CHAPTER 1: Bio-based Polymers and Materials

Biomaterials have gained attractiveness in the last decades due to both ecological and economic concerns. Increased pollution, and especially visible pollution, has first driven the scientific and industrial communities to look at biofragmentable and biodegradable substitutes for traditional petroleum-based non-biodegradable materials. Then the dramatic increase of oil prices before the economic crisis of 2007 influenced the move from the biodegradable to the bio-based. Finally, the compliance of the obtained materials with thermo-mechanical constraints has turned interest to the partially bio-based materials.

Bio-based materials can be obtained mainly by two different ways: the direct production of polymers or the production of bio-based monomers and their further (bio)chemical polymerization. The direct production of biopolymers can be achieved by microorganisms (polyhydroxyalkanoates, PHA), by algae (alginate etc.), by superior plants (pectin etc.) or by several types of producers, e.g. cellulose is produced by superior plants but also by bacteria, chitosan is produced by crustacean but also by fungi.

Whatever the producer of biopolymers, the main difficulty is to trigger its composition. Indeed, the obtained material has to comply with the thermo-mechanical constraints of its anticipated usages, and these characteristics are strongly related to the monomeric composition of the polymer and its size. This common problem of biopolymers does not have a unique solution. In the case of PHA, several microorganisms can produce the same polymer and the modification of the feeding substrates influences the monomeric composition of the polymers produced by the same microorganism, whereas important differences are found in the monomeric composition of the poly/oligo-saccharides produced by different (micro)organisms. The regulation of the size of the biopolymers appears even more complicated; although PHA can be obtained with important masses, polysaccharides are actually mainly oligosaccharides. In any case, the main difficulty consists in making the (micro)organism perform the biosynthesis we want consistently and repeatedly.

To circumvent this problem, one can be tempted to make more controlled chemical polymerization with the bio-based monomers. Thus the production of bio-based monomers was also developed. However, the polymerization of bio-based monomers often asks for more development, as in the case of polylactic acid (PLA) and of polybutylene succinate (PBS); moreover, the thermo-mechanical needs for the expected applications are hardly reached with these polymers. Therefore, two more options can be foreseen: the production of partially bio-based materials (Sorona^®) or the production of bio-monomers identical to the already existing and improved petroleum-based (ethylene, isobutylene, caprolactam etc.).

In this chapter we discuss the main biomaterials produced by these different methods as well as the achieved improvements and remaining bottlenecks in their production, modifications and applications.

Polyhydroxyalkanoates (PHA) were discovered in 1926 by Maurice Lemoigne¹  as energy storage materials in Bacillus megatherium and Bacillus mesentericus vulgatis. Still, they had to wait until the 1960s and for Cupriavidus genera (previously referred as Hydrogenomonas, Alcaligenes, Ralstonia and Wautersia) to be extensively studied.^2–4  Indeed, the accumulation of the PHA by this genera has appeared to be more effective. Moreover, the first petrol crisis and further ecological issues increased the awareness and the interest in the bio-based materials.

Several PHA producing microorganisms as well as several types of PHA were discovered.^5–8  The whole PHA family can be sub-divided into three main categories: the short-chain length PHA (PHA_SCL), the medium-chain length PHA (PHA_MCL) and rarer PHA (Figure 1.1). The structural differences inside the PHA family imply deep differences in their thermo-mechanical properties. Thus, PHA_SCL mainly composed by polyhydroxybutanoates (PHB) and poly(hydroxybutanoate-co-valerate) (PHBV), are crystalline polymers, which are rather brittle and stiff, with high melting points (near 160–180 °C) and low glass transition temperature (between −5 and 0 °C), whereas the PHA_MCL are thermoplastic elastomers with low crystallinity and tensile strength with high elongation to break (400–700%).^9,10 

PHA_SCL are the most studied biopolymers among the PHA family. Numerous improvements of their production have been achieved during these last decades. These improvements mainly concerned the selection of wild-type strains (121 g L⁻¹ of PHB was thus achieved using Cupriavidus genera¹¹ ), the engineering of strains (161 g L⁻¹ of PHB was reported with E. coli (XL1-Blue) strain¹¹ ), the feeding strategy ‘nutrient limited’ versus ‘nutrient sufficient’ conditions,^12,13  with the latter having been recently proved to be most efficient for the main producing strains (33 times productivity enhancement¹⁴ ).

