Fully Biobased Aliphatic Anionic Oligoesters : Synthesis and Properties

Biobased aliphatic sulfonated oligoesters with 10 to 30% of sulfonated units were synthesized by melt polycondensation of biobased monomers such as diethyl succinate, z-octadec-9-enedioic acid, dimer fatty acid, sodium (sulfonated dimethyl succinate), and various diols like 1,4-butane diol and isosorbide. Structural characterization of the resulting oligoesters was determined by H NMR spectroscopy and MALDI-TOF MS technique. Showing a regular structure, the nature of the different expected species present in the macromolecular structure allowed the detection of etherification and cyclisation side reactions. The study of the thermal properties indicates that the resulting oligoesters are amorphous or semicrystalline that essentially depend on the nature of monomers. Films of oligoesters treated in acidic, basic, and natural media at 37°C indicate that the remaining weight depends essentially on the composition of oligoesters. Finally, sulfonated oligoester was used to prepare a biobased poly(ester-urethane) thermoset, a material having tunable properties.


Introduction
The sulfonated polyesters constitute the most studied ionic polymer family.Indeed, the incorporation of sulfonated groups into the structure of aromatic polyesters, mainly PET and PBT, has been the subject of many researches for nearly three decades [1][2][3].PET containing small amounts of sulfonated units has been known for a long time.They have been exploited by DuPont as textile fibers with improved dye ability to cationic dyes [4].The pioneer works were interested in the preparation of sulfonated PET containing Na+, K+, and Cs+ counterions [5][6][7][8][9][10].In particular, they were interested in the evolution of the thermal behavior and the crystallinity following the modification of the metallic counterion nature associated with the sulfonate group and the content of sulfonated units inside the copolyesters.The melt polycondensation of dimethyl terephthalate (DMT), dimethyl 5-sodiosulfoisophthalate (Na-DMSI), and 1,2-ethanediol has been successfully realized by Fleury et al. [11] who show that the formation of diethylene glycol (DEG) and triethylene glycol (TEG) units by etherification reaction was inevitable but can be used to tune the thermal properties, stability, and mainly the hydrolytic degradation of these polymers.In addition to the etherification side reactions, Guo et al. [12] have mentioned that the molecular weight of copolyester ionomers depends on the sulfonate content.Therefore, the copolyesters that had the highest sulfonate content led to the lowest value of molecular weight.Sulfonated polyesters based on 1,4-butane diol (BD) and 1,6-hexane diol (HD) [13][14][15][16] were also prepared; these studies clearly show the major role of the sulfonated unit content on the copolymer chain mobility and as a consequence on Tg and hydrolytic degradation behavior.The replacement of petrochemical feedstocks by their biobased counterpart product from renewable resources in aromatic and aliphatic copolyesters has benefits relating to positive environmental impacts such as reduced carbon dioxide emissions.In this context, our team has been interested in the preparation of new biobased sulfonated copolyesters from furanic monomer derived from furfural, which can be easily obtained by chemical treatment of vegetal biomass.The original properties of these materials involving thermal stability, water sorption, and hydrolytic degradation help them to be good materials for a variety of demanding applications [17].In the last few years, the synthesis of biobased aliphatic polyesters such as poly(butylene succinate) (PBS) and poly(butylene adipate) (PBA) received great interest from several research projects [18,19] due to their excellent physical properties.They are among the most promising materials for biomedical applications and environmental protection.Particular interest has been given to aliphatic polyesters based on ionomers because of their potential biological properties, such as biocompatibility and biodegradability, and their relatively easy synthesis.Two main methods have been applied to prepare these materials: (i) by polycondensation of succinic acid or adipic acid and sodium (sulfonated dimethyl succinate) (Na-DMSS) obtained by sulfonation of dimethyl maleate (DMM) and various diols [20][21][22][23][24][25] and (ii) by postsulfonation of unsaturated polyesters.The second method was carried out to prepare surfactants by sulfonation of unsaturated poly(silicone-esters) [26,27].A more detailed study was realized to describe the sulfonation of unsaturated polyesters based on dimethyl maleate and 1,2-ethane diol (ED) by the addition of sodium bisulfite in an aqueous-alcoholic medium [28].Resulting materials are soluble in an aqueous-organic medium and exhibit micellar behavior in an aqueous medium which depends on the concentration of the sulfonated units.It is also noted that the presence of anionic aliphatic unit modifies the thermal and mechanical properties of the polyesters.Recently, a series of sulfonated poly(butylene succinate) (PBS) ionomers containing up to 14 mol% of sulfonated succinate units have been synthesized by Bautista et al. [29].They report the synthesis, NMR characterization, and mechanical and thermal properties of PBS ionomers.The hydrolytic degradation of these ionomers was also performed in an aqueous medium at different pH to estimate their susceptibility to hydrolysis in different conditions.
This work is aimed at studying and characterizing fully biobased aliphatic sulfonated oligoesters based on diethyl succinate (DES), 1,18-octadec-9-enedioic acid with z configuration (C18), hydrogenated dimer fatty acid (DA), sodium (sulfonated dimethyl succinate) (Na-DMSS), and two biosourced diols: 1,4-butane (BD) diol and isosorbide (Is).The objective is to evaluate the effect of the nature of the aliphatic monomers on both thermal properties and hydrolytic degradation of these new oligoesters.Deep characterization by 1 H NMR and MALDI-TOF analysis [30,31] and hydrolytic stability of these materials both in aqueous and oxidative media were investigated [32].Finally, in view of designing new ionic and biobased thermosets, we have tested one oligoester as macrodiol in a polyurethane formulation and evaluated the thermal and hydrolysis properties of the corresponding cross-linked material.

