Interplay of the Main Chain, Chiral Side Chains, and Solvent in Conformational Transitions:  Poly{[(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene}

Light scattering, sedimentation equilibrium, viscosity, circular dichroism (CD), and UV absorption (UV) measurements were made on dilute solutions of poly{[(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene} (PRS) as functions of molecular weight. From light scattering and viscosity data, PRS is found to be a very stiff polymer of persistence length q as large as 103 nm at 25 °C, essentially a 73 helix found in the solid state; q increases only gradually with lowering temperature between −15 and 25 °C. The CD data show that PRS undergoes a conformational transition around 3 °C in isooctane (transition temperature T c). The CD signal is largely positive at low temperatures, passes through zero at T c, and becomes largely negative at higher temperatures; T c is independent of sample's chain length N. This is a highly cooperative helix (M)-to-helix (P) transition depending remarkably on N, as PRS is substantially rodlike. The CD data are converted to the fraction f P of P helix as a function of N and analyzed successfully by a statistical mechanical theory based on a helix reversal model, where a polymer chain consists of M and P helices intervened by helix reversals, with the result that the free energy difference ΔG h between P and M shows a temperature dependence similar to that of 2f P − 1, whereas the helix reversal energy is substantially constant at 1.2 × 104 J mol-1; the latter value means that the helix reversal occurs only once in 100 Si units or less. This ΔG h change and solvent dependence of T c are explained by a double-well potential for the rotation about Si−Si bonds, which incorporates into ΔG h the solvent interactions with the helical grooves of side chains surrounding the main chain. Detailed features of UV absorption spectra at different temperature and molecular weights are also presented.