Also, the growth and the PHA accumulation on wastes and by-products have been paid important attention in recent years, in order to enhance the economic and sustainable efficiencies. Thus, different alternative substrates were tested – such as vinasse,¹⁵  oil palm frond juice,¹⁶  soybean oil,¹⁷  waste glycerol¹⁸  and other by-products from the biodiesel industry.^19,20  Unfortunately up to now these strategies have not shown comparable productivities as an artificial carbon source (only 67.2 g L⁻¹ of PHB were produced when soybean oil was used as a substrate).

Even if the Cupriavidus genera remains predominant in the production of PHA_SCL, other genera were also discovered and studied in recent years: Bacillus cereus,²¹  Brevundimonas vesicularis,²²  Sphingopyxis macrogoltabia,¹⁸  Nostoc muscorum,²³  Synechocystis sp.,²⁴  Herbaspirillum seropedicae,²⁵  Haloferax mediterranei²⁶  etc.

The important issues of the control of the biopolymer composition, the relative abundance of the 3-hydroxybutanoate (3-HB) and 3-hydroxyvalerate (3-HV), was also addressed by different feeding strategies, namely the choice of the 3-HV inducing substrates^27–30  (up to 80% of 3-HV content was obtained with 1 g L⁻¹ mixture of levulinic acid and sodium propionate³¹ ) as well as the choice of one-time initial versus sequential addition of those substrates, the latter been found more efficient.^32–35 

Finally, bioprocess improvements such as solid-state fermentation (SSF),^36,37  continuous and two-stage culture systems,³⁸  down-stream processing (DSP)³⁹  and purification⁴⁰  were studied.

PHA_MCL are mainly produced by the Pseudomonads.⁴¹  They are usually synthesized as copolymers of two or three or even more monomers, obtained by β-oxidation of fatty acids used as feeding substrates, the monomeric parts usually bear n±2 carbons. One noticeable exception to this general rule is the recently reported Pseudomonas mendocina strain able to produce pure homopolymers of poly-3-hydroxyoctanoates (PHO).⁴² 

Although several strategies, such as multiple nutrient limitation,^43,44  batch and chemostat strategies^45,46  or strain engineering,^47–49  were attempted for improving the PHA_MCL productivity, it remains rather low compared to the results of PHA_SCL. Thus PHA production of 0.2 g L⁻¹ h⁻¹ was observed for Pseudomonas oleovorans grown on octanoic acid,⁵⁰  or 47% of PHA inside the cells of Pseudomonas putida grown on 11-phenoxydecanoic acid, or 53–58% of the conversion of raw materials by Comomonas testosterone grown on vegetable oil.⁵¹ 

With PHA_MCL presenting more interesting thermo-mechanical properties and PHA_SCL being more easily produced it was tempting to try to combine the advantages by synthesizing the PHA_SCL-co-PHA_MCL. The most studied among these copolymers is poly(3-hydroxybutanoate-co-3-hydroxyhexanoate) (P(HB-co-HHx)). Aeromonas caviae seems to be one of the rare bacteria to naturally produce such copolymers, the main results being obtained with engineered strains. The best results so far (up to 70% of 3-HHx content) was obtained with the Cupriviadus necator engineered with the Rhodococcus aetherivorans PHA synthase, grown on crude kernel oil.