2.
2. Synthesis of Sodium (Sulfonated Dimethyl Succinate) (Na-DMSS).Sodium (sulfonated dimethyl succinate) (Na-DMSS) was prepared according to the procedure described elsewhere [33].First, dimethyl maleate (DMM) (10 g) and NaHSO3 were dissolved in a mixed methanol/water (50/50 v/v) and were submitted to reflux at 80 °C for 8 h.Then, the reaction mixture was filtered in order to remove traces of NaHSO 3 and evaporate to dryness.The residue was dissolved in DMSO, the liquid phase was precipitated with a large amount of acetone, and the precipitate was collected by filtration and dried under vacuum at 80 °C for 48 h.The resulting copolymers were used for characterization without any purification.
2.3.5.Synthesis of Poly(ester-urethane) Network (Cross-Linked PBSu 70 Ss 30 or cPBSu 70 Ss 30 ).The poly(ester-urethane) network cPBSu 70 Ss 30 was synthesized in a way that the ratio of OH / NCO = 1 09.First, 0.495 g (0.258 mmol) of PBSu 70 Ss 30 oligoester with Mn = 1920 g/mol and 0.0346 g (0.25 mmol) of TMP has been dissolved in a limited quantity of DMF (80 : 20 wt% : wt%) at 120 °C under a nitrogen 3 International Journal of Polymer Science atmosphere, while the mixture was stirred for a few minutes.Then, 0.338 g (0.517 mmol) of HDMI was added.After isocyanate addition, the mixture was stirred again for a few minutes and poured in a rectangular silicone mold and heated at 130 °C for 48 under vacuum in order to eliminate DMF and air bubbles.
In the extractible rate, the resulting cPBSu 70 Ss 30 was put in DMF for 24 h at ambient temperature.Then, the sample was filtered and dried under vacuum at 120 °C for 48 h in order to eliminate the traces of DMF.The extractible rate determined according to equation (1) was around 7.5%.
where m i and m f are the initial mass of matter and the final mass of dry matter, respectively.The MALDI-TOF MS spectra were recorded using 10 g/L solution of sample and in DMSO mixed with 2-(4-hydroxyphenylazo)-benzoic acid of matrix solution (45/5 v/v) for PBSu 70 Ss 30 and PBISu 70 Ss 30 .For PBDA 70 Ss 30 , the analysis was recorded using 10 g/L solution of sample in CHCl 3 /THF (50/50 v/v) mixed with 2-(4-hydroxy-phenylazo)-benzoic acid of matrix solution (45/5 v/v).
Differential scanning calorimetry experiments (DSC) were performed with TA Instruments Q 22.The samples were cut and placed in a close hermetic aluminum capsule and were tested under the dry nitrogen condition, at a heating rate of 10 °C/min.Tg was taken at the inflection point and Tm at the minimum of melting endotherms.
The hydrolytic degradation essays were performed with films of oligoesters and cPBSu 70 Ss 30 : after immersion in water (pH = 4 35, pH = 7 4, and pH = 11 5) at 37 °C for one month, the samples were rinsed thoroughly in water and dried to constant weight.Sample weighting measurements were used to follow the rate of hydrolysis.
The oxidative degradation was carried out with films of oligoesters in a solution composed of 30 vol% H 2 O 2 and in a solution composed of 20 vol% H 2 O 2 in combination with 0.1 M CoCl 2 for 15 days at 37 °C [34].Sample weighting was used to follow the rate of oxidative degradation.

