Biobased polymers from lignocellulosic sources

ABSTRACT The ongoing threat of global warming has resulted in numerous attempts to reduce greenhouse gas emissions and impede its ramifications. Replacing fossil fuels in products with renewable biobased materials is currently one approach to tackle the climate change crisis. Lignocellulose is the most abundant natural biomass source and is a potential candidate to replace the non-renewable resources currently made with petroleum-based products. Cellulose, hemicellulose, and lignin, which together make up lignocellulose, are all suitable choices for the creation of biobased materials. This review aims to highlight some of the recent efforts towards synthesizing renewable biobased polymers from raw lignocellulose, as well as refined cellulose, hemicellulose, and lignin sources while identifying some of the current applications to which they are suited. GRAPHICAL ABSTRACT


Towards a greener chemistry
For the last 100 years, there has been a heavy dependence on the use of fossil fuels such as oil, gas, and coal to generate energy and produce petroleum-based chemicals (1).It was only when the detrimental environmental impacts of fossil fuels reached a critical level in the 1980s that there was a call to phase out these resources from industrial use.Since then, a shift to greener alternative materials and methods by both government and industries has slowly become more mainstream (2).The concept of green chemistry involves acquiring the knowledge and skills to identify alternatives that help reduce or eliminate dangerous materials deemed harmful to people and the environment during the synthesis, manufacturing, and application of chemical products (3,4).A green chemistry approach involves making products that consume a minimum amount of energy and produce little waste material (5).The twelve principles of green chemistry were developed by Paul Anastas and John Warner in 1998, who outlined a framework for making chemical processes or products greener throughout each step of their life cycle (3,6).One green principle that is important to combat the negative environmental effects caused by petroleumbased products is the use of renewable feedstocks.

Renewable feedstocks
Renewable feedstocks utilize raw materials that are renewable rather than depleting for the creation of products whenever it is technically or economically feasible (6).This entails replacing non-renewable materials such as oil with renewable resources to create sustainable materials that can potentially be recycled into new products or that are biodegradable and can naturally decompose with no toxic effects on the environment (7).
One example of a material that is renewable is biomass, which is derived from living organisms such as plants and animals.Incorporating biomass into the synthesis of new materials has led to the classification of materials that are considered 'biobased' (8).This biomass can be derived from various plant-based sources, including agricultural crops, forestry-based materials, or organic waste from industrial and municipal workplaces (9).The partial or complete replacement of petroleum-based materials with biobased ones allows for the creation of products that are deemed more sustainable; they do not deplete non-renewable resources while mitigating any harmful effects towards the environment (5).Examples of plant-based materials used in biobased products are listed below.

Food-based materials
Agricultural crops, or food-based materials, are derived from plants that are generally rich in carbohydrates and are used for food production.A few examples of food-based materials are summarized in Figure 1, which include compounds such as fatty acids, vegetable oils, carbohydrates, chitin, and chitosan.Fatty acids are long carbon chains containing terminal methyl and carboxylic acid groups that act as metabolic fuel for the transportation and storage of energy (10).Vegetable oils are natural oils extracted from seeds, fruits, or plants that can include palm, soybean, canola, sunflower, coconut, corn, wheat, and olive oil (11,12).Carbohydrates are a class of macromolecules present in living organisms that are classified into three main groups (monosaccharides, oligosaccharides, and polysaccharides) based on their size (13,14).Chitin is the second most abundant polysaccharide that is a main component in the exoskeletons of invertebrates, such as arthropods and mollusks, and it can also be found in the cell wall of algae and fungi.Chitosan is also a linear polysaccharide that is synthesized by alkaline deacetylation of chitin, where most of the acetamide groups are converted into primary amines.Like chitin, chitosan can also be found in the cell walls of select fungi (15)(16)(17).
In recent years, however, there has been a growing consensus amongst scientists to forgo food-based materials to avoid competition with food production and intensified farming methods to meet these demands (18).Therefore, this review will be intentionally limited to biobased polymers derived from specific forestry-based materials.

Forestry-based materials
Forestry-based materials come from plants not eligible for food production, such as those that come from trees.Some examples of forestry-based materials used to synthesize biobased products are highlighted in Figure 2 and include compounds such as terpenes, terpenoids, and rosin acids.Terpenes and terpenoids are synthesized from plants such as trees, fruits, and flowers, or obtained from essential oils from plants such as lemon grass, mints, and orange trees.The main difference between the two is that terpenes are made purely from hydrocarbons while terpenoids contain at least one oxygen group within their structure in the form of alcohols or carbonyl groups (19,20).Rosins can be classified into three types: wood, tall oil, and gum.Wood rosins come from old pine stumps, tall oil rosins are made from the distillation of crude oil from the wood pulp manufacturing Kraft process, and gum rosins, which are also the most common of the three, is the non-volatile fraction of pine resin from the extraction of terpenes, with turpentine being the volatile fraction (21,22).

