Conformational Energies and Unperturbed Chain Dimensions of Poly(phenylmethylsilylene) and Poly(silastyrene)

Molecular mechanics techniques were employed to calculate the molecular structure and conformational energies of model compounds for poly(phenylmethylsilylene) [-SiPhMe-] and poly(silastyrene) [-SiPhH-SiH₂-], in both isotactic and syndiotactic stereochemical forms. The structural and conformational energy data provided were used to calculate, by application of rotational isomeric state (RIS) theory, the unperturbed chain dimensions, given as the characteristic ratio C_n- <r²)₀/nl², and its temperature coefficient d(ln C_n)/dT_K. Both iso and syn [-SiPhMe-] overwhelmingly prefer the trans, planar zigzag, conformations, at the exclusion of the corresponding gauche states. Corresponding C_n=400 values are 274 and 240 for iso and syn [-SiPhMe-], respectively, the large values being indicative of an essentially fully extended, rodlike chain. For [-SiPhH-SiH₂-], the iso form strongly prefers trans states over gauche states while the syn form conversely prefers gauche states over trans states. In both cases, the chains favor that conformational state providing maximal attractive overlap between the substituent phenyl groups. Values of C_n=400 are 19 and 79 for the iso and syn [-SiPhH-SiH₂-], respectively. Calculated C_n values qualitatively agree with experimental values published for related polymers.