Enzymatic ring-opening DOI: Polymerization of lactones by lipase catalyst: Mechanistic aspects

Mechanistic aspects of lipase-catalyzed ring-opening polymerization (ROP) of lactones to give polyesters are discussed from accumulated experimental data and new insight. Comparison of the ROP reactivity by lipase catalyst with the anionic ROP reactivity by a metal-catalyst clearly demonstrates the characteristics of lipase catalysis; the larger ring-sized monomers with lower ring strain showed higher polymerizability than medium ring-sized ones, in contrast to the anionic ROP showing the reverse direction where the ring strain of monomer is operative. The enzyme-catalysis involves an acyl-enzyme intermediate formation as a key-step. From the copolymerization results a new mechanism is proposed, that involves the formation of the acyl-enzyme intermediate (acylation step) and/or the nucleophilic attack of the propagationg alcohol end to the carbonyl carbon of the intermediate to open the monomer ring (deacylation step) as the rate-determining step. The structure of the propagating alcohol end (primary or secondary) affects much on which step is more operative.