Lignocellulosic biomass
While terpenes and rosin acids are suitable materials to produce biobased products, lignocellulose has gained considerable attention since it is the most abundant biomass source in the world (23).It can be isolated from various waste streams such as agriculture, forestry, and the paper industry, giving it the potential to be a renewable carbon source to phase out hydrocarbons (24).It is complex in its structure, with multiple layers and varying compositions and arrangements of biopolymers that define the cell wall of a plant cell (25).Lignocellulose is composed of cellulose, hemicellulose, and lignin that are rigidly connected through non-covalent bonding and covalent crosslinking (Figure 3) (26).
Lignocellulose is typically resistant to biological or chemical digestion due to factors such as the crystallinity of cellulose, the degree of lignification, and the overall structural features of the cell wall.As a result, it is a necessity for lignocellulose to undergo a pre-treatment process to make it acceptable for use in the biorefinery process, which converts biomass to fuel or energy, or to separate it into its respective components.Since the composition of lignin, cellulose, and hemicellulose varies with each lignocellulose source, the pre-treatment method that is selected will also depend on these compositions.There are four main pre-treatment methods that can be carried out on lignocellulosic materials: physical, chemical, physicochemical, and biological.Common processes for each pre-treatment method are as follows (26): 1. Physical: milling, microwave, extrusion, and ultrasonication 2. Chemical: alkaline and acid hydrolysis, ionic liquids, and organosolv processes 3. Physicochemical: steam and ammonia fiber explosion, CO 2 explosion, and liquid hot water 4. Biological: whole cell and enzymatic

Cellulose
Cellulose is the most abundant linear polysaccharide found in nature.It consists of repeating cellobiose units (Figure 4) that are held together by glycosidic bonds (22).While the cellulose content varies depending on the biomass feedstock, it roughly accounts for 40-50 wt% of the lignocellulose (1).
The hydroxyl groups in cellulose help contribute to its hydrophilicity and biodegradability to bind the molecules together.Cellulose can exist in either crystalline or amorphous form, with the latter being more susceptible to degradation using enzymes or chemical reagents due to poorly organized hydrogen bonding networks (25).While the actual ratio of amorphous and crystalline segments of cellulose varies at the macroscopic level of a microfibril, they tend to prefer a more ordered environment, resulting in cellulose fibers with reduced chemical reactivity and enhanced mechanical properties (22).

Hemicellulose
Hemicellulose is the second most abundant natural polysaccharide that shares some structural similarities with cellulose.It accounts for 25-30 wt% of the lignocellulose biomass and is generally lower in molecular weight than cellulose (1,27).Hemicelluloses are typically composed of pentoses such as xylose and arabinose (Figure 5(a)) and hexoses such as glucose, mannose, and galactose (Figure 5(b)).Depending on the hemicellulose source, their backbone usually consists of xylose units for hardwood or mannose and glucose units for softwood.Hemicelluloses can bind to each other and to cellulose and lignin units in lignocellulose through hydrogen and covalent bonding.Through these interactions they contribute to the rigidity and flexibility of the cell wall (25).

Lignin
Lignin is a complex aromatic macromolecule that accounts for roughly 15-20 wt% of the lignocellulose and contains a non-crystalline and irregular 3-D structure held together by ether and carbon-carbon linkages, although the former is the dominant linkage for regular lignin (25).Lignin is derived from three phenylpropanoid monomer units or monolignols, which are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 6).These can be further broken down to form the base aromatic ring structures containing p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) subunits in the polymer, which consist of one phenol group, one phenol and one methoxy group, and one phenol and two methoxy groups, respectively (28).Lignin composition differs depending on the biomass source, where softwood lignin typically contains more G units and hardwood lignin has more S units.These differences in the overall composition have an affect on its reactivity, though it contributes to the structural strength and stiffness of the cell wall (1,25).

Research scope
The overall scope is to provide an overview of the various biobased polymers derived from lignocellulosic  biomass sources within the last 5 years and explore some of their current applications.The biomass sources investigated include the three main components of lignocellulose: cellulose, hemicellulose, and lignin.This review is intended to highlight some of the interesting research currently being conducted on renewable biobased polymers from lignocellulosic sources.

Polymers from renewable resources
While there are many monomers and polymers that have been prepared from lignocellulosic biomass and its three main components (24,29), this section will focus on select polymers and polymer blends that incorporate at least one of these biomass sources in their structure.Green chemistry metrics of some of these syntheses will also be analyzed.

