratios in the final polymer and the initial feed.The presence of polyesters containing odd numbers of lactate units indicates the occurrence of transesterification reactions during the ring-opening-oligomerization [180].The authors found that molecular weights were mainly limited by cyclization [182].A control experiment employing preformed poly(L-lactic acid) resulted in the formation of a block-type polyester [182].
The M n values for isophthalic acid based polyesters ranged between 11,000 and 21,000 g/mol, while phthalic acid based polyesters exhibited M n values between 17,000 and 46,000 g/mol.This discrepancy is due to a higher inclination of isophthalic acid to form cyclic polyesters.Polyesters with a fumaric acid repeat unit were prone to crosslinking under the described reaction conditions, which, combined with cyclization, resulted in relatively low M n values (6,000-10,500 g/mol).All polyesters showed polydispersities between 4 and 6, which is due to the formation of low molecular mass species as a result of cyclizations.
ε-Caprolactone is a long chain a,b-type monomer which was once an important precursor to caprolactam.Poly(ε-caprolactone), one of the first synthetic aliphatic polyesters, obtained by ring opening polymerization of ε-caprolactone, is currently being investigated as a constituent of tunable biomaterials for drug release and other biomedical applications.
Several researchers attempted the copolymerization of ε-caprolactone with isosorbide and a diacid.One approach was reported by Scheme 8. Synthesis strategy using presynthesized poly((R)-3-hydroxybutyrate-co-isosorbide) and lactide by Casarano et al. [177].The cyclic pre-polymer was synthesized in a dilute solution using suberoyl chloride, as previous studies showed that these conditions favor the formation of cyclic structures [185].The subsequent copolymerization was conducted in bulk with a CPIS:ε-caprolactone ratio of 1:2.The authors tested a variety of catalysts, which showed fundamentally different reactivity.Sn(Oct) 2 with a benzoyl alcohol initiator only proved successful at high temperatures (200 • C), as transesterification was too slow otherwise.2,2-Dimethyl-2-stanna-1,3-dioxepane, a cyclic tin alkoxide, proved much more efficient in transesterifications, as it catalyzed the partial degradation of the prepolymer by back-biting more effectively.This allowed for the composition ratios in the polymer to approach the feed ratios.The rare-earth metal alkoxides Y(OiPr) 3 and La(OiPr) 3 proved to be inefficient in transesterification, although they were the best initiators for the ringopening polymerization.Unfortunately, only the intrinsic viscosity of the resulting copolyesters was determined as an indicator of the degree of polymerization.The values varied between 0.15 and 0.46 dL/g, with T g values between − 48 and +5 • C.
Wang et al. explored a more rigid copolyester by using 2,5-furandicarboxylic acid as the diacid monomer in a recent study (Scheme 10) [186].
In the first step, OH-functionalized poly(isosorbide-2,5-furanoate) oligomers were synthesized by melt esterification (3 h at 210 • C) between isosorbide and 2,5-furandicarboxylic acid (2 equiv.isosorbide) using Ti(OBu) 4 catalysis (0.2 mol% of FDCA).In the next step, varying amounts of ε-caprolactone (50-90 mol% of total FDCA) were added together with another portion of Ti(OBu) 4 catalyst (0.2 wt% of ε-caprolactone) and after transesterification for 3 h at 220 • C, a vacuum of 0.1 mbar was applied for 3 h at 250 • C to conduct polycondensation (Scheme 10).The authors reported M n values ranging from 13,800 to 54,300 g/mol with PDI values between 2.32 and 2.41, with increasing molecular weights at higher ε-caprolactone loadings.It was observed that around 20 mol% of ε-caprolactone was lost during polycondensation, so incorporation ratios were always significantly lower than feed ratios.The products had T g values between − 9.9 and 132.1 • C; no melting temperatures were observed by DSC.For an overview of the mechanical properties of the copolyesters, see Table 24.An increase in FDCA content was found to improve the tensile modulus and tensile strength while decreasing the elongation at break of the respective materials.
The presented studies on lactic acid copolyesters with isosorbide show that various synthetic strategies have already been explored.The most promising approach to generate high molecular weights appears to be the generation of isosorbide-lactic acid oligomers which undergo subsequent polycondensation with diacid chlorides.This limits the potential for larger scale applications of these copolyesters due to the drawbacks of polycondensations with acid chlorides discussed earlier.The potential problems encountered if diacids or diesters were to be used for a more scalable process, can be seen in a recent publication on the incorporation of PLA into PET by melt copolymerization [187].It was found that temperatures ≥ 200 • C result in significant decomposition of PLA units during polycondensation.These temperatures, however, are necessary for efficient transesterification of isosorbide containing polyesters, as reaction rates of the secondary alcohols of isosorbide at T ≤ 200 • C are usually low.This makes the synthesis of lactic acid containing isosorbide polyesters by melt polymerization challenging.The resulting polyesters would likely have different final properties, as the commonly observed cyclization during solution polymerization does not occur readily in melt polycondensations.
Thermally more stable a,b monomers like ε-caprolactone were shown to yield copolyesters with isosorbide and FDCA by melt polymerization.

