# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 8 Lipase-Catalyzed Synthesis of Sustainable Polyesters and Polyamides from Biobased Monomers
### Sweet Polyesters Derived from Carbohydrates  
#### 8.5.1 Sugar and Sugar Alcohol-Based Polyesters  
Sugars and sugar alcohols can be used as starting materials for the production of biobased linear and branched functional polyesters which have various potential applications, for example, they can be used as coating materials, biodegradable and bioresorbable polymers, and optically active polymers [112].  
It is difficult to synthesize polyesters with sugar and sugar alcohol units via conventional techniques, as tedious protection and de-protection steps are normally required, to prevent the gelation during the polymerization. However, these multiple-functional monomers can be directly polymerized via enzymatic polymerization, due to the highly regio-selectivity of the enzymes.  
The lipase-catalyzed polymerization with D-sorbitol was first reported by [PERSON] et al. (Scheme 45) [288]. Sorbitol-based polyesters with \(\overline{M_{\mathrm{n}}}\)'s of around 3000-12,000 g/mol were produced in moderate yields (around 40%-85%). Moreover, \({}^{1}\)H- and \({}^{13}\)C-NMR study indicated that the primary hydroxyl groups of D-sorbitol were exclusively esterified during the enzymatic polymerization. However, to compensate the low catalytic reactivity of N435 in the polar aprotic solvent acetonitrile, activated divinyl sebacate and high concentration of N435 (76 wt %) were used.  
In addition, divinyl sebacate and other activated monomers including bis(2,2,2-trifluoroethyl)malonate, bis(2,2,2-trifluoroethyl)glutamate and divinyl adipate, were enzymatically polymerized with disaccharides (sucrose, trehalose, and lactose), D-sorbitol and D-mannitol in acetonitrile. [289] It was found that N435 was able to differentiate the five tested carbohydrates, as the molecular weights of the resulting polyesters showed the following array: disaccharide-based polyesters < mannitol-based polyesters < sorbitol-based polyesters.  
Unactivated diacid monomers were also used as starting materials for the enzymatic synthesis of sorbitol-based polyesters. [PERSON] et al. [280, 281] reported the N435-catalyzed polycondensationof D-sorbitol, 1,8-octanediol and adipic acid in bulk (Scheme 46). Poly(sorbityl adipate) and poly(octamethylene adipate-_co_-sorbityl adipate)s were produced, with \(\overline{M_{\mathrm{n}}}\)'s and dispersities of around 7000-20,300 g/mol, and 1.6-3.3, respectively. In addition, the molar percentage of sorbityl units in the _co_-polyesters can be tunable from 0% to 100%. Moreover, the NMR analysis revealed that N435 showed highly regio-selectivity (\(\geq\)85% \(\pm\) 5%) towards the primary hydroxyl groups of d-sorbitol at 1- and 6-positions [280].  
In the same laboratory, the enzymatic polycondensation of adipic acid and 1,8-octanediol with different sugar alcohols was investigated (Scheme 46) [290]. Sweet _co_-polyesters with \(\overline{M_{\mathrm{w}}}\)'s ranging from 11,000 and 73,000 g/mol were obtained; and no correlation was observed between the reactivity of the tested sugar diols and their chain length. However, N435 showed the highest reactivity towards d-mannitol; and the _co_-polyester containing d-mannitol units possessed the highest degree of branching.  
Recently, sorbitol-based, hydroxy-functional polyesters were successfully synthesized via the N435-catalyzed _co_-polymerization (Scheme 47) [291]. The \(\overline{M_{\mathrm{n}}}\) of the resulting _co_-polyesters was successfully controlled at around 4000-8000 g/mol, by tuning the reaction time, enzyme concentration and reaction stoichiometry. However, only maximum 53% of the added d-sorbitol was incorporated into the final products, even though its feed ratio was quite low (\(\leq\)5 mol %). Besides, the obtained polyesters displayed suitable properties for being used as solvent-borne coating resins.  
**Scheme 47.** N435-catalyzed _co_-polymerization of d-sorbitol, alkane-\(\alpha\),\(\omega\)-aliphatic linear diols and diacids or diesters in bulk.