Synthesis and self-assembly of a novel Y-shaped copolymer with a helical polypeptide arm

A novel biodegradable Y-shaped copolymer, poly(l-lactide)2-b-poly(γ-benzyl-l-glutamic acid) (PLLA2-b-PBLG), was synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG–NCA) with centrally amino-functionalized poly(l-lactide), PLLA2-NH2, as a macroinitiator in a convenient way. The Y-shaped copolymer and its precursors were characterized by 1H NMR, FT-IR, GPC, WAXD and DSC measurements. The self-assembly of the PLLA2-b-PBLG copolymer in toluene and benzyl alcohol was examined. It was found that the self-assembly of the copolymer was dependent on solvent and on relative length of the PBLG block. For a copolymer with PLLA blocks of 26 in total degree of polymerization (DP), if the PBLG block was long enough (e.g., DP = 54 or more), the copolymer/toluene solution became a transparent gel at room temperature. In benzyl alcohol solution, only PLLA2-b-PBLG containing ca. 190 BLG residues could form a gel; those with shorter PBLG blocks (e.g., DP = 54) became nano-scale fibrous aggregates and these aggregates were dispersed in benzyl alcohol homogeneously. Copolymers with short PBLG blocks behaved like a pure PLLA both in toluene and in benzyl alcohol. These experimental results were discussed and explained by virtue of the helical conformation of PBLG and the interactions between the solvents and the PLLA and/or PBLG segments.