Equilibrium ring concentrations and the statistical conformations of polymer chains: Part 5. Stereoisomeric cyclics in poly (phenylmethylsiloxane) equilibrates

Abstract The molar cyclization equilibrium constants K x for cyclic phenylmethylsiloxanes, [C 6 H 5 (CH 3 )SiO] x ( x = 3–50), have been measured in the undiluted polymer at different temperatures and in solution in toluene. The K x values for [C 6 H 5 (CH 3 )SiO] x in the bulk polymer at 383 K are the same, within experimental error, as those found previously for the corresponding cyclics [CH 3 CH 2 (CH 3 )SiO] x and it is concluded that phenyl and ethyl groups have similar influences on the statistical conformations of siloxane chains. The weight fraction of cyclics in a high molecular weight undiluted phenylmethylsiloxane equilibrate at 383 K is 0.30 and this weight fraction increases to 0.90 when equilibration is carried out in the presence of 47% by volume of toluene. The K x values for the cyclic tetramer and pentamer in the latter equilibrate are nearly twice as large as their values in the former, whereas the K x values for the large cyclic phenylmethylsiloxanes are the same in both. The configurational isomers of the cyclic trimer, tetramer and pentamer were separated completely by gas-liquid chromatography. They were found to be present in the equilibrates in proportions close to those expected from the random cyclization of an atactic polymer. The configurational isomers of [C 6 H 5 (CH 3 )SiO] 3 and the isomer of [C 6 H 5 (CH 3 )SiO] 4 with all its phenyl groups in cis positions were the only stereoisomers that were found to be strained relative to the open siloxane chains.