Polyesters
Polyesters are one of the most prevalent biobased polymers that can be synthesized from biomass sources (30).The most common type of polyester material is polyethylene terephthalate (PET) (Figure 7), which can be found in materials such as clothing fibers and plastic packaging (31,32).
As PET is almost always derived from oil-based materials, recycling waste PET will help address current environmental issues that arise from simply discarding and incinerating it after use.Xu et al. were successfully able to make composites using waste PET combined with biopolymer methyl cellulose (MC), which has the hydrogen atoms of the hydroxyl groups of cellulose replaced with methyl groups (Figure 8) (33).PET and MC were dissolved in hexafluoro isopropanol (HFIP) to form a 1.1 wt% solution, where it was then film cast into a glass mold and confirmed to be a full blend of PET and MC using solid state 13 C NMR analysis.Although HFIP is not a green solvent, it was used to ensure the solubility of both PET and MC to form a homogenous solution.It was found that adding 3 % PET gave the highest tensile strength of the composites, although further increasing the PET loading increased the elongation at break.Various MC/PET blends also showed higher thermal decomposition temperatures than MC but lower than PET using thermogravimetric analysis (TGA) (33).
A study by Manker et al. was able to prepare various biobased polyesters from modified hemicellulose sugars that are biodegradable and have good mechanical properties (34).Birch wood lignocellulose was treated with glyoxylic acid and a strong acid to separate it into its main components via aldehyde-assisted fractionation.The cellulose and lignin were then isolated to leave the hemicellulose portion, particularly the 5-carbon sugar xylose (Figure 9).The reaction between xylose and glyoxylic acid with sulfuric acid produced the diacid precursor diglyoxylic acid (DGAX), which was then refluxed with methanol to form dimethylglyoxylate xylose (DMGX).Polymerization via melt condensation with Lewis acid catalysts of DMGX with various aliphatic diols ranging from C 2 -C 6 formed the desired polyesters,    collectively referred to as poly(alkylene xylosediglyoxylates) (PAX) (34).
DSC analysis revealed a range of glass transition temperature (T g 's), with the hexylene polyester (PHX) having the lowest T g at 72 °C and the ethylene polyester (PEX) having the highest T g at 137 °C.The smaller alkylene chain polyesters demonstrated significantly higher T g 's compared to most bioplastics such as PET (65-75 °C) and polylactic acid (PLA) (55-65 °C).The tensile strength showed that PHX and the pentylene polyester (PPTX) had the highest tensile strength and ductility, with the butylene polyester (PBX) possessing good tensile strength and reduced flexibility, whereas PPX and PEX were too brittle for analysis (Table 1).This showed that the PAX polymers are able to withstand high temperatures and processing without significant degradation while the long alkylene chain polyesters demonstrated the best mechanical properties (34).
Since most plastic waste ends up in landfills, Manker et al. wanted to ensure that their polyester plastics could naturally degrade in aqueous environments (34).They performed hydrolysis reactions on PHX plastics at room temperature under acidic and neutral conditions and at 37 °C under neutral conditions.The pH 2 and 7 conditions at room temperature showed ∼55 % decrease in the molecular weight with no mass loss after 77 days, where they continued to dissolve in the aqueous solutions.By 173 days, the plastic in the pH 2 water completely dissolved, but there was still a significant amount of undissolved plastic left in the pH 7 water.At elevated temperatures, however, the plastics were completely dissolved by day 20.Using HPLC analysis, the dissolved plastics were found to contain a mixture of DGAX and 1,6-hexanediol.Further hydrolysis would degrade the mixtures to xylose, glyoxylic acid, and more 1,6-hexanediol, which are all biodegradable and non-toxic (Figure 10) (34).
Kim and Chung prepared a lignin-containing copolymer grafted to poly(ethylene brassylate) (PEB), which is derived from castor oil and is relatively inexpensive (35).The lignin was modified using sebacic acid, N,N ′ -Dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) to attach on carboxylic acids groups (Figure 11(a)) and the PEB was separately polymerized using 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD) as an initiator and catalyst (Figure 11(b)).The modified lignin and PEB (1:3 ratio) were then added together in DMF at 80 °C for 2 days to generate the lignin-graft-PEB copolymer in an 87 % yield (Figure 11(c)).A blended polymer was also prepared by reacting lignin and PEB under similar reaction conditions for only 30 minutes, resulting in a polymer that lacked a covalent linkage between  lignin and PEB from the ester groups as confirmed by NMR (35).
From the DSC data, the lignin-graft-PEB polymer displayed a higher T g value at 12 °C compared to the blended polymer at −17 °C.Due to the lack of covalent bonding, the blended polymer exhibited two melting temperatures (T m 's) and two crystallization temperatures (T c 's), whereas the grafted copolymer only showed one of each.The TGA data showed that the blended copolymer exhibited a decomposition temperature of ∼150 °C compared to the grafted copolymer at ∼390 °C, suggesting the covalent linkage contributes to the improved thermal stability of the polymer.Comparing the tensile strengths from the stress-strain curve of the grafted copolymer, PEB homopolymer, and blended polymer revealed values of 7.34 ± 1.04, 1.88 ± 0.53, and 1.06 ± 0.36 MPa, respectively.Similar to what was observed from the thermal properties, the addition of covalent linkages in the lignin-graft-PEB copolymer resulted in significantly improved mechanical properties, making it a promising sustainable polymer (35).

Polyamides
Polyamides are semicrystalline thermoplastics that generally have excellent thermal and mechanical properties, relatively good adhesion properties, and are typically easy to melt (36).Most polyamides are semicrystalline, with their thermal stabilities being determined by the T g and T m .Amorphous polyamides also exist, although they are typically less common than semicrystalline polyamides (37).One of the most common  synthetically-made polyamides is nylon 6,6 (Figure 12), which can be found in toothpaste fibers and gas pipes to name a few (7).
Melt blending can be done by mixing two or more materials in the melt without the addition of any other chemicals (38).It has been observed that when two polymers are miscible in the melt, there is a decrease in the crystallinity of the polymer due to the addition of a new component, resulting in a more amorphous polymer and a more depressed T m , whereas immiscible polymer blends see no change in the T m .When polymer blends have both hydrogen bonding and semicrystalline properties, they tend to exhibit larger T m depressions than those without hydrogen bonding, thereby showing that melt blending has an effect on hydrogen bonding within these polymer systems (39).Polymer blending is typically done using a twin screw extruder, where the materials are mixed and melted together before being extruded through a cylinder with rotating screws (40).Twin screw extrusion has also been found to be more efficient at mixing and melting lignocellulose compared to milling techniques (23,41,42).
A study performed by Muthuraj et al. aimed to investigate blending hardwood lignin with various synthetic polyamides to optimize the melt process and explore their properties and miscibility (38).Lignin and the respective polyamides were oven dried and then melt blended using a twin screw extruder with wt% blends of 30:70, 40:60, 50:50, 60:40, and 70:30 of the lignin and polyamide, respectively.The polyamides selected for the study included PA1012, PA1010, and PA11 (Figure 13) (38).
Muthuraj et al. were unable to observe a noticeable T g in any of the blended samples by DSC, so they were obtained using dynamic mechanical analysis (DMA), where the tan delta peak was used for the second order thermal transition of the samples.The blends each showed two T g 's: one for the lignin (which has a T g of ∼108 °C) and one for the respective polyamide (which all have T g 's of ∼60 °C), which suggests that these blends were not miscible at the molecular level (Table 2).However, in each case, the blends all had two T g 's that were significantly higher than their respective starting polymers.Compared to the starting polyamide and lignin T g 's, there was a significant increase with the 50/ 50 blend of PA11 (T g 's of 81 and 134 °C; + 23 and +26 °C), followed by the PA1010 blend (T g 's of 85 and 110 °C; + 30 and +2 °C) and the PA1012 blend (T g 's of 69 and 118 °C; + 9 and +10 °C), indicating that PA11 is more compatible with lignin compared to the other two polyamides.This was also confirmed using scanning electron microscopy (SEM), where a higher lignin loading (70:30) in PA1010 and PA1012 blends showed significantly higher particle sizes ranging from 2-2.5 microns compared to the 50/50 blends which were ∼0.5 microns.Conversely, there was no noticeable difference in particle size with a higher lignin loading of the PA11 blend and it maintained its size of ∼0.5 microns (38).
Zhang et al. investigated improving the thermal stability of wood flour lignocellulose, which is finely pulverized wood that looks like sawdust, by melt blending it with PA6 (Figure 13) and boric acid, which is typically used as a flame retardant and preservative for wood composites (Figure 14(a)) (43).First, the wood flour was immersed in a 5 wt% solution of boric acid for 2 hours, after which it was oven dried for a full day.The   PA6 was mixed with varying amounts of borated wood flour (0-50 wt%) using a twin screw extruder, where the composites underwent mechanical and thermal stability testing (43).DSC analysis of the borated wood flour (BWPA) composites showed that each of the composites exhibited a T m and a T c , indicative of the alpha and gamma crystalline structures of PA6.A similar trend was also observed for the wood flour (WPA) composites.The T m 's of the BWPA's were also observed to be higher than the WPA's when the wood content was greater than 40 wt %.This is significant since the decomposition of wood flour produces organic volatiles that have negative environmental effects, and the boric acid was shown to slow down the thermal degradation of the wood flour, thereby mitigating the environmental concerns (43).It was also observed from the flexural strength testing that there was an increase in the overall flexural strength of the composites by ∼30 MPa for WPA's and ∼40 MPa for BWPA's with increasing wood content (Figure 14  (b)).On the other hand, tensile strength testing showed there was a decrease in the overall tensile strength by ∼10 MPa for both WPA's and BWPA's with increasing wood content.In both cases, the WPA's typically had higher flexural and tensile strength testing compared to the BWPA's, showing that although the addition of boric acid promotes the thermal stability of wood flour, it does decrease the overall composite strength (43).