Other rare PHA are mainly composed of P3HA-co-P4HA^52–54  and the thiopolyesters polyhydroxybutanoate-co-polymercaptoprionate (PHB-co-PMP),^55–57  however, the whole PHA family contains more than hundred different polymers, and is still growing.^58,59 

Applications of PHA have evolved. Initially foreseen applications in packaging have been recently replaced by more promising and cost-compatible medical applications. Numerous devices (patches, scaffolds etc.), wound management tools (suture, dressings), drugs delivery systems and pro-drugs were based on these biopolymers.^60–63 

The observed shift in the applications of PHA has significant importance on the production of those polymers. More particularly the high purity required for the final products designed for medical application may not be compatible with the use of wastes and by-products as raw materials. Thus, in the near future we will observe a shift from the study of cheap raw materials (in order to lower the overall cost of PHA^64–66 ) to purification processes in order to separate the PHA from the enzymes and proteins linked to the PHA granules inside the cells.^67–69 

Although present worldwide, the total industrial production of PHA remains tiny (Table 1.1). However, very recently one of the main historical producers of PHA, Metabolix, has achieved for the first time a $3.6 million profit in 2011.⁷⁰  It now becomes reasonable to foresee more success stories in the future for this family of biopolymers.

Polysaccharides and oligosaccharides are widely produced in nature. Animals and plants are the most important producers in terms of volume, whereas microorganisms produce much wider diversity. Also, the small-scale structure, the proportion of different sugars and the type of linkage play an important role in the final properties of these biopolymers. Thus, applications of these poly- and oligo-saccharides are also very different, from low- to high-added value products.

Plants are the most important producers of polysaccharides. Their main products are cellulose, hemicellulose, starch, inulin and pectin. Cellulose is by far the most abundant renewable polymer available worldwide, its occurrence was estimated at some 10¹¹ –10¹² tons per annum.⁷¹ 

Cellulose and starch are both homopolymers, only composed by d-glucose units (Figure 1.2). The only difference between them is the type of linkage between the sugar units. Cellulose is composed by β-d-glucose units, whereas starch has α-d-glucose units. This tiny structural difference makes a huge difference in the properties of these two polysaccharides. Starch is digestible product used by many organisms on earth, whereas the digestion of cellulose is very difficult as it is often requiring both physical and chemical steps. Starch, used by humans for centuries for food and feed (as well for its nutritional value as thickener and emulsifier),⁷²  has found application also in textile and paper industries and as a biodegradable packaging material.^73–75  More recently it has been investigated as a first-generation bio-fuel, but raised ethical issues due to the famine problem in different parts of the world. On the other hand, the traditional usage of cellulose was in the paper and textile industries,⁷⁴  but more technical applications have been recently explored using nanocrystals⁷⁶  or grafted cellulosic materials.^77–79  Nowadays it is also abundantly studied as a second-generation bio-fuel.

Inulin is another homopolymer produced by plants. It is composed mainly of fructose units, even if a starting glucose moiety can be present (Figure 1.3). The main producers of inulin are either chicory and artichokes or biocatalytically synthesized fructo-oligosaccharides.⁸⁰  Inulin is mainly used in the food industry for both its nutritional and technological advantages. Indeed, being composed of fructose units, inulin is hardly hydrolysed during the digestion process.^81,82  The non-food applications of inulin have also been recently investigated; until now they concern merely modified inulin, thus carboxymethylinulin (CMI) was successfully used as dispersing agent and dicarboxyinulin (DCI) as a builder or co-builder in detergent formulation to replace polyacrylates.⁸³ 

Pectins are polysaccharides bearing methyl-esterified galacturonic acid and rhamnose units.⁸³  The proportions of these units as well as the presence of other constitutive units depend on the plant from which the studied pectin was isolated.⁸⁴  Pectins are biosynthetically produced in the Golgi apparatus of plants. Although many plants are able to produce pectins, industrial pectin is mainly extracted from citrus peel and apple pomace under mildly acidic conditions. The main applications of pectins remain in the food industry as gelling agents, while more technical applications could arise from more homogeneous polysaccharides obtained by chemical modification or gene technology.^85,88 

Chitin and chitosan are the main polysaccharides produced by animals. Those molecules are part of insects’ and crustaceans’ exoskeletons,⁸⁶  even if some mushrooms and fungi⁸⁷  are also able to produce them. Chitin and chitosan are aminoglucopyranans composed of N-acetylglucosamine (GlcNAc) and glucosamine (GlcN) units (Figure 1.4). Currently the most important source of chitin and chitosan remains chemical processing of the waste fraction of the shellfish industry, even if some biotechnological and entomological studies are in progress. The main applications of these polysaccharides are, until now, based on their antimicrobial properties as applied to either the food or the cosmetic industries.⁸⁸ 

