Interplay of the Main Chain, Chiral Side Chains, and Solvent in Conformational Transitions: Poly{[(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene} Akio Teramoto,* ,², | Ken Terao, ², |,⊥ Yoshimi Terao, ², | Naotake Nakamura, ², | Takahiro Sato, ‡,| and Michiya Fujiki §,| Contribution from the Research Organization of Science and Engineering and Faculty of Science and Engineering, Ritsumeikan UniVersity, 1-1-1 Nojihigashi, Kusatsu 525-8577, Japan, Department of Macromolecular Science, Osaka UniVersity, 1-1 Machikaneyama-cho, Toyonaka 560-0043, Japan, NTT Basic Research Laboratories, 3-1 Wakamiya, Morinosato, Atsugi 243-0198, Japan, and CREST-JST (Japan Science and Technology Corporation), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed June 25, 2001 Abstract: 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 7 3 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 ∆G 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 × 10 4 J mol -1 ; the latter value means that the helix reversal occurs only once in 100 Si units or less. This ∆G 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 ∆G 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. Introduction There are a number of linear polymers capable of forming helical conformations and undergoing a thermal and/or solvent- induced transition from one conformation to another, always containing one helical conformation, for example, helix to random coil. Indeed, the helix is one of the most important conformations common to biopolymers and synthetic poly- mers. 1,2 Examples of such polymers are polypeptides, 3-5 polyisocyanates, 6-8 polysilylenes, 9-12 polyacetylenes, 13 and so forth. This transition is unique for its molecular-weight depen- dence, and theoretically, all these polymers are regarded as linear cooperative systems whose molecular-weight dependent con- formations are formulated on the basis of a linear Ising model. 1,2 On the other hand, different polymers and solvents show different transition curves, and chemistry plays a crucial role. 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