# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 8 Lipase-Catalyzed Synthesis of Sustainable Polyesters and Polyamides from Biobased Monomers
### Sweet Polyesters Derived from Carbohydrates
#### 8.5.2 Polyesters Based on Rigid Sugar Derivatives  
DAHs (isosorbide, isomannide and isoidide) and the diacetalized monomers 2,3:4,5-di-O-methylene-galactaric acid (Glux diacid) and 2,4:3,5-di-O-methylene-d-glucitol (Glux diol), are rigid compounds which are derived from sugars. They are good candidates for polyester synthesis, rendering polyesters with high values of \(T_{\mathrm{g}}\) and better thermal stability. Isosorbide was enzymatically polymerized with various aliphatic diacid ethyl esters in the presence of N435 (Scheme 48), as reported by [PERSON] et al. [292]. The solvent-free enzymatic polymerization gave low molecular weight poly(isosorbide adipate) (\(\overline{M_{\mathrm{w}}}\leq\) 3800 g/mol), as the hydroxyl groups of isosorbide can be condensed by N435. However, high molecular weight isosorbide polyesters with \(\overline{M_{\mathrm{w}}}\)'s of up to 40,000 g/mol were produced via the enzymatic polymerization by azeotropic distillation; and significantly higher molecular weight products can be obtained by decreasing the concentration of reactants in the reaction media. Meanwhile, it was found that the suitable solvents for the enzymatic polymerization were cyclohexane, cyclohexane/benzene (6:1, _v/v_) and cyclohexane/toluene (6:1, _v/v_). Furthermore, the enzymatic azeotropic polymerization in cyclohexane/toluene gave isosorbide polyesters with a higher \(\overline{M_{\mathrm{w}}}\) when the chain length of the tested aliphatic diacid ethyl esters increased from C4 to C6. However, by further increasing the chain length from C6 to C8, C10 and 12, isosorbide polyesters with lower molecular weights were produced. The same research group also investigated the N435-catalyzed azeotropic polycondensation of isosorbide/isomannide and diethyl adipate with different fractions of unsaturated diesters (Scheme 49) [293]. No homo-polyesters and _co_-polyesters containing itaconate units were obtained via the enzymatic polymerization. However, high molecular weight isosorbide/isomanide-based unsaturated _co_-polyesters were produced from the enzymatic azeotropic polymerization with the other tested unsaturated diesters, with \(\overline{M_{\mathrm{w}}}\)'s of up to 15,900 g/mol. Moreover, Michael additions of water to C=C bonds occurred during the enzymatic polymerization; and the corresponding polymers can undergo additional reactions through hydroxyl pendant groups. Besides, the enzymatic polymerization involving isosorbide gave much higher molecular weights products, which suggested that N435 prefers isosorbide over isomannide. This is however in contract with the result reported by [PERSON] et al. [294]. They investigated the N435-catalyzed polymerization of succinic acid with isomannide, isosorbide or isoidide in toluene/_tert_-butanol; and found that N435 showed preference for isomannide over isosorbide and over isoidide. They attributed this to the preference of N435 for the _endo_-hydroxyl groups, which is due to the fact that the transition state of esters with _exo_-hydroxyl groups does not form all the required hydrogen bonds for catalysis. A series of Glux diacid- and Glux diol-based polyesters were synthesized via the N435-catalyzed polycondensation of diethyl sebacate and 1,4-BDO with Glux diacid/Glux diol (Scheme 50) [295]. The \(\overline{M_{w}}\), reaction yield and intrinsic viscosity of the resulting polyesters were around 10,000 g/mol, 30%-70%, and 0.3-0.44 dL/g, respectively. In addition, the molecular weights and reaction yields of the obtained Glux diacid-based polyesters were lower than those of the synthetic Glux diol-based polyesters; and these two parameters decreased monotonically with increasing the Glux content in both Glux diacid- and Glux diol-based polyesters. Moreover, no polyester was obtained from the enzymatic polymerization of Glux diacid and 1,4-BDO. This can be explained by the bulky bicyclic structure next to the carboxylate groups, which hinders the access of this group to the active site of the enzyme CALB.