Ring opening polymerization of α‐amino acid N‐carboxyanhydrides catalyzed by rare earth catalysts: Polymerization characteristics and mechanism

Five rare earth complexes are first introduced to catalyze ring opening polymerizations (ROPs) of γ‐benzyl‐ L ‐glutamate N ‐carboxyanhydride (BLG NCA) and L ‐alanine NCA (ALA NCA) including rare earth isopropoxide (RE(OiPr) 3 ), rare earth tris(2,6‐di‐ tert ‐butyl‐4‐methylphenolate) (RE(OAr) 3 ), rare earth tris(borohydride) (RE(BH 4 ) 3 (THF) 3 ), rare earth tris[bis(trimethylsilyl)amide] (RE(NTMS) 3 ), and rare earth trifluoromethanesulfonate. The first four catalysts exhibit high activities in ROPs producing polypeptides with quantitative yields (>90%) and moderate molecular weight (MW) distributions ranging from 1.2 to 1.6. In RE(BH 4 ) 3 (THF) 3 and RE(NTMS) 3 catalytic systems, MWs of the produced polypeptides can be controlled by feeding ratios of monomer to catalyst, which is in contrast to the systems of RE(OiPr) 3 and RE(OAr) 3 with little controllability over the MWs. End groups of the polypeptides are analyzed by MALDI‐TOF MS and polymerization mechanisms are proposed accordingly. With ligands of significant steric hindrance in RE(OiPr) 3 and RE(OAr) 3 , deprotonation of 3‐NH of NCA is the only initiation mode producing a N ‐rare earth metallated NCA ( i ) responsible for further chain growth, resulting in α‐carboxylic‐ω‐aminotelechelic polypeptides after termination. In the case of RE(BH 4 ) 3 (THF) 3 with small ligands, another initiation mode at 5‐CO position of NCA takes place simultaneously, resulting in α‐hydroxyl‐ω‐aminotelechelic polypeptides. In RE(NTMS) 3 system, the protonated ligand hexamethyldisilazane (HMDS) initiates the polymerization and produces α‐amide‐ω‐aminotelechelic polypeptides.