Controlled Lipase-Catalyzed Synthesis of Poly(hexamethylene carbonate)

Abstract

High molecular weight (Mw > 25 000) poly(hexamethylene carbonate) (PHC) with polydispersity (Mw/Mn) ≤ 2.2 was successfully synthesized via copolymerization of diethyl carbonate with 1,6-hexanediol catalyzed by immobilized Candida antarctica Lipase B (CALB). Because diethyl carbonate is highly volatile, polymerizations were performed by a first stage oligomerization at low vacuum (600 mmHg pressure) followed by a second stage polymerization under high vacuum (1−5 mmHg pressure). Enzymatic polycarbonate synthesis is preferably performed in solution (e.g., in diphenyl ether), although it also proceeds in solventless reactions albeit at reduced rates. Synthesized PHC contains hydroxyl and ethyl carbonate terminal groups. Influence of regulating the ratio of diethyl carbonate to 1,6-hexanediol in the monomer feed on polymer end-group structure was determined. Reaction conditions and monomer feed ratios resulting in PHC with exclusively hydroxyl or ethyl carbonate termini were established. The ability to synthesize PHC products with desired end-group structure is critical to their potential use as macromers in, for example, polyurethane synthesis. Mechanistic features of enzymatic polycarbonate synthesis were elucidated, which explain why high molecular weight PHC can be prepared at high (e.g., 4:1) diethyl carbonate to diol monomer feed ratios whereas conventional chemically catalyzed step-polycondensation reactions of AA−BB type monomers require a 1:1 monomer feed ratio. That is, enzyme-catalyzed polycondensations between dialkyl carbonate and diol proceed via two pathways: (i) reaction between hydroxyl and carbonate end groups with elimination of alcohol and (ii) transesterification between two carbonate end groups with elimination of dialkyl carbonate.