Conformational analysis of methyl-phenyl-siloxane chains

The conformational energy of chain fragments of poly(methylphenylsiloxane) is studied, considering contributions from bond stretching, bond angle bending, bond torsion, and van der Waals interactions. Energy is minimized without constraints, so that minima correspond to fully relaxed conformations. The energy minima of this phenyl-substituted polymer differ notably from those of the parent poly(dimethylsiloxane), due to the importance of the attractive interactions between pairs of phenyl groups. For two repeat units (diad), the stable conformations occur when the pair of phenyl rings is coupled parallel, face-to-face. For three repeat units (triad), the stable conformations occur also when a pair of phenyl rings is coupled parallel, face-to-face, but not necessarily those in consecutive units. Instead, the chain can coil to couple together the phenyl rings of alternate units, leaving uncoupled the phenyl ring in between. This coiling of the chain to yield a stable conformation is a genuine triad effect that cannot be predicted from the diad energies. In longer chain sequences, the contribution of this coiled triad conformation is significant.