Compared to higher plants and animals, microorganisms are characterized by a wide diversity of the poly- and oligo-saccharides they produce (Table 1.2). Among the microorganisms, bacteria have the widest spread of possibilities. Polysaccharides produced by bacteria are mainly extracellular polysaccharides (EPS),⁸⁹  also called exopolysaccharides, whereas those produced by algae are mainly cell wall and structural constituents.

Intracellular bacterial polysaccharides have not yet found proper applications; they are, however, extensively studied as storage materials similar to those in humans (glycogen⁹⁰ ) and as specific targets for the drug attacks of pathogens (murein,⁹¹  teichoic and teichonic acids⁹² ).

The main applications of EPS are in the food and cosmetic industries as thickeners, gelling agents and emulsifiers or in pharmacy and medicine,^93–95  with some of them being used as the active principles (schizophyllan⁹⁶ ).

Some of the microbial polysaccharides are also produced by higher organisms, such as cellulose in plants or chitin and chitosan in animals; however, microbial production is often better controlled and offers the possibility of higher-added value applications.^89,97 

Among other applications we can underline are the use as oil-drilling agents (dextran and derivatives,⁹⁸  xanthan,⁹⁹  sphingan,¹⁰⁰  scleroglucan¹⁰¹ ), immobilization supports (curdlan,¹⁰²  alginate¹⁰³ ), cement-based materials (sphingan⁹⁹ ), adhesives (pullulan¹⁰⁴ ) and can sealing (alginate¹⁰⁵ ).

Several other biopolymers are directly synthesized in nature, such as proteins, poly(amino acid)s, lignin, humic substances or sporopollenin. Until now, they are under-used and under-studied compared to the previously detailed two main families of directly produced biopolymers. Thus in this paragraph we will only mention some recent developments in the field of proteins and poly(amino acid)s, and lignin.

Proteins are composed of amino acids linked by peptide bonds. Two main biosynthetic pathways for protein production have been identified so far: the ribosomal and the non-ribosomal–multi-enzyme paths.¹⁰⁶  The proteins are mainly heteropolymers composed by a variety of amino acids; however, three poly(amino acid)s can be obtained through the multi-enzyme pathway: cyanophycin (aspartic acid–arginine dipeptide), ε-poly-l-lysine and poly-α,β-aspartic acid (Figure 1.5).

The main applications of proteins are in the nutraceutical¹⁰⁷  and pharmaceutical¹⁰⁸  industries. The specific antimicrobial properties of ε-poly-l-lysine promoted its utilization in the food industry in Japan,¹⁰⁹  whereas poly-α,β-aspartic acid is mainly used as a polydispersant in detergents.¹¹⁰ 

Some proteins were studied for a long time for their materials applications: soy protein,¹¹¹  wheat gluten¹²⁰  and collagen (the denatured form being called gelatine).^112,113  In all the cases, the stability of the proteins and their sensitivity to moisture require strengthening by plasticization, compatibilization, cross-linkage^114,115  or production of protein–nanoclay composites.¹¹⁶ 

Lignin is, besides cellulose and hemicellulose, the third main constituent of plants. Lignin possesses a very complex, cross-linked, polyphenolic structure.¹¹⁷  Despite its really important chemical potential as the sole abundant source of bio-based aromatic compounds, the inherent difficulties of the purification and homogenization of lignin severely limits its widespread usage.¹¹⁸ 

Current studies also cover the purification^119,120  and the de- and re-polymerization of lignin,^121–123  as its application in the materials industry.¹²⁴  In this fast-evolving context, the drawback is that the price of lignin, previously merely considered as waste or by-product, rose, whereas valuable applications are not yet clearly identified.^125,126 

In the category of the polymers produced from bio-based monomers, the polyesters used to be more popular. Thus, historically, the main studied monomers were bi-functional molecules, such as lactic acid, an α-hydroxy acid able to self-condense for the production of polylactic acid (PLA); 1.3-propanediol (PDO) leading to Dupont's Sorona^® after condensation with terephthalic acid and succinic acid, foreseen to be a key bio-based building block and leading to polybutylene succinate (PBS) after condensation with 1,4-butanediol.

