The Mechanism of Enzyme-Catalyzed ROP

The mechanism of enzyme-catalyzed ROP was first proposed by Uyama et al. [54]. Here, the catalyhc site of lipase is the serine residue, which forms a complex with the lactone, leading to the formation of an enzyme-monomer complex (EMC). The rate-determining step in the overall polymerization was believed to be the formation of EMC (also known as an enzyme-activated monomer ). 

In the propagation step, the nucleophilic attack of the terminal hydroxyl group of the co-hydroxyl carboxylic acid on EMC leads to the formation of a one unit more-elongated polymer chain. 

In contrast to this proposal, MacDonald et al. [64] su ested that the chain propagation of polymerization was a slow step, while Gross et al. [65] found that the monomer conversion followed first- order law and was independent of both the type (water, butanol and butyl amine) and concentration of the nucleophile. Yet another group suggested a complex mechanism of polymerization involving ring-opening and linear condensation polymerization [66]. 

The main feature of enzyme catalysis involves the formation of an acyl-enzyme intermediate as a key step, although whether this step or the subsequent propagation step is rate-determining remains the subject of controversy. Recently, Kobayashi [17] suggested that the acylation step (formation of the acyl-enzyme intermediate) and/or a nucleophilic attack of the propagating alcohol end to the 

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