Poly(oleic diacid-co-glycerol): Comparison of Polymer Structure Resulting from Chemical and Lipase Catalysis

This study compares the synthesis and structure of poly(oleic diacid-co-glycerol) that results by using immobilized Candida antarctica Lipase B (Novozym 435, N435) and dibutyl tin oxide (DBTO) as catalysts. By using N435 catalysis and an oleic diacid to glycerol molar ratio of 1.0:1.0, the resulting polyester number-average molecular weights (M_n) were 6000 g/mol at 6 h and 9100 g/mol at 24 h with low branching degree (Den% of glycerol 13%−16%). ¹³C NMR spectra of these polyesters revealed their chain-ends consist exclusively of monosubstituted glycerol units. Further diversification in polymer structure was achieved by using N435 catalysis and by changing the feed ratio of oleic diacid to glycerol from 1.0:1.0 to 1.5:1.0 in 0.1 increments. Resulting polyesters were not cross-linked (no observed gel fraction), had similar M_n values (generally between 4800 and 6000 g/mol), but differed in dendritic unit content, glycerol unit degree of substitution, and end-group structure (monosubstituted glyercol versus carboxyl end-groups). In contrast, by using DBTO as catalyst and an oleic diacid to glycerol molar ratio of 1.0:1.0, polyester M_n of 1700 g/mol was obtained at 6 h and, thereafter, a gel was formed due to cross-linking. As a consequence of N435’s ability to deter cross-link reactions owing to steric hindrance at the active site, a family of unique, soluble, hyperbranched copolyesters was formed.