More recently, some other monomers have been studied. The attraction of production of well-known materials from renewable feedstock led to studies of the use of ethanol for the production of bio-based polyethylene (PE), of caprolactam and muconic acid for the production of polyamides (PA) and of isobutylene for the synthesis of polyisobutylene.

Lactic acid used to be an important molecule for the chemical and food industries for centuries. It is produced through anaerobic fermentation by many bacteria. Traditionally its main applications are in the food industry where it is used as a natural acidifying agent.^127,128  More recently, the scope of its applications was significantly enlarged by the synthesis of polylactic acid (PLA) as a new biodegradable and bio-based bioplastic.¹²⁹ 

The synthesis of PLA from lactic acid cannot really be achieved by simple condensation, which mainly leads instead to the oligomers. Therefore a more original way of synthesizing PLA was achieved by ring-opening polymerization (ROP) starting from the lactide, the lactic acid dimer (Figure 1.6), with the lactide itself being obtained by the partial de-polymerization of the oligomers.¹³⁰ 

Lactic acid bears an asymmetric carbon and is mainly produced by bacteria at nearly enantiopure (S) (l) configuration. The improvement of thermo-mechanical properties of PLA and more specifically of its heat resistance require the combination of pure P(l)LA and P(d)LA in so-called stereocomplexes.¹³¹  The production of the (R)-lactic acid was therefore studied, even if until now the strains producing this enantiomer are rarely reported and the production remains lower than for the (S) counterpart.¹³² 

Recycling currently appears to be a more sustainable approach for the management of the end of life of PLA than composting. Thus, de- and re-polymerization techniques were studied.¹³³  These studies have faced the racemization issue,^134,135  therefore desymmetrization approaches were studied. The stereoselective oxidation to pyruvic acid unfortunately leads to the loss of 50% of the desired product^136,137  or to the reduction of the same enantiomer of the lactic acid.¹³⁸  Biocatalytic discrimination using the Candida antarctica lipase B (CALB) remained unsuccessful.¹³⁹  Finally, kinetic resolution using (R)-myrtenol or the separation of the diastereoisomers obtained with (S)-2-methylbutanol as a chiral auxiliary¹⁴⁰  were found to be the most promising at the current stage.

PLA suffers from its cost and also some weaknesses in thermo-mechanical properties (mainly heat resistance); however, it is considered as one of the most promising bioplastics for the substitution of the petroleum-based polymers in materials and packaging applications.^141–143  The main industrial PLA companies are Nature Works, invested by Cargill and PTT Global Chemicals, with the announced production of 140 000 metric tons of PLA per year; as well as the traditional lactic acid producers such as Purac through Synbra and the collaboration with Sulzer Chemtec and Galactic through its joint venture with Total, Futerro (1500 tons per year).

1,3-Propanediol (PDO) is one of the oldest known products of anaerobic fermentation. Numerous wild-type genera, Klebsiella,¹⁴⁴  Enterobacter,¹⁴⁵  Citrobacter,¹⁵⁴  Clostridium,¹⁴⁶  Lactobacillus,¹⁵⁷  convert glycerol to PDO (Figure 1.7). Most of them are class II pathogens, i.e. opportunistic pathogens. Also the production of PDO by wild-type strains leads to the concomitant production of several by-products, such as 2,3-butanediol, lactate, acetate, formate, ethanol etc. Finally, despite the assumption of the imminent over-production of glycerol due to the extensive use of biodiesel, PDO remains more expensive than glucose.

Several studies were accomplished in order to achieve production of PDO by engineered strains. Two different strategies were attempted: expression of genes for the glycerol production from glucose in PDO-producing strains and the expression of the PDO-producing genes in the glycerol-producing strains. The first strategy was found to be more efficient and has been applied industrially by Genencor,^147–149  a Danisco's subsidiary.