Polycarbonates
Polycarbonates are one of the main industrial plastics along with PET, polyvinyl chloride (PVC) and polypropylene (PP).Their applications vary in sectors such as transportation, packaging, and healthcare.One of the most well-known ways to make polycarbonates are to react bisphenol A (BPA) with phosgene (Figure 15).However, polycarbonates made in this way are petroleum-based and there are safety and environmental issues concerning the use of phosgene (44).
Koelewijn et al. were able to synthesize a biobased derivative of BPA from lignin and use it to synthesize thermoplastic polycarbonates (45).To form the biobased BPA, they reacted 4-n-propylguaiacol (4PG) with a 37 wt % solution in water of formaldehyde in the presence of HCl, which resulted in the formation of m,m'-bisguaiacol (Figure 16).They also reacted 4-methyl and 4-ethylguaiacol (4MG and 4EG, respectively) under similar reaction conditions.The BPA derivatives were then reacted with triphosgene, a slightly greener and safer alternative to phosgene, to form the desired thermoplastic polycarbonates (45).
While regular polycarbonates from BPA are amorphous in nature, polycarbonates prepared from 4MG were more crystalline and exhibited more solubility issues despite their good thermal properties (46).Koelwijn et al. analyzed powder X-ray diffraction (PXRD), DSC, and TGA to observe the properties of their biobased polycarbonates (Table 3).The PXRD data revealed  that increasing the alkyl chain length led to polymers that were increasingly amorphous and less crystalline.From the DSC data, the T g and T m of 4PG was significantly lower than that of 4EG and 4MG as well as that of petroleum-based polycarbonates from BPA, which have a T g of 145-150 °C and a T m of 225 °C.Although there was no significant trend in the TGA data, there appeared to be a slightly higher thermal stability with 4PG and 4EG compared to 4MG.In addition, 4PG had greater solubility in CHCl 3 , DCM, and THF, which suggests that it could be used for in-situ polymerization in the future (45).
Another study by Zhang et al. adapted a similar approach from their previous research, using borated wood flour as their lignocellulosic feedstock source to make biobased composites of polycarbonates as opposed to polyamides (Figure 17) (47).Here, the wood flour was added to a 5 wt% boric acid solution for 2 hours and mixed with polycarbonate using a twin screw extruder (47).
The DSC results showed that increasing the amount of wood flour content from 0-40 % led to a decrease in the T g from 140.4 to 127.1 °C, and the lack of a T m confirmed the amorphous natures of these polymers.It was suggested that this was due to the hydroxyl groups of the wood flour reducing the crosslinking density of the polymer, resulting in a lower T g .The tensile and flexural strengths revealed that the flexural strength was independent of the wood flour content, while the tensile strength showed a slight decrease with increasing wood flour content, thereby showing that the addition of boric acid reduces the strength of the wood flour (47).