Results and Discussion
3.1.Copolymerization.A set of novel biobased aliphatic oligoesters: series of sulfonated poly(butylene succinate) (PBSu x Ss y ), sulfonated poly(butylene z-octadec-9-enedioate) (PBC18 x Ss y ), sulfonated poly(butylene dimer acid) (PBDA x Ss y ), and sulfonated poly(butylene-co-isosorbide succinate) (PBISu x Ss y ), containing various concentrations of sodiosulfosuccinate units were prepared according to a heating melt process with two successive steps: transesterification/esterification and polycondensation (Scheme 1).In the first one, the blend of monomers was mixed at a temperature raised to 190 °C in a presence of catalyst: tetrabutoxytitanium (Ti(OBu) 4 ) or zinc acetate (Zn(OAc) 2 ) (Table 1).The temperature was maintained for 6 h to complete the transesterification reaction by evaporating by-products: methanol, ethanol, and water, coming from the reaction between diols and diesters (methyl and ethyl) and diacids.The second step was then performed at a higher temperature (220 °C) under vacuum in order to release the excess of diol and thus displace the reaction equilibrium for getting higher molecular weight.This latter step was also catalyzed by an addition of Ti(OBu) 4 .These conditions (limited temperature and titanium catalyst) were chosen not only for avoiding the degradation of the sulfosuccinate moieties by β-elimination and formation of the maleate or fumarate units [20] but also for facilitating the polycondensation of diacids and isosorbide which are less reactive than diester monomers and 1,4-butanediol [35,36].In particular, the introduction of isosorbide together with 1,4-butanediol has allowed the release of the excess of BD during the polycondensation step [1].

