NAM-TMS Mechanism of α-Amino Acid N-Carboxyanhydride Polymerization: A DFT Study

The normal amine mechanism via the proton-transfer route (NAM-H) is widely accepted for the synthesis of polypeptides with nonionic initiators. Besides proton transfer, the trimethylsilyl (TMS) group transfer process has been found in living/controlled polymerization initiated by N-TMS amine in experiments, but the corresponding mechanism has never been proposed. In this work, we employed density functional theory (DFT) with the solvation model to investigate the details of the TMS-transfer mechanism, defined as NAM-TMS, for the ring-opening polymerization of α-amino acid N-carboxyanhydride. The TMS transfer process of NAM-TMS is thermodynamically more favored than the NAM-H mechanism according to the lower addition energy barrier observed. The rate-determining step (RDS) in NAM-TMS is the decarboxylation step, i.e., the release of CO₂, rather than carbonyl addition in NAM-H because of the low dipole stable precursor enlarged energy gap of decarboxylation. It is the first calculation evidence supporting decarboxylation as RDS in the NAM mechanism.