Conformational fluctuations of helical poly(dialkyl silylene)s in solution

On the basis of the broken wormlike chain model, the persistence length q of helical polymers is formulated in terms of the internal rotation potential E(phi), probabilities p(P) and p(M) of the right- and left-handed helical states, the average length 1 of a helix sequence, and the kink angle thetav at the helix reversal. Here, p(P), p(M), and (l) can be calculated from the free energy difference 2DeltaG(h) between the right- and left-handed helical states and the free energy DeltaG(r) of the helix reversal; 2DeltaG(h) can be calculated from E(phi). With assumption of an empirical functional form of E(phi) along with using DeltaG(r) determined experimentally, q and 2DeltaG(h) are calculated for two optically active polysilylenes, poly [n-hexyl-(S)-3-methylpentylsilylone] (polysilylene 1) and poly[(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene] (polysilylene 2), and the results are compared with experimental data obtained in previous work to determine parameters in E(phi) and thetav. From the E(phi) determined, standard deviations ((phi - phi(0))(2))(1/2) of the torsional fluctuation in the dihedral angle are estimated for the two polysilylene chains in solution. The considerably flexible polysilylene 1 has larger theta(y) and ((phi - phi(0))(2))(1/2) than the rigid polysilylene 2. While the chain flexibility of polysilylene 2 is mainly determined by the torsional fluctuation, both kink due to the helix reversal and torsional fluctuation are important in the chain flexibility of polysilylene 1. The potential E(phi) determined is compared with a result of molecular mechanics. It is shown that the ultraviolet absorption spectrum of polysilylene correlates not to the torsional fluctuation but to the helix reversal (or (l)).