Characterization of Aliphatic Biobased
Oligoesters.The chemical structure of oligoesters was investigated by 1 H NMR and the MALDI-TOF MS method.As an example, the typical 1 H NMR spectrum of PBSu 70 Ss 30 is given Figure 1.It reveals the presence of the characteristic signals of the succinate proton H1 at 2.57 ppm and those of sulfonated units H4 (-CH2) and H5 (-CH-) at 2.79 and 3.70 ppm, respectively.This is different to what has been described by Ishida et al. [20,25] who reported the presence of the -CH-signal at 6.77 ppm while the 1 H NMR spectrum of Na-DMSS highlighted the presence of the -CH-signal at 3.70 ppm.The weak pic at 1.48 ppm is related to the 1,4-butanediol internal groups -CH2-CH2-CH2-CH2-OH.The singlets at 1.62 and 4.03 ppm are associated with H3 (-CH2-CH2-) and H2 (-CH2-O-) protons of 1,4-butanediol integrated into the macromolecular chains.Finally, the small signal at 4.13 ppm is associated to the -CH2-O-adjacent to the sulfonated units.One can point out that no signal of maleate proton was detected in the 1 H NMR spectrum of PBSu 70 Ss 30 as well as that of the PBSu x Ss y series.This indicates that β-elimination reaction of sulfosuccinate units and consecutive isomerization of maleate to fumarate  2)) confirming that the experimental molar fraction is close to the theoretical one (Table 2).The number average molecular weights (Mn) calculated by 1 H NMR are ranged between 6970 and 11459 g mol -1 .One can remark that the Mn slightly increases with the increase of the sulfonated unit content as already reported in the literature for poly(ethylene furoate-co-ethylene isophthalate sodiosulfonate) (PEFSI) copolyesters [17].
x y Scheme 2: Possible side reactions involved in PBSu 70 Ss 30 oligoester. 6 International Journal of Polymer Science The MALDI-TOF MS spectrum of PBSu 70 Ss 30 oligoester is depicted in Figure 2. Five different peaks were detected and noted: L x,y , L x,y ′ , L x,y ″ , C x,y , and C x,y ″ with L and C for linear and cyclic whereas x and y are succinate and sulfonated repeating units.
More precisely, L x,y may correspond to linear oligomers containing hydroxyl end groups, as, for instance, the structure of L 3,1 in Figure 3.
An increase of 172 uma corresponding to the molar mass of one butylene succinate unit leads to the L 4,1 also detected in the spectrum as well as the L 5,1 .One can highlight that the L 5,0 and the L 6,0 were detected which means that a part of the oligoester chains was not sulfonated.Finally, even if the MALDI-TOF method is not quantitative, the peaks L x,y having the highest intensity mean that their concentrations are logically higher than those of the other structures.As mentioned above, a gap of 172 uma is observed in Figure 2 stating the presence of L 4,1 ′ oligoesters.However, one can assume that both oligoesters are in a less concentration than L x,y oligoesters because of the weak intensity of signals.
In the series of peaks, noteC x,y was assigned to cyclic oligomers which were generated during the polycondensation reaction (Scheme 2) [37].Finally, some other species: L x,y ″ and C x,y ″ resulting from side etherification reactions: etherification coupling between two 1,4-butanediol or one 1,4-butanediol and one hydroxyester end chain, already mentioned in the literature when 1,4-butanediol is used as comonomer [38] (Scheme 2), were highlighted in the MALDI-TOF MS spectrum.However, similar structures were not detected in the 1 H NMR spectra probably suggesting that their concentrations are too low, below 1 mol %, to be detected.With respect to specific 1 H NMR signals for the PBC18 70 Ss 30 series (Figure 5), one can point out that the aliphatic proton H5 from the C18 units is visible at 1.29 ppm whereas those of protons in the α position of ethylenic proton H4 are located at 1.99 ppm, the ethylenic proton H1 being located at 5.33 ppm.The signals at 2.26 and 1.54 ppm were attributed, respectively, to H7 and H6 located in the α and β position of the ester carbonyl groups integrated in fatty acid units.The difficulty to well esterify was stressed by the presence of the methoxy end 9 International Journal of Polymer Science groups (CH 3 -O-) at 3.60 ppm.However, such feature did not impact the good correspondence between the theoretical diacid C18/sulfosuccinate ratio and the experimental one (equation ( 3)).In addition, the level of molecular weight which ranged between 6740 and 8105 g•mol -1 is similar to the previous series (Table 2).These oligoesters are no longer soluble in DMSO, probably because it has undergone a change over time, so the MALDI-TOF analysis was not possible.The presence of the dimer acid units in PBDA 70 Ss 30 oligoester (Figure 6) was set by the existence of characteristic 1 H NMR peaks of the following cyclic protons: H7 The MALDI-TOF analysis (Figure 7) shows the presence of cyclic oligoesters C x,y in Figure 8.
Finally, with the respect to oligoester PBISu 70 Ss 30 (Figure 11), the isosorbide proton signals were clearly detected at 5.11 for H14 and at 5.02 for H13, H10, and H9 which were situated at 4.75 and 4.44 ppm; the signals from 3.71 to 3.83 ppm were attributed to H11 and H12 incorporated into the oligoester structures.The isosorbide end groups H11e and H12e were set at 3.30 and 3.38 ppm.H10e was situated at 4.15 ppm.The signal at 4.10 ppm was assigned for H9e protons.H14e and H13e were situated at 4.67 and 4.39 ppm, respectively.The final compositions ((diacid)/(Ss), (butandiol)/(Is)) and molar masses of oligoesters PBISu x Ss y were not determined because of the complexity of the 1 H NMR spectra.However, the MALDI-TOF analysis (Figure 12) also confirmed the presence of isosorbide unit within the polymer chain.In addition, the MALDI-TOF spectra highlighted that all oligomers are dihydroxy functionalized and that the hydroxyl functions come in majority L(3,0,0,1) Na+ L′(3,0,0,1) Na+ L(0,3,1,0) Na+ L(0,4,1,0) Na+ L′(4,0,0,1) Na+ L′(5,0,0,1) L n,m,y,z corresponds to linear species oligomer containing 1,4-butanediol end groups with n and y succinate and sulfonated repeating units associated to 1,4-butanediol, m and z succinate and sulfonated repeating units associated to isosorbide.L n,m,y,z ′ corresponds to linear species oligomer containing isosorbide end groups with n and y succinate and sulfonated repeating units associated to the 1,4-butanediol, m and z succinate and sulfonated repeating units associated to isosorbide.L n,m,y,z ′ oligoesters with isosorbide end groups in Figure 14.These results confirm the low reactivity of isosorbide due to the steric hindrance and the nature of hydroxyl function; here secondary alcohol knowing that the reactivity of hydroxyl functions on C2 and C5 is close even if not strictly equivalent [39].

