Asymmetric polymerizations of chiral 4‐benzyl‐2‐ethynyloxazoline with rhodium catalyst and chiroptical properties of the polymers

Abstract

Optically active 4-alkyl-2-ethynyloxazoline derivatives (BnEOx) were polymerized with rhodium catalysts. The polymerization in toluene produced polymer with the highest absolute values of specific rotation ([α](D) = -77.3°). The yields, molecular weights, and specific rotations of poly(BnEOx)s were influenced by polymerization conditions. The copolymerization with phenylacetylene (PA) was effective to increase the molecular weight of the copolymer. It is interesting to note that the copolymers exhibited positive specific rotations ([α](D) = +4.7° to +62.5°) despite the fact that [α](D) s of BnEOx and the homopolymer are negative sign. The chiroptical properties were investigated by the chiral/achiral copolymerization of BnEOx with PA. The copolymerizations of BnEOx with PA gave copolymers containing higher order structure such as one-handed helical conformation. Furthermore, induced Cotton effects were observed in the π-π* transition region of conjugated main chain depending a complex of these polymers with zinc triflate salt in tetrahydrofuran solution, indicating the formation of chiral supramolecular aggregates.