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Enzyme-Catalyzed Polymerization Mechanism of Aliphatic Polyester Investigated by 1H NMR and MALDI-TOF Mass Spectrum

Enzyme-catalyzed polymerization mechanism of adipic acid and hexanediol was investigated by proton nuclear magnetic resonance (1H NMR) and MALDI-TOF mass spectra. When the stoichiometric substrates were used for the reaction, 1H NMR measurements showed that the hydroxyl terminated product was preferentially formed at the early stage, then with reaction time it changed to carboxyl terminated product. This process was also confirmed by MALDI-TOF mass spectra. First the chain propagation follows the step-growth mechanism accompanying the sequential addition of a diacid-diol unit. In this case, an enzyme-substrate complex is attacked by hydroxyl group of substrates, and as a result the hydroxyl terminated product was preferentially formed. And at the later stage of the reaction, the transesterification plays an important role to form the carboxyl terminated product. When an excess diacid was used, the end group structure showed the same behavior as the stoichiometric condition, i.e., the predominant end group is consisted of the hydroxyl group at the initial stage then it rapidly changed to the carboxyl group although the time to change to carboxyl terminated product became shorter. In case of the condition of an excess diol, the reaction was avoided at the early stage because no enzyme forms a complex with diacid.