Selective Interaction between Helical Polymer Backbones and Chiral Graft Polymer Chains in Graft Copolymers

The helical polymers, comprising (R)-(-)- or (S)-(+)-2-methylpiperazinium-2,4-pentadienylideneammonium 7,7 ',8,8 '-tetracyanoquinodimethane anion radical, namely, P(R;TCNQ(-center dot)) and P(S;TCNQ(-center dot)), were employed for the synthesis of copolymers grafted with chiral polymer chains. The AIBN-initiated polymerization of N-acryloyl-d-alanine methyl ester (d-AME) and N-acryloyl-l-alanine methyl ester (l-AME), followed by the treatment with P(R;TCNQ(-center dot)) and P(S;TCNQ(-center dot)), resulted in the formation of the graft copolymers with polyolefin graft chains, namely, P(R;D), P(R;L), P(S;D), and P(S;L), through radical coupling between the terminal radical of the growing polyolefin and TCNQ(-center dot). The circular dichroism (CD) measurements revealed that the helical conformation of the polymer backbone remained in P(R;D) and P(S;L) but decreased in P(R;L) and P(S;D). Density functional theory (DFT) calculations showed that the conformational fitting between the polymer backbone and the graft polymer chain in graft copolymers played an important role in preserving the helical polymer backbone structure.