Polyurethanes
Polyurethanes are a versatile class of polymers that are used as foams, elastomers, adhesives, and coatings to name a few.These products are used extensively in markets that include construction, automotive, and packaging (48).Polyurethane foams are generally prepared in a two-step process (49).The first step is the gelation reaction, where the hydroxyl groups of a polyol react with a suitable diisocyanate (O = C = N) to form the urethane linkage, which consists of a carbamate ester (Figure 18(a)).In the second step, the diisocyanate groups react with water to generate carbon dioxide as the blowing agent and forms a urea linkage that provides covalent and hydrogen bonding sites (Figure 18(b)) (48).Combining the urethane and urea linkages from the reaction of diisocyanates forms the desired polyurethane foams, with the rigidity or flexibility of the foam depending on the type of diisocyanate used (Figure 18(c)) (49).Since most polyols, diisocyanates and polyurethanes are derived from petroleum-based products, it is important to consider greener alternatives to non-renewable fossil fuels (50)(51)(52).A study by Amran et al. prepared rigid polyurethane foams using lignocellulosic and cellulosic biomass to synthesize biobased polyols in the formation of the foams using a method known as liquefaction (53).Liquefaction involves the use of solvents and high temperatures to depolymerize biomass materials into smaller fragments in the liquid form.Their lignocellulose and cellulose sources were oil palm empty fruit bunch fiber (EFB) and empty fruit bunch fiber-based cellulose (EFBC), and they used a mixture of polyethylene glycol (PEG) and glycerol (Gly) (PEGGly) as the choice solvent for liquefaction.The liquefaction process involved the addition of the respective biomass source (EFB or EFBC), PEGGly, and sulfuric acid in a glass reactor at varying temperatures and times, where they found that the ideal liquefaction conditions for EFB and EFBC were 175 °C for 90 minutes and 175 °C for 180 minutes (53).
One of the identifying properties of these foams is the hydroxyl or OH number, which is a measure of the concentration of hydroxyl groups on the polyol.At the optimal liquefaction conditions, the OH number of the EFB and EFBC polyols reached 228.08 and 270.49mg KOH/g, which were not quite as high as the original PEGGly polyol at 372.60 mg KOH/g.In general, OH numbers are affected by the viscosity and molecular weight of the polyols used.It was found that EFB produced polyols with the highest viscosity and molecular weight, whereas EFBC produced polyols with the lowest viscosity and molecular weight compared to the PEGGly polyol, with the latter being due to severe transesterification from the longer liquefaction time (53).
To prepare foams, the optimized EFB or EFBC polyols were mixed with a DABCO catalyst and water followed by the addition of polymeric methylene diphenyl isocyanate (pMDI).The compressive strength of the foams was measured to be 41 MPa for the EFB foam and 46.8 MPa for the EFBC foam, which were both lower than the PEGGly foam at 52.6 MPa.When looking at the SEM micrographs of the foams and their respective cell sizes, the EFB foams were larger in size, whereas the EFBC foams were similar to the PEGGly foams.This demonstrated that the cellulose-based polyol exhibited properties closer to that of the petroleum-based polyol compared to the lignocellulose-based polyol and could be used as a biobased alternative (53).
A study by the Shmulsky group used surface functionalized lignin to synthesize rigid polyurethane foams, with the idea being that functionalizing the lignin will increase the number of hydroxyl groups for enhanced reactivity with isocyanates (Figure 19) (54).To prepare the premix, which is when the materials are added together prior to the start of the reaction, excess pMDI was mixed with lignin and silicone oil.The solution was then heated in an 80 °C water bath for an hour to form the functionalized lignin-pMDI prepolymer.To make the foams, a separate mixture containing a polyol, catalyst Dabco 33-LV, and blowing agent 1,1,1,3,3-pentafluorobutane was prepared.The polyol mixture was then added to either the premix or the prepolymer, transferred to an open mold, and allowed to rise at room temperature.The foams contained anywhere between 0-40 % of the regular lignin premix (L-RPU) or the surface functionalized lignin prepolymers (SFL-RPU) (54).
Certain properties of the premixes, prepolymers, and foams were analyzed.Both the polyol and lignin were found to have similar OH numbers (∼292 and ∼297 mg KOH/g, respectively), which would make lignin a good replacement candidate to petroleum-based polyols in polyurethane foams, but the higher phenolic OH content in lignin would lower its reactivity.The prepolymers were found to be slightly more viscous than their respective premixes, which suggests that the lignin was functionalized with the pMDI, and the viscosity increased with increasing lignin content.The TGA graphs of the L-RPU's and the SFL-RPU's showed that they all degraded within the same temperature range of 150-165 °C, and the amount of mass lost was decreased with increasing lignin content.The compressive strength of the SFL-RPU's was slightly higher than the L-RPU's, although both foams showed increasing values up until 20 wt%, where a decrease in these values continued with increasing lignin content.This was confirmed by looking at the SEM micrographs, where the lignin foams containing up to 20 wt% showed homogenous cell structures and increasing it further resulted in the formation of more defective cells, although the SFL-RPU's exhibited more distorted cells.Overall, this work shows that the improved thermal stabilities and mechanical properties between the control foam and the lignin foams arise from the increase in the crosslinking density of the foam and lignin matrices and the good dispersity of lignin in the cell structures, and that up to 30 wt% of surface functionalized lignin can potentially substitute petroleumbased polyols to form rigid polyurethane foams (54).

Green chemistry metrics
For some of the synthetic processes previously described, green chemistry metrics involving the E factor (E), atom economy (AE), mass intensity (MI), reaction mass efficiency (RME), effective mass yield (EMY), and solvent intensity (SI) were calculated following previously described methods (Table 4) (55,56).Only reactions involving heavy synthesis were analyzed using green chemistry metrics, thereby excluding the polymer blends as well as the polyurethane foam composites previously mentioned.These metrics were calculated based on the information provided by the authors on the quantities of reagents used.For a complete analysis of how these values were obtained, please refer to the supplementary.
Only the solvents that were directly involved in the synthesis of the materials were used, while those used for extractions or precipitations were disregarded to provide a fairer comparison among the reactions.Therefore, some of the values reported herein, such as for E and SI, should actually be higher when work-up solvents are also considered.
The study by Manker et al. previously mentioned the greenness of all three of their reactions by maintaining high AE, which were all calculated to be greater than 95 %.The low E, MI, and SI also agree with their claim, although their RME and EMY values could be better in some of their syntheses (34).In comparison, the study by Kim and Chung also obtained high AE for all three steps, although the first step involving the synthesis of modified lignin was found to be the least green due to the high amount of waste generated from solvent use and unwanted by-products (35).The study by Koelewijn et al. appeared to be the least green overall compared to the other two reactions.A high AE was obtained to prepare the m,m'-bisguaiacol, but was reduced to less than 60 % to make the polycarbonate due to the chlorine groups on triphosgene.The E and MI values were also high due to the amounts of reagents and solvents used in the synthesis, and the toxicity of some of the materials resulted in low EMY values (45).Overall, these reactions have some good green chemistry metrics but there is always room for improvement to make them better.

