A Model Chiral Graft Copolymer Demonstrates Evidence of the Transmission of Stereochemical Information from the Side Chain to the Main Chain on a Nanometer Scale

A model chiral graft copolymer, poly(phenylacetylene)-g-poly(n-hexyl isocyanate) (PPA-g-PHIC), in which a chiral moiety is located at the end of each PHIC side chain, was synthesized. First, chiral PHIC macromonomers with a phenylacetylene end group were synthesized via living anionic polymerization using the functional initiator sodium N-(4-ethynylphenyl)benzamide (Na-4EPBA) and then end-capped using the chiral terminator (S)-2-acetoxypropionyl chloride ((S)-Ct). The molecular weights (MWs) of the PHIC macromonomers were controlled based on the feed ratio of the monomer to the initiator. Subsequent polymerization of PHIC macromonomers using Rh+(2,5-norbornadiene)[(eta(6)-C6H5)-B-(C6H5)(3)] (Rh(nbd)BPh4) catalyst generated chiral PPA-g-PHIC graft copolymers with varying graft strand lengths. Chiral macromonomers and graft copolymers were characterized by SEC-MALLS, NMR, and CD spectroscopy. This model chiral graft copolymer provided an excellent example of the transmission of stereochemical information from the side chain to the main chain, as a preferred helicity was induced in the PPA backbone of the graft copolymer even when chiral moieties were separated from the main chain by nanometer scale distances (5.4-13 nm). Furthermore, CD spectroscopy clearly showed that the CD intensity of the PPA main chain was directly dependent on the CD intensity of the optically active PHIC side chain determined by the strand length.