Thermal Properties of Aliphatic Biobased Oligoesters.
Each DSC measurement was performed with a heating rate of 10 °C/min and a first heating cycle in order to reduce the presence of absorbed water molecules which could plasticized the polymer followed by a second one for evaluating the thermal characteristics.Thus, the data reported in Table 3 and Figure 15(a) shows that sulfonated poly(butylene succinate) (PBSu x Ss y ), having 0 to 20% of sulfonated units have a semicrystalline character and a glass transition temperatures increasing from −38 to −24 °C with the increasing of sulfonated unit content.This is in good agreement with those ionomers containing up to 14 mol% of sulfonated units [29].Moreover, the authors attributed the increasing of Tg to the intermolecular ionic interactions between the sulfonates units.This trend is also consistent with that of Bougarech et al. [17] who have reported on the role of sulfonated moieties which rigidify the oligoester backbone limiting the flexibility of the macromolecular chains and thus leading to higher Tg.For PBC18 x Ss y oligoesters, DSC traces (Figure 15(b)) highlighted their semicrystalline character which clearly indicates that the aliphatic units promote the arrangement of macromolecular chains so the crystallization phenomenon occurs.The melting point decreased from 18 to 6 °C with the increasing of the sulfonated content.On the other hand, both melting and recrystallization enthalpies appear to strongly depend on the ionic content (Table 3).It is noteworthy to indicate that the presence of the ionic groups reduces the regularity of macromolecular backbone giving birth to a decrease in melting temperature, and melting and recrystallization enthalpies as for the PBS ionomer [29].Tg values of PBC18 x Ss y oligoesters were ranged from −68 to −63 °C that is lower than their homologous PBSu x Ss y .Indeed, the length of the aliphatic units of the fatty acid helps the segmental rotation of macromolecular chains and thus leads to a decrease of Tg.Concerning the PBDA x Ss y , the bulky character of the cyclic units gave rise to amorphous structures (Figure 15(c)) with Tg ranging between −56 and −54 °C, the effect of sulfonated units being quite limited.The incorporation of isosorbide in the oligoester structures also generated amorphous oligomers and led to the increasing of the glass transition temperatures with a value up to 12 °C for the PBISu 70 Ss 30 .This was because the two fused tetrahydrofuran rings to impart a stiffness in the macromolecular backbone as already reported with other copolyesters [40].
Thermal decomposition of biobased sulfonated oligoesters was evaluated by TGA measurements under nitrogen atmosphere.Thermal gravimetrical traces from 30 °C to 600 °C are reported in Figure 16, and the temperatures for 5% weight loss (Td (5%)) are gathered in Table 3 to compare their thermal stabilities.First, it clearly appears that thermal stability of oligoesters decreased with the increasing of the sulfonated unit content probably because of a possible acidic character of these units.Then, the thermal degradation depends obviously on the overall composition.Thus, stability of PBSu x Ss y oligoesters (between 268 and 260 °C) was found lower compared to that of the PBC18 x Ss y and PBDA x Ss y ones which showed stability up to 312 °C and 353 °C, respectively.As the trend for chain scission should be higher in polyesters having a higher number of methylene groups [41], one can suppose that the presence of double bond in C18 units and the cyclic character of HDA have limited the thermal degradation by favoring cross-linking reactions.This is not the case for PBISu x Ss y containing isosorbide units in their macromolecular backbone which can undergo a hydration reaction at a high temperature, leading to 1,4-sorbitan formation.Thus, the thermal degradation was close to that of PBSu x Ss y oligoesters [42].
The remaining weight of all oligoesters after the degradation at 500 °C increased with the sulfonated content (Table 3).Indeed, PBSu 70 Ss 30 , PBC18 70 Ss 30 , PBDA 70 Ss 30 , and PBISu 70 Ss 30 show a remaining weight value much greater than their homopolyesters.The remaining weight of PBSu x Ss y goes from 2.7% to 24%.Residual weight depends also on the chemical structure of oligoesters.In fact, oligoesters based on C18 and HAD presented a lower