Applications
The polymers previously mentioned are some examples of biobased materials that can be used in various sectors (7).As there are many possible uses for these green polymers, this section will focus on certain applications of lignocellulosic biomass materials and provide some examples of each.

Food packaging
Plastics amount to more than 30 % of the total market for packaging materials, and 99 % of plastic bags are currently derived from petroleum-based products (57).These plastics account for 20 % of the total oil consumption and contribute to the emission of greenhouse gases.As of 2018, 33 % of plastics were recycled, 25 % ended up in landfills, and the remainder was incinerated, thus demonstrating a need to move towards biobased plastics (32).The composites previously mentioned used common plastic materials such as polyesters (33,34), polyamides (38,43), and polycarbonates (47), although other plastics that can incorporate lignocellulosic biomass include polyvinyl alcohol (PVA) (58) and polyethylene (PE) (59).
Biobased plastics, particularly those from lignocellulosic biomass, can also be used as packaging since their biodegradability can help reduce environmental pollution (50,60).The food industry is one of the largest consumers for manufacturing plastics, as the films used to package food help protect it from external environmental factors while also maintaining their mechanical properties.Hemicellulose films have been considered as potential alternatives to the petroleumbased films as they are more environmentally friendly, but they tend to have poor mechanical and barrier properties (61).Hemicellulose films that have been physically   or chemically modified were found to improve the barrier properties to comparable levels as petroleumbased films, although they still had much lower mechanical properties (62)(63)(64)(65)(66). Cellulosic films have also been explored as potential alternatives for food packaging materials due to their enhanced mechanical, barrier, and biodegradation properties (67,68).Select improved properties of the films with the addition of cellulose nanocomposites include increased tensile properties up to 42 %, decreased water vapor permeability up to 28 %, and decreased oxygen permeability up to 21 % (69).Future investigations of using lignocellulosic biomass as biodegradable food packaging should focus on improving the cost and energy effectiveness, conducting proper biodegradation studies, and improving the compatibility of these films with other materials such as plasticizers or adhesive agents to further improve their properties (57,69).

Hydrogels
Hydrogels are 3-D crosslinked networks of polymers that are soluble in water and can be tailored into a variety of physical forms such as nanoparticles, films, and coatings.This allows them to be used in various applications such as drug delivery, tissue engineering, and regenerative medicine (70)(71)(72).
Kalinoski and Shi developed hydrogels made from combinations of cellulose, hemicellulose, lignin, or pure lignocellulose sources such as sorghum and poplar (73).This was achieved by dissolving certain amounts of biomass in the ionic liquid, 1-ethyl-3-methylimidazolium acetate, mixing them until dissolution had occurred, casting them in a mold and immersing them in a 1:1 water/ethanol solution (Figure 20), where they were later freeze dried for characterization purposes.The poplar hydrogel was found to have the highest compressive strength, retain the most water, and exhibit good antimicrobial properties with a significant decrease in Escherichia Coli growth after 24 hours (73).
Yan et al. were able to synthesize multifunctional lignin-based hydrogels with alkali lignin, polyacrylic acid, and a Sn metal catalyst (74).The lignin and Sn (II) chloride were added to deionized water along with polyacrylic acid and ammonium peroxydisulfate and mixed to form the hydrogels.When trying different metals, they found that the Sn 2+ metal formed the best hydrogels based on their adhesive strength, with Fe 3+ forming the second-best hydrogels.The lignin-tin hydrogels were also found to exhibit high transmittance, resulting in a very transparent hydrogel that was able to completely self-heal after 30 minutes (Figure 21(a)).The lignin-tin hydrogels also exhibited good conductivity and tensile strength even after being broken and healing (Figure 21(b)), allowing these materials to potentially be used in self-robotics and stretchable biobased electronics (74).

