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 ≤ x ≤ 0.5) ( 1 ) with coiled conformations and poly[(( S )-2-methylbutyl(phenyl)-silylene) x - co -( n -hexyl( m -tolyl)silylene) 1 - x ] (0 ≤ x ≤ 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 = Δε/ε) 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.