Degradation in Aqueous Solutions of Aliphatic Biobased
Oligoesters.The solubility of oligoesters was first tested at room temperature and/or upon heating in water and organic solvents having various polarities.Most of the sulfonated oligoesters were not soluble in water.Thus, in the PBSu x Ss y series, only the PBSu 60 Ss 40 with the highest content of sulfonate group was partially soluble in water and all oligoesters from the PBISu x Ss y series were hydrosoluble which confirms the hydrophilic character of the isosorbide units already highlighted in the literature.Indeed, isosorbide was used for preparing hydrotropes [43].Moreover, these results confirm that the macromolecular design of water-soluble ionic polyesters or oligoesters needs at least two hydrophilic monomers [11].
To assess their sensitivity to hydrolysis in acidic, neutral, and alkaline media, films of PBSu 80 Ss 20 , PBC18 80 Ss 20 , and PBDA 80 Ss 20 oligoesters were soaked in water.The temperature of bath was fixed at 37 °C over a period of four weeks.The data presented in Figure 17 show that the remaining weight (RW%) of PBSu 80 Ss 20 was decreased both in acid and basic media, 76 and 83, respectively.Contrarily, the degradation at pH 7.4 was minor with a RW% of 94.These values should be compared with those reported by Bautista et al. [29] in the case of high molecular weight polyester having a close chemical composition (PBSu 85 Ss 15 ).Data are different within a particular higher degradation level at basic pH (RW% of 60 instead of 83) and at neutral pH (RW% of 85 instead of 94).This might be caused by difference in the 13 International Journal of Polymer Science hydrolysis protocol and/or by the molar mass effect.Nevertheless, our results are expected because ester bonds are generally more sensible to hydrolysis at extreme pH [44].
The hydrolytic degradation of oligoesters based on C18 was high in basic conditions.However, in neutral conditions, hydrolytic degradation stayed higher than in acidic media.The 1 H NMR analysis of the degraded sample in acidic media logically revealed the presence of acid end groups at 12 ppm, proving the cleavage of the macromolecular backbone by hydrolysis of ester functions.In addition, a residual resonance with very low intensities was also detected at 5.31 ppm and attributed to ethylenic protons.Thus, the 1 H NMR spectra suggested that in acid conditions, the ethylenic double bonds were affected by migration in order to form carbocation located at the 4-or 5-position on a fatty acid backbone.Probably, an intramolecular cyclization of a fatty acid carboxyl group with carbocations took place to form γ-and δ-stearolactone [45] because of the presence of novel peaks at 4.62 ppm (Figure 18).The mechanism of the formation of γand δ-stearolactone is detailed in Scheme 3 [46].
Finally, oligoester based on dimer acid HDA (PBDA 80 Ss 20 ) remained as it is with nonvariation of RW% even after four weeks in water, whatever the pH.These data clearly illustrate the protective role of the dimer acid which hampers the ester accessibility by water molecules due to its hydrophobic character.
PBSu 80 Ss  In that case, one can assume that macromolecular chain's scissions occurred concomitantly via hydrolysis and oxidative reactions.In H 2 O 2 /CoCl 2 solution, the remaining weight was close to 100 wt% (films PBSu 80 Ss 20 and PBC18 80 Ss 20 attained nearly 94% and 96%, respectively) suggesting that production of reactive hydroxyl radicals by the oxidation reaction may lead to cross-linking because in the same pH, one can observe a higher degradation [34].