Flexible electronics
Flexible electronics integrate electronics based on organic and/or inorganic compounds on thin metal substrates or flexible plastics that possess properties such as good portability, flexibility, and transparency.Flexible electronics allow rigid materials to bend and deform without fracturing and usually require soft materials as their building blocks, including organic small molecules and inorganic nanomaterials (75)(76)(77).One such application of flexible electronics is smart textiles, which are materials that are woven or knitted on from fibers such as yarn or filaments, that can interact with both the user and the environment.This combination of flexible and wearable electronics opens the door for possible applications in sports, medical care, or military uses (78)(79)(80).
Supercapacitors are the main component for these electrochemical storage devices as they have high power density, good performance cycles, and fast charging cycles.Allowing these normally rigid materials to become more flexible would allow them to be used in wearable electronics.Cellulose has been considered a good candidate to prepare electrode materials in flexible supercapacitors since it is relatively inexpensive, has good flexibility and is very light weight (78).
A study by Wang et al. used a knitted Modal textile, which is a semi-synthetic cellulose fiber made from spinning reconstituted cellulose for their flexible electronics (81).The pristine Modal textile had a weft knitted pattern and underwent thermal treatment in a tube furnace at temperatures ranging from 600 to 1050 °C at a rate of 3 °C/min, stabilized for three hours, and cooled to room temperature.This process resulted in the formation of the stretchable (carbonized Modal textile CMT), where it maintained its flexibility even after reaching high temperatures.Using this CMT, they were able to turn them into two flexible electronics: a stretchable supercapacitor and a wearable heater (Figure 22) to highlight the potential for their materials.The CMT supercapacitor exhibiting high electrical conductivity and the CMT heater reaching stabilizing temperatures within 4 s, and both demonstrated good tensile strengths before and after testing their properties (81).
Liu et al. were able to develop sustainable, transparent, flexible, and conductive cellulose/zinc oxide (ZnO)/aluminum-doped zinc oxide (AZO) [CZA] films to be applied to flexible electronics (82).Here, the cellulose was the transparent substrate, the AZO was the conductive layer, and the ZnO was the interface buffer layer, with the fabrication process shown in Figure 23(a).The addition of ZnO was found to significantly improve the conductivity and transmittance of the film compared to just the cellulose/AZO film.In their case, the ZnO helps to alleviate the incompatibility between the organic cellulose and the inorganic AZO.They then used the CZA film to form an electroluminescent device, with the CZA acting as the transparent conductive substrate, the copper-doped ZnS as the photoactive layer, the BaTiO 3 as the dielectric layer, and the silver as the back electrode (Figure 23(b)).Using this structure, they were able to fabricate a 'scissors hand' pattern-based luminous film which demonstrated electroluminescence even when bent up to 90°(Figure 23(c)).This could potentially allow these films to be used in other bendable and flexible devices such as solar cells, OLEDs, or electrochromic windows (82).

3-D printing
3-D printing is becoming more dominant in science and engineering laboratories.It has been used to design, prototype, and print materials for parts of lab equipment and materials for teaching purposes.However, there are certain limitations to this method, particularly with the solvent compatibility with the desired print materials.There are also certain factors that need to be considered, such as the cost of materials, the print resolution and speed, and a certain level of expertise in the technology for a design to be successful.However, there are certainly more advantages of utilizing 3-D printing of materials compared to traditional printing methods (83)(84)(85)(86).
Cellulose has been granted considerable attention for 3-D printing, although in most cases it requires a significant amount of chemical modification and post printing processes, whereas hemicellulose has been relatively underexplored in this field.Work done by Gökçe et al. demonstrated a novel approach that allowed hemicellulose to be 3-D printed without any significant chemical modifications and without needing to blend it with any other polymer (87).Hemicellulose was extracted from corn cobs and ground up into a fine paste, where they were then mixed either with water or a 10 wt% solution of NaOH and mixed anywhere from 60-80 °C, with varying water content ranging from 62-68 %.The hemicellulose pastes then underwent 3-D printing with various shapes (Figure 24).The optimal parameters for their 3-D printed structures were with water content at 65% when heated to 80 °C.The different structures could be printed at higher or lower water contents by adjusting the parameters of the 3-D printer and were shown to be reproducible, although their mechanical properties were still inferior compared to materials made from extrusion or solvent casting methods (87).
Shao et al. synthesized microfibrillated cellulose (MFC)/lignosulfonate (LS) /cellulose powder (CP) (MFC/ LS/CP) carbon structures that demonstrated electrically conductive and mechanically resistant properties that were used for 3-D printing (88).They first obtained a MFC hydrogel, to which they added the LS and CP powders and mixed them together into a slurry.The slurry was added to a 3-D printer to form three different 3-D objects, including a dog bone, a circle, and a gridlock circle (Figure 25(a)).The 3-D printed  structures also underwent pyrolysis up to 1000 °C, where they managed to retain their overall shape (Figure 25 (b)).The resulting 3-D materials maintained good mechanical and high electrical conductivity before and after pyrolysis, possibly opening the door for these materials to be used in electrodes (88).

