SYNTHESIS AND CONFORMATIONAL STUDY OF OPTICALLY-ACTIVE POLY(PHENYLACETYLENE) DERIVATIVES BEARING A BULKY SUBSTITUENT

Three optically active phenylacetylenes having a chiral carbamoyloxy g roup at the para or meta position were synthesized, polymerized, and c opolymerized with various optically inactive comonomers with transitio n metal catalysts such as [RhCl(NBD)](2) (NBD: norbornadiene) in THF i n order to explore the role of the bulkiness of side groups on the phe nyl moieties during the formation of a helical conformation. [RhCl(NBD )](2) produced high molecular weight polymers and copolymers in THF. T he yields and molecular weight of the obtained copolymers were influen ced by the bulkiness of the comonomers. AU polymers showed broad elect ronic absorptions up to ca. 570 nm, and the H-1 MMR spectra indicated that the polymers had a cis-transoidal structure. The chiral homopolym ers and copolymers showed an intense induced CD in the UV-visible regi on depending on the position of the substituents of the chiral residue s and the bulkiness of the achiral comonomers. The polymers having chi ral carbamoyloxy groups at the para position on the phenyl groups show ed very intense induced CD bands in the UV-visible region with negativ e 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 i n the bulkiness of the substituents of the comonomers. The polymers wi th a chiral substituent at the meta position showed a very weak induce d CD in the UV-visible region. However, a copolymer with the phenylace tylene having a bulky tert-butyldiphenylsiloxy group at the para posit ion exhibited a very intense induced CD which was almost a mirror imag e to those of the chiral homopolymers in the 300-450 nm wavelength ran ge. These results suggest that the copolymer may possess a reversed he lical structure. The sign and specific rotation at 780 nm of the para- substituted polymers and copolymers were also influenced by the bulkin ess of the substituents of the comonomers; the specific rotation of th e copolymer of 1 or 2 with the phenylacetylene having bulky tert-butyl diphenylsiloxy 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 bas is of molecular mechanics and molecular dynamics calculations of model polymers.