Toward the Preparation of Poly(ester-urethane) Network.
The preparation of sulfonated aliphatic oligoesters α and ω hydroxyl functionalized represents an opportunity to develop thermosets that are biosourced and have tunable properties [47].Thus, a poly(ester-urethane) thermoset (cPBSu 70 Ss 30 ) was performed by reacting together the oligoester PBSu 70 Ss 30 and 4,4-methylenebis(cyclohexyl isocyanate) (HMDI) in a presence of trimethylolpropane chosen as the cross-linker.The quantity of reagent was calculated in order to have a stoichiometry OH/NCO close to one.After blending, the components were cured in a silicon mold at 130 °C under vacuum for 48 h.Such conditions were chosen in order to facilitate the evaporation of the DMF previously added to ensure the miscibility between the reagents.The resulting material was not soluble in DMF, and the formation of network structures can be assumed because the extractible rate was 7.5 wt%.In addition, the glass transition temperature of the cPBSu 70 Ss 30 was much higher compared to the PBSu 70 Ss 30  3).That increase of Tg is obviously due to the presence of the urethane linkage together with the tridimensional cross-linked structures which can increase the rigidity of the macromolecule backbone [48,49].The hydrolytic degradability of that cross-linked material was also determined in water at various pH and at 37 °C and were found in the same range as a non-cross-linking material.In this case, cPBSu 70 Ss 30 is more degradable in acid and alkaline than neutral media because of the presence of ester bonds which are more sensible to hydrolysis in acid and alkaline conditions.

Conclusion
A series of biobased oligoesters containing up to 30% of sulfonated units, namely, sodium sulfonate succinate, were successfully prepared by melt polycondensation reaction using biobased monomers including isosorbide,  16 International Journal of Polymer Science hydrogenated dimer acid, and C18 diacid (1,18-octadec-9-enedioic acid).The structures of the resulting oligoesters were studied by 1 H NMR and their regular structures were evidenced.Thus, the MALDI-TOF MS analysis showed the presence of expected linear species but also cyclic species resulting from cyclisation reactions commonly observed in polycondensation.It was also highlighted that end groups are in majority hydroxyl but that acid functions can also be detected.The study of the thermal properties clearly confirms the role of the oligoester compositions.Thus, a high content of sulfonate unit as well as bulky units such as isosorbide and hydrogenated dimer acid leads to amorphous polymers whereas oligoesters containing a low level of sulfonated units and long alkyl chains are semicrystalline.This study also clearly underlines that water solubility is only achieved with oligoesters having both sulfonated and isosorbide units or very high content of sulfonate moieties (>30 mol%).The degradation in aqueous solution films of oligoesters was also correlated with the composition, the most hydrophobic oligoester being more stable.Finally, a thermoset was prepared using sulfonated oligoesters, opening the way to prepare biobased poly(ester-urethane) materials having tunable properties.