Overview of mechanical properties of polyesters with isosorbide as a comonomer
Table 24 shows an overview of the mechanical properties of isosorbide copolyesters.Note that the mechanical properties of neither isosorbide homopolyesters (with isosorbide as the only diol) nor polyesters based on isomannide or isoidide have been characterized at the time this review was written.
The mechanical properties of isosorbide copolyesters depend on the comonomers in the respective material.Copolyesters with aromatic diacids and rigid diols (PEIT, PCIT, PITT) are relatively stiff, whereas materials with aliphatic diacids and long chain diol comonomers are more flexible (PBIS, PEIS).Isosorbide incorporation typically increases tensile modulus and tensile strength (compare PET and PEIT), although these trends are not always clear due to lower molecular weights at higher isosorbide contents (see PEI 23 T).Isosorbide incorporation also increases the materials' processing window due to a lowering of melting temperatures and an increase in thermal decomposition temperatures.

Concluding remarks
Polyesters containing 1,4:3,6-dianhydrohexitols, specifically isosorbide, bear a tremendous potential for future large scale applications in packaging and thermoplastics.This is due to their favorable thermal and mechanical properties induced by the rigid bicyclic structure of 1,4:3,6-dianhydrohexitols.
The influence of 1,4:3,6-dianhydrohexitol moieties in polyesters on hydrolysis and biodegradability vary depending on the polyester composition.For the former it was found that 1,4:3,6-dianhydrohexitol incorporation into copolyesters, both aromatic and aliphatic, generally improves hydrolytic degradability.Biodegradability on the other hand depends on many factors such as polyester structure (aliphatic vs. aromatic), rigidity, (semi)crystallinity and the conditions (enzymes, temperature, polymer morphology) used to study biodegradation.Different observations have been reported depending on the use of aliphatic or aromatic diacids and the chain length of diol comonomers, which means a general trend cannot easily be identified.The (bio)degradability of new, biobased materials an important aspect to consider due to the low degradability of many established fossil-based polymers in natural environments, which leads to their bioaccumulation.
Most recent publications focus on the synthesis and characterization of polyesters with isosorbide.Increasing availability of isomannide and isoidide by epimerization of isosorbide will hopefully result in more research on the melt polymerization of these monomers Despite many recent efforts to synthesize isosorbide based polyesters by melt polymerization, high molecular weights can only be reached after prolonged polycondensation times.This is due to the unreactive secondary alcohols of isosorbide.Some authors explore new polymerization techniques in the form of stoichiometric additives with some success.Recent results report higher molecular weights than initial studies from the 2000s, but novel catalytic systems tailored to the polyesterification of unreactive secondary alcohols cannot be found in the academic literature.Most publications rely on catalysts established in the framework of PET synthesis.Some promising examples using diphenyl esters to compensate for the low reactivity of isosorbide during polyesterification are scattered throughout the patent literature.
Polyester compositions with isosorbide as a property-enhancing comonomer, such as PEIT, can already be obtained with high molecular weights by melt polymerization on large scales.Some promising polyesters like poly(isosorbide succinate) however have not yet been synthesized with sufficiently high molecular weights.This also means that past reports on the properties of these polyesters usually underestimate their thermal and mechanical properties, which can be seen by some large variations of these values between publications using different synthetic methods.The negative influence of low molecular weights on the thermal and mechanical properties of polymers is widely known [38] but often not reflected in publications.This also means that the thermal and mechanical properties of many 1,4:3,6-dianhydrohexitol-containing polyesters presented here are not the "true" values of the same material with higher molecular weights.The true potential of many polyesters with 1,4:3,6-dianhydrohexitols is therefore not yet discovered due to the lack of adequate synthetic methods.
