Polymerization of α-amino acid N-carboxyanhydrides catalyzed by rare earth tris(borohydride) complexes: Mechanism and hydroxy-endcapped polypeptides

Abstract

In this work, rare earth tris(borohydride) complexes, Ln(BH₄)₃(THF)₃ (Ln = Sc, Y, La, and Dy), have been used to catalyze the ring-opening polymerization of γ-benzyl-L-glutamate N-carboxyanhydride (BLG NCA). All the catalysts show high activities and the resulting poly(γ-benzyl-L-glutamate)s (PBLGs) are recovered with high yields (≥90%). The molecular weights (MWs) of PBLG can be controlled by the molar ratios of monomer to catalyst, and the MW distributions (MWDs) are relatively narrow (as low as 1.16) depending on the rare earth metals and reaction temperatures. Block copolypeptides can be easily synthesized by the sequential addition of two monomers. The obtained P(γ-benzyl-L-glutamate-b-ε-carbobenzoxy-L-lysine) [P(BLG-b-BLL)] and P(γ-benzyl-L-glutamate-b-alanine) [P(BLG-b-ALA)] have been well characterized by NMR, gel permeation chromatography, and differential scanning calorimetry measurements. A random copolymer P(BLG-co-BLL) with a narrow MWD of 1.07 has also been synthesized. The polymerization mechanisms have been investigated in detail. The results show that both nucleophilic attack at the 5-CO of NCA and deprotonation of 3-NH of NCA in the initiation process take place simultaneously, resulting in two active centers, that is, an yttrium ALA carbamate derivative [H₂BOCH₂(CH)NHC(O)OLn] and a N-yttriumlated ALA NCA. Propagation then proceeds on these centers via both normal monomer insertion and polycondensation. After termination, two kinds of telechelic polypeptide chains, that is, α-hydroxyl-ω-aminotelechelic chains and α-carboxylic-ω-aminotelechelic ones, are formed as characterized by MALDI-TOF MS, ¹H NMR, ¹³C NMR, ¹H–¹H COSY, and ¹H–¹³C HMQC measurements. By decreasing the reaction temperature, the normal monomer insertion pathway can be exclusively selected, forming an unprecedented α-hydroxyl-ω-aminotelechelic polypeptide. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012