Data Availability
The figures and schemes used to support the findings of this study are included within the article.

Conflicts of Interest
The authors confirm that this article content has no conflict of interest.

Figure 12 :
Figure12: The MALDI-TOF MS spectrum of PBISu 70 Ss 30 .L n,m,y,z corresponds to linear species oligomer containing 1,4-butanediol end groups with n and y succinate and sulfonated repeating units associated to 1,4-butanediol, m and z succinate and sulfonated repeating units associated to isosorbide.L n,m,y,z ′ corresponds to linear species oligomer containing isosorbide end groups with n and y succinate and sulfonated repeating units associated to the 1,4-butanediol, m and z succinate and sulfonated repeating units associated to isosorbide.

Figure 15 :
Figure 15: DSC curves of biobased oligoesters: (a) DSC curves of PBSu x Ss y , (b) DSC curves of PBC18 x Ss y , (c) DSC curves of PBDA x Ss y , and (d) DSC curves of PBISu x Ss y .

Figure 16 :
Figure 16: TGA curves of biobased oligoesters: (a) TGA curves of PBSu x Ss y , (b) TGA curves of PBC18 x Ss y , (c) TGA curves PBDA x Ss y , and (d) TGA curves of PBISu x Ss y .

Table 2 :
Characterization of oligoesters: initial and final composition and molecular weight (Mn).
[29]acid unit/sulfosuccinate unit molar ratio in oligoesters determined by1H NMR.b Molecular weight (Mn) determined by 1 H NMR. c End groups not detected by 1 H NMR. International Journal of Polymer Science conformation at temperature above 160 °C did not occur in our conditions[29].1HNMR peak integrations allowed the quantization of succinate and sulfonated monomer units in oligoesters of the PBSu x Ss y series (equation ( Figure7: The MALDI-TOF MS spectrum of PBDA 70 Ss 30 .L x,y corresponds to linear species oligomer containing hydroxyl end groups with x and y fatty acid and sulfonated repeating units.C x,y corresponds to cyclic species oligomer with x and y fatty acid and sulfonated repeating units.L x,y ′ corresponds to linear species oligomer containing ester end groups with x and y fatty acid and sulfonated repeating units.L x,y ″ corresponds to linear species containing acid end groups with x and y fatty acid and sulfonated repeating units.

Table 3 :
Thermal properties of biobased oligoesters.Crystallization enthalpies were registered by DSC.International Journal of Polymer Science residue value than PBSu x Ss y .For PBISu x Ss y , the remaining weight slightly increased compared to their homologous PBSu x Ss y .
a Degradation temperature at which a 5% weight loss was observed in TGA.b Remaining weight at 500 °C.c Glass transition temperature taken as the inflection point of the DSC traces.d Melting temperatures were registered by DSC. e Melting enthalpies were registered by DSC.f 20 , PBC18 80 Ss 20 and PBDA 80 Ss 20 films underwent an oxidative degradation for 15 days at 37 °C in two solutions composed of (i) 30 vol% H 2 O 2 and (ii) a combination of 20 vol% H 2 O 2 and 0.1 M CoCl 2 , respectively.The data presented in Table 4 show that practically, films of PBDA 80 Ss 20 were very stable in both oxidative solutions which was expected because of the hydrophobic character of that oligoester.Contrarily, a fall in sample weight took place for PBSu 80 Ss 20 and PBC18 80 Ss 20 in a solution