Lipase-Catalyzed Synthesis of Aliphatic Poly(carbonate-co-esters)

Candida antarctica Lipase B (CALB) was found to catalyze condensation reactions of diethyl carbonate (DEC), diester, and diol to form aliphatic poly(carbonate- co -esters). Poly(butylene carbonate- co -butylene succinate), poly(BC- co -BS), and poly(hexamethylene carbonate- co -hexamethylene adipate), poly(HC- co -HA), with about 1:1 mol/mol ester-to-carbonate repeat units, were prepared by maintaining the diol/diester monomer feed ratio (mol/mol) at 2 to 1. By varying the [DEC] to [1,4-butanediol minus diethyl succinate] ratio from 1:1 to 4:1, poly(BC- co -BS) with terminal groups of up to 96 mol % hydroxyl and 90 mol % ethyl carbonate plus ethyl ester, respectively, were prepared. Weight-average molecular weights ( M w ) of poly(BC- co -BS) and poly(HC- co -HA), without product fractionation, reached 26 000 and 59 000 at 80 and 90 °C, respectively. Low polydispersities of poly(HC- co -HA) ( M w / M n ∼ 1.5) indicate that the polymerization proceeds with chain selectivity . NMR analysis revealed poly(BC- co -BS) copolymers contain carbonate and ester repeat units randomly distributed along polymer chains. CALB-catalyzed reactions between DEC and propyl propionate (PP) gave ethyl propionate (EP), ethyl propyl carbonate (EPC), and dipropyl carbonate (DPC). A reaction pathway to form these products was proposed to involve three equilibrium reactions for which equilibrium constants were determined. Furthermore, CALB-catalyzed carbonate−ester transacylation reactions at 95 °C, for 3 and ≥21 h, between poly(butylene carbonate) (PBC, M w = 13 800) and poly(butylene succinate) (PBS, M w = 23 400), gave block copolymers with an average segment length of 2.8 repeat units and random sequences, respectively.