Lipase-catalyzed synthesis of aliphatic poly(carbonate-co-esters)

Condida 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 suceinate] 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,) of poly(BC-co-BS) and poly(HC-co-HA), without product fractionation, reached 26 000 and 59 000 at 80 and 90 degrees C, respectively. Low polydispersities of poly(HC-co-HA) (M-w/M-n similar to 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 degrees 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.