Cooperative preferential helical ordering in poly(alkylarylsilylene) copolymers

We prepared two series of poly[((S)-2-methylbutyl(phenyl)silylene)(x)-co-(methylphenylsilylene)(1-x)] (0 less than or equal to x less than or equal to 0.5) (1) with coiled conformations and poly [((S)-2-methylbutyl(phenyl)-silylene)(x)-co-(n-hexyl(m-tolyl)silylene)(1-x)] (0 less than or equal to x less than or equal to 0.5) (2) with stiffer conformations; however, we could not produce the poly[((S)-2-methylbutyl(phenyl)silylene)] homopolymer due to steric overcrowding. We conclude that even optically inactive 1 with x = 0 and 2 with x = 0 adopt helical conformations with an equal quantity of P-and M-screw-senses by means of UV, circular dichroism (CD), fluorescence (FL) spectra, FL anisotropy, and molecular mechanics calculations. We found that the preferential screw-sense is subject to a noticeable cooperative induction effect in both copolymer systems as the chiral silylene moiety increases. This is based on a quantative analysis of the dissymmetry ratio (g = Delta epsilon/epsilon) as a function of chiral molar composition. However, the helical cooperative effects are markedly different between 1 and 2, which is believed to arise from their differences regarding chain stiffness, global conformation, and persistence length.