Further, DuPont performed the polymerization of PDO with terephthalic acid (Figure 1.8) to obtain Sorona^®.¹⁵⁰ 

This partially bio-based polymer is mainly used in different fibre applications such as floor coverings and sportswear. The PDO success story is considered as an important milestone from both industrial and societal points of view because it led to the investigation of other possibilities for the coupling of bio-based and traditional monomers (see below for PBS) with further consideration of the importance of the bio-based content in polymeric materials (controlled by the ASTM D 6866 norm, for example), and to the ultimate acquisition of Danisco with its Genencor subsidiary by DuPont in 2011.¹⁵¹ 

Succinic acid can be produced by anaerobic fermentation of several wild-type strains: Anaerobiospirillum,¹⁵²  Propionibacterium,¹⁵³  Escherichia,¹⁵⁴  Pectinas. Succinic acid is considered as one of the major bio-based platform chemicals (Figure 1.9). Its applications are foreseen as well in commodity chemicals (THF, hydroxysuccinimide etc.), as organic acids (malic, fumaric, itaconic etc.) or polymers (polybutylene succinate, PBS).¹⁵⁵ 

Therefore, several academic and industrial studies were launched for the improvement of the production of succinic acid. Several enzymes involved in the metabolic pathway for the production of succinic acid, fumarate reductase, PEP carboxylase, malate deshydratase, were cloned and overexpressed in E. coli or in S. cerevisiae; improvements and mutations of wild strains were also performed.

Currently, the main industrial programs aiming at the production of succinic acid are headed at 2000 ton-scale by BioAmber (a joint venture of ARD and DNP), at 100 000 ton-scale by Reverdia (a joint venture of Roquette and DSM) and at 60 000 ton-scale by Succinity (a joint venture of Purac and BASF). However, the current market of succinic acid (produced from fossil raw materials) is only 15 000 tons per year. The industrial companies are clearly anticipating the enhancement of the demand for bio-based succinic acid, mainly due to the PBS applications.

The production of PBS remains mainly at the research and development stages. The direct condensation of succinic acid and 1,4-butanediol encounters the same problems as in the PLA case discussed above. To circumvent these issues, different strategies were adopted: the two-stage melt polycondensation (esterification and polycondensation), the condensation–extension approach using hexamethylene diisocyanate as extension agent. This latter strategy gave the most promising results until now with the M_n and MW of 40 000 and 100 000 g mol⁻¹ respectively.^156–158  The use of cyclic monomers was yet poorly explored and mainly used in enzymatic polymerizations.^159–161 

Ethanol is one of the oldest biotechnological products used by humans, even ancient Egyptians were drinking a sort of beer obtained by alcoholic fermentation. More recently ethanol was involved in first- and then second-generation biofuels. The overall sustainability and economic viability of these approaches remain doubtful. However, the production of bio-ethanol in important amounts led to its consideration for bio-based plastics production. Thus, the dehydration of ethanol was extensively studied and improved to produce ethylene, while the further polymerization to polyethylene (PE) and utilization are well known in the plastic industry.¹⁶²  Furthermore, bio-ethanol was also used for the production of partially bio-based polyethylene terephthalate (PET).¹⁶³ 

Braskem is currently the main actor for the production of bio-PE.¹⁶⁴  The Braskem's bio-PE is produced from sugar cane ethanol; in order to satisfy its production capacity of 200 000 tons per year, it uses arable land estimated at 0.02% of all the available land in Brazil. Further developments in bio-based polyolefin concern bio-polypropylene (PP) obtained through the metathesis of the ethylene dimer.

The polyamide-6 (PA-6) was first synthesized from caprolactam by Wallace Carothers in 1935 when working for DuPont. Then, it became one of the most popular polymers worldwide, reaching an annual production of nearly 2 billion tons. Its main producer is the Dutch DSM. Thus it is understandable that this company was pioneering the biotechnological approach for the production of bio-caprolactam from the α-ketoglutarate (Figure 1.10).

