Synthesis and Conformational Study of Optically Active Poly(phenylacetylene) Derivatives Bearing a Bulky Substituent

Abstract

Three optically active phenylacetylenes having a chiral carbamoyloxy group at the para or meta position were synthesized, polymerized, and copolymerized with various optically inactive comonomers with transition metal catalysts such as [RhCl(NBD)] 2 (NBD : norbornadiene) in THF in order to explore the role of the bulkiness of side groups on the phenyl moieties during the formation of a helical conformation. [RhCl(NBD)] 2 produced high molecular weight polymers and copolymers in THF. The yields and molecular weight of the obtained copolymers were influenced by the bulkiness of the comonomers. All polymers showed broad electronic absorptions up to ca. 570 nm, and the 1 H NMR spectra indicated that the polymers had a cis-transoidal structure. The chiral homopolymers and copolymers showed an intense induced CD in the UV-visible region depending on the position of the substituents of the chiral residues and the bulkiness of the achiral comonomers. The polymers having chiral carbamoyloxy groups at the para position on the phenyl groups showed very intense induced CD bands in the UV-visible region with negative and positive Cotton effects. The main chain of the polymers seems to be chiral, probably based on a predominant one-handed helical sense. The magnitude of the CD of the copolymers increased with an increase in the bulkiness of the substituents of the comonomers. The polymers with a chiral substituent at the meta position showed a very weak induced CD in the UV-visible region. However, a copolymer with the phenylacetylene having a bulky tert-butyldiphenylsiloxy group at the para position exhibited a very intense induced CD which was almost a mirror image to those of the chiral homopolymers in the 300-450 nm wavelength range. These results suggest that the copolymer may possess a reversed helical structure. The sign and specific rotation at 780 nm of the para-substituted polymers and copolymers were also influenced by the bulkiness of the substituents of the comonomers ; the specific rotation of the copolymer of 1 or 2 with the phenylacetylene having bulky tert-butyldiphenylsiloxy groups at the para position showed a specific rotation about 2 times larger than that of the homopolymers. The steric effects of bulky side groups on helical conformation was discussed on the basis of molecular mechanics and molecular dynamics calculations of model polymers.