Biobased Polyol-Polyesters via Enzyme-Catalysis

By using mixtures of natural polyols and other bio-derived building blocks, polar polyols are partially or completely solubilized resulting in highly reactive condensation polymerizations. Furthermore, organic solvents and activation of diacids are not needed. Polymerizations are performed at temperatures between 60 and 95 °C, in-vacuo (to remove water), and give products of high molecular weight with narrow polydispersities. The method offers simplicity, mild reaction conditions, and the ability to incorporate a wide range of renewable polyols into polyesters without protection-deprotection steps. 

In one example, CAL-B-catalyzed bulk polymerization of sorbitol and adipic acid proceeded with high regioselectivity (85%) at primary hydroxyl groups to give a water-soluble product with Mn 10 880 and Mw/Mn of 1.6 (Kumar et al., 2003). Also, our laboratory reported the synthesis of hyperbranched glycerol copolyesters by heating ternary monophasic mixtures of glycerol (B¢B2) with adipic acid (A2) and 1,8-octanediol (B2) monomers.35a Variation of reaction time and glycerol in the monomer feed gave copolymers with degrees of branching varying from 0% to 58% (Kulshrestha et al., 2005). An important finding in studies with glycerol copolymerizations is that the kinetic products of polymerizations are linear structures due to enzyme regioselectivity.

Also, Novozyme 435 catalyzed condensation copolymerization of adipic acid, 1,8-octanediol and sorbitol, using a monomer feed ratio of 50 to 35 to 15 (mol/mol), respectively, at 90 °C for 42 h gave a fully organosoluble sorbitol copolyester with Mw and Mw/Mn of 117 000 and 3.4, respectively. Such polymerizations carried out with organometallic catalysts gel at much lower molecular weights due to crosslinking reactions that are largely avoided due to enzyme regioselectivity. 

In another study, trimethylolpropane (TMP) was found to be a suitable polyol monomer for introduction of branch points along chains during Novozyme-435-catalyzed bulk polycondensations (70 °C, 42 h) (Kulshrestha et al, 2007). Variation of TMP in the monomer feed gave copolymers with degree of branching (DB) from 20 to 67%. In general, molecular weight decreased with increased TMP in the monomer feed. This trend may be explained by: (i) Increased branching results in more compact and dense structures that decrease chain end binding at the active site, and (ii) formation of trisubstituted TMP units results in a deviation from 1:1 in reactive carboxyl and hydroxyl groups in the condensation polymerization.