It is worth noticing the retrosynthetical approach of this synthesis, as the first patented step was the cyclization of 6-aminocaproic acid to caprolactam,¹⁶⁵  followed by the transformation of α-ketopimelate into 6-aminocaproic acid¹⁶⁶  and finally the elongation of α-ketoglutarate to the α-ketopimelate.¹⁶⁷  This latter step is of particular interest, as it uses the iteration of acetyl-CoA's addition–dehydration–hydrogenation–decarboxylation cascade ending by the addition of one carbon atom to the main skeleton, and can be followed until α-ketosuberate (three carbons) have been added to the initial chain (Figure 1.11).

Adipic acid is the most important commercial aliphatic dicarboxylic acid. Its main application is the synthesis of the polyamide-6,6 (PA-6,6), another important polyamide first synthesized by Carothers in the early 1930s. The direct biotechnological access to adipic acid from sugars remains challenging. It is hypothesized that 2-oxoadipate, an intermediate in the l-lysine biosynthesis, could be used; however, the subsequent elimination of the keto-group still needs to be confirmed. Besides, it can also be obtained from the degradation of cyclohexane, caprolactam and long-chain dicarboxylic acids, or nitrile hydrolysis of adiponitrile¹⁶⁸  (Figure 1.12).

The uncertainty about direct biotechnological access to adipic acid offers a route to the exploration of possible biosynthesis of its intermediates – glucaric and muconic acids.^169,171  The final conversion of these intermediates to adipic acid is until now performed only by chemical means. An interesting point is the possibility to produce the muconic acid from aromatics,¹⁷⁰  thus combining depollution and the synthesis of useful chemicals.

Until now, the bio-based monomers were composed by hydrocarbons bearing oxygen or nitrogen heteroatoms, with pure hydrocarbons being only obtained from fossil feedstock. This assertion is no longer true. French biotechnological company Global Bioenergies has recently patented a biotechnological process for the production of isobutylene, the monomer leading to the polyisobutylene widely used for its gas barrier properties (Figure 1.13). Besides the impressive technological breakthrough of this process, whose economic feasibility has yet to be discussed, the described procedure is very elegant, as the product is recovered in gas form from the reaction media, thus avoiding its saturation.

Further developments of pure hydrocarbon monomers leading to bio-isoprene are currently under development by Ajinomoto (teaming with Bridgestone), DuPont (teaming with Goodyear), Glycos biotechnologies and Aemetis. Bio-based butadiene is currently under development by Genomatica and Global Bioenergies in collaboration with Lanza Tech.¹⁷¹ 

After the academic community's fixation on biodegradable products, bio-based products became more attractive. Recycling appears to be more sustainable than composting as an end-of-life solution. This twist is extremely important for future research and industry directions. Thus, low-added value applications, such as packaging, are hardly considered any longer. Biomaterials whose main attractiveness was based for years on biodegradability and did not show any particular thermo-mechanical properties (PLA) are currently under extensive research for more high-added value applications.^172–175 

The difficulty of producing tailor-made 100% bio-based biomaterials (PHA, polysaccharides etc.) led to the investigation of partially bio-based versions of already well-established petroleum-based polymers (PET) or the development of new ones (PBS, Sorona^®). It is worth noticing that even if at the current stage of knowledge these materials look like a tiny compromise between sustainability and economic efficiency, the development of genetic tools allows hope in further full bio-based versions of these materials.¹⁷⁶ 

Recent breakthroughs also concern the development of pure bio-based products identical to the petroleum-based materials (PE, PP, PA-6, PA-6,6, polyisobutylene etc.). The economic outcomes of these initiatives are clearly dependent on the global energy supply. The recent enhancement of shale gas production, including the production of so-called ‘wet gases’ such as butane, propane and ethane, will deeply influence the economic perspectives of bio-PE and bio-PP.¹⁷⁹ 

Finally, neither price nor pure ideals can be considered as valuable driving forces in the present-day ever-evolving globalized world. The intrinsic performance and special characteristics of the materials could be more valuable guidelines for the mid-term research and investments.