Organic-Inorganic interfaces
Hybrid materials are materials that are comprised of both inorganic and organic components.They have been used for applications in fields such as energy storage, the environment, and bio-electronics to name a few (89,90).The area where two immiscible phases come together, specifically between the organic and inorganic phases, is known as the hybrid interface.These interfaces are typically used to improve interface properties, such as the adhesive strength or hydrophobicity, that organic or inorganic components alone cannot do (91)(92)(93).
Bismuth(III) oxybromide (BiOBr) has a uniquely layered structure that has stacked layers of [Bi 2 O 2 ] 2+ that are interlaced with halogen ions [Br 2 ] 2-.BiOBr possesses photocorrosion tolerance and chemical inertness, allowing it to be used as a photocatalyst for wastewater remediation among other applications (94,95).Organicinorganic hybrid photocatalysts have also been investigated for the incorporation or conversion of biomass to produce hydrogen sources such as H 2 and H 2 O 2 (96).
A study by Gopakumar et al. looked into producing heterogeneous photocatalysts derived from hydrolysis lignin to generate H 2 O 2 from seawater as an alternative to traditional synthetic methods that tend to generate a chemical waste and require a lot of energy (Figure 26) (97).The BiOBr semiconductor nanosheets were hydrothermally grown under alkaline conditions on the hydrolysis lignin to produced the desired photocatalyst that will be used to generate H 2 O 2 .Hydrolysis lignin was selected as the lignin source since it can withstand dissolution in alkaline conditions (97).
They first tested the photocatalytic H 2 O 2 production of various BiOBr (BOB) catalysts grown on different substrates, including lignin (LBOB), chitosan (CBOB), and graphene (GBOB) by adding NaCl to pure water to mimic the amount of saline in seawater, which is roughly 0.6 M NaCl.It was found that the LBOB catalyst produced the most H 2 O 2 compared to the other catalysts at ∼3200 µM, and the concentration of H 2 O 2 continued to increase after 6 hours.They also looked at the effects of the LBOB catalyst on H 2 O 2 production with different metal and non-metallic ions that might be found in seawater, such as Ca 2+ , Mg 2+ , K + , and SO 4 2-.Using salts ranging from 0.01-0.04M of CaCl 2 , MgCl 2 , KCl and Na 2 SO 4 in pure water, the H 2 O 2 concentration was found to increase with an increase in the salt concentration.Assessments of the LBOB catalyst in actual seawater generated 1710 µM of H 2 O 2 , compared to 580 µM of H 2 O 2 in pure water, suggesting the phenol groups of lignin get ionized and act as an electron sink when surrounded by metal ions.The LBOB catalyst was then studied for its recyclability in natural seawater.Under a closed system of an O 2 atmosphere, H 2 O 2 production remained at 1710 µM for five reaction cycles, while in an open air system, H 2 O 2 production jumped to 4085 µM due to the adsorption-desorption abilities of the O 2 molecules on the LBOB catalyst surface, showing that it can be recycled at least five times with no decrease in catalytic activity (97).
A study by Onwumere et al. looked into using a hybrid cellulose-bismuth oxybromide membrane to  For the removal of organic pollutants, they looked into removing rhodamine B (RhB), an amphoteric dye that is hazardous to aquatic organisms and causes eye and skin irritation (99).They studied the photocatalytic degradation of RhB on its own and with either the cellulose membrane, the Bi 4 O 5 Br 2 /BiOBr membrane, or the  CM/Bi 4 O 5 Br 2 /BiOBr membrane.It was found that both the cellulose membrane and the Bi 4 O 5 Br 2 /BiOBr membrane showed some decrease in the RhB concentration over time, but using the CM/Bi 4 O 5 Br 2 /BiOBr membrane was found to completely degrade the RhB dye after 100 minutes, thereby demonstrating its strong photochemical response to potentially remove other organic dyes (98).
For the removal of inorganic pollutants, they investigated removing Co(II) and Ni(II) ions, which in excessive quantities with other heavy metal ions can cause serious environmental problems (100).They compared the adsorption abilities at concentrations of 20 and 50 mg/ L for both the cellulose membrane and the CM/Bi 4 O 5 Br 2- /BiOBr membrane and found that 70-80 % of the Co(II) ions were adsorbed within the first 5 hours of contact with the metal solutions, while only 50-60 % of the Ni (II) ions were adsorbed in the same time frame.For both membranes, 94-100 % of the metal ions were removed after 96 hours had passed.Based on this, they calculated the adsorption capacity for the cellulose membrane to be 28.

Conclusion
The replacement of non-renewable petroleum-based products with renewable biobased materials shows promise as a solution to the climate change crisis.This review aimed to highlight the significance of lignocellulose, cellulose, hemicellulose, and lignin in polymer products and current applications from within the last five years to yield various biobased materials and implement them in academic and industrial settings.Lignocellulosic biomass materials have been incorporated into or blended with polymers such as polyesters, polyamides, polycarbonates, and polyurethanes, with applications in food packaging, hydrogels, flexible electronics, and 3-D printing.Although these are only a select few of the polymers and applications that were listed, there will be more attention geared towards synthesizing biobased products and using them for novel and current applications soon.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1.Examples of food-based materials used for the synthesis of biobased products.

Figure 2 .
Figure 2. Examples of forestry-based materials used for the synthesis of biobased products.

Figure 9 .
Figure 9. Synthesis of PAX polyester from the esterification of xylose from ref. (34).

Figure 15 .
Figure 15.General synthesis of polycarbonates from BPA and phosgene.

Figure 18 .
Figure 18.(a) Gelation reaction between a diisocyanate and polyol to form urethane linkages.(b) Blowing reaction between a diisocyanate and water to form urea linkages.(c) Reaction between urethane and urea linkages with diisocyanates to form polyurethane foams.

Figure 22 .
Figure 22.Fabrication of a stretchable supercapacitor and thermal-therapy device using weft-knitted Modal fabric as the electrodes.Adapted from ref. (81) with permission.Copyright 2017, American Chemical Society.
remove pollutants from wastewater by adsorbing metal ions in inorganic pollutants and perform photocatalytic decomposition on organic pollutants(98).The cellulose membrane was prepared using an Experimental Paper Machine (XPM), and the Bi 4 O 5 Br 2 /BiOBr heterojunction was prepared by mixing Bi(NO 3 ) 3 with KBr and 1 M NaOH, transferred to a steel autoclave, and underwent hydrothermal treatment.To form the cellulose-bismuth oxybromide (CM/Bi 4 O 5 Br 2 /BiOBr) membrane, the Bi 4 O 5- Br 2 /BiOBr solution was combined with fragments of the cellulose membrane, where SEM micrographs confirmed the successful functionalization of the cellulose and Bi 4 O 5 Br 2 /BiOBr at the organic-inorganic interface (98).

Figure 24 .
Figure 24.3-D printed shapes from hemicellulose paste, including (a) a hollow cube before and (b) after drying for 24 hours; (c) a flower before and (d) after drying for 24 hours; (e) a three-layered grid.Adapted from ref. (87) with permission.Copyright 2020, American Chemical Society.
7 and 29.7 mg/L for Co(II) and Ni(II), respectively, and 37.3 and 30.2 mg/L for Co(II) and Ni(II), respectively for the CM/Bi 4 O 5 Br 2 /BiOBr membrane.This demonstrated that the CM/Bi 4 O 5 Br 2 /BiOBr membrane had a higher affinity for Co(II) ions than for Ni(II) ions (98).

Figure 26 .
Figure 26.Interface of a heterogeneous photocatalyst derived from hydrolysis lignin supported by BiOBr to produce H 2 O 2 from seawater.Adatped from ref. (97) with permission.Copyright 2022, American Chemical Society.