There is currently only a limited amount of rigid (T g > 60 • C), biobased polyesters commercially available.Especially amorphous materials with a high T g are in demand for applications that do not allow the use of semicrystalline materials (e.g.transparent packaging) [90].1,4:3,6-dianhydrohexitols have a rigid molecular structure which enables the synthesis of polyesters with T g values higher than most other biobased polyesters while their bent structures usually disrupt chain alignment which leads to an amorphous material at higher 1,4:3,6-dianhydrohexitol loadings.To reach T g values higher than 60 • C, varying amounts of 1,4:3,6-dianhydrohexitols are necessary depending on the corresponding diacid used.With aromatic diacids, up to 30 mol% of 1,4:3,6-dianhydrohexitols are sufficient to obtain rigid materials.With aliphatic diacids it is often necessary to use 1,4:3,6-dianhydrohexitols as the sole diol moiety to obtain a material with a high T g , which makes the synthesis of these materials more challenging.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(50:50 to 30:70).In order to obtain higher molecular weights, ethylene glycol was added twice during transesterification, as the reaction temperature of 200 • C led to EG volatilization.A higher succinic acid content decreased the T g without noticeably affecting the reactivity.The incorporation of isosorbide into the polyesters is low compared to previous examples on PCIT.The polyesters exhibited good biodegradability under composting conditions (58 • C), as 40% of PEI 30 ST 70 degraded within 50 days compared to 70% degradation of poly(1,4-butylene adipate-co-terephthalate). The polyesters also showed similar or superior light transmittance (PEI 30 ST 70 69.7%transmittance) in the range from 380 to 780 nm compared to other biodegradable polyesters such as PLA (87.5% transmittance) and poly(1,4-butylene adipate-co-terephthalate) (44.9% transmittance).
products.This was attributed to thermal degradation of ester end groups during polycondensation.The products exhibited a very favorable processing window due to high melting temperatures and thermal stability up to 405 • C. Incorporation of isosorbide improved the mechanical properties with regard to elongation at break (205% to 265.9% compared to 135% for poly(1,10-decamethylene-2,5-furanoate) (PDF)) and tensile strength (16.66 to 20.53 MPa compared to 11.4 MPa for PDF)

Fig. 44 .Scheme 10 .
Fig. 44.Synthesis of a range of polyesters by solution polymerization with acid chlorides starting from oligomers formed by ring opening of isosorbide with lactide (see Scheme 7).
• C. For other examples from the same work with 1,4-butanediol as a comonomer, see paragraph 3.1.2.
Unless otherwise noted, the values in upcoming tables were determined by the same methods.asample codes denote the feed ratio of IS to TPA. b composition ratio determined by 1 H NMR. c M n determined by GPC.d T g determined by DSC.

Table 4
Synthetic conditions and properties of PEIT with varying amounts of isosorbide.If no reaction times are given, the authors ended the process when a certain torque value of the stirrer was reached.
a Isosorbide amounts are given with respect to total molar amount of diols.bMolarSb: M (M = Li, Al) ratio.D.H.Weinland et al.
a Polycondensation time necessary to reach 30 Nm torque.D.H.Weinland et al.

Table 9
Comparison of the properties of an uncured resin obtained from recycled PET flakes and a commercially available product (Crylcoat 1771-3 from Allnex Industries).The values have been determined byGioia et al.

Table 24
Mechanical properties of isosorbide copolyesters discussed in this review.The bottom entries show reference values of established materials as determined in the indicated publication.Composition before curing: 18.0 wt% itaconic acid, 41.6 wt% phthalic anhydride, 30.1 wt% 1,2-propylene glycol, 10.2 wt% isosorbide.