CONFORMATIONAL-ANALYSIS OF CYCLIC POLY(METHYLPHENYLSILOXANE) - EXCIMER-FORMING SITES AND SIDE-GROUP ROTATION

A Monte Carlo simulation method (MC) that uses the rotational isomeric state model (RIS), modified to consider fluctuating rotational angles and excluded volume interactions, is applied to cyclic poly(methylphenylsiloxane) (c-PMPS) with 50% meso diads, to determine the percentage of conformers that are excimer-forming sites (EFS) in the average equilibrium conformation. Calculations were performed as a function of temperature (5-60-degrees-C) and of the number of backbone bonds (N) in the range 8-60. The EFS population of cycles with N < 20 shows the opposite trend to that of linear PMPS; it increases with N. For large cycles with N > 20, EFS level off to the same value as for the homologous linear PMPS (about 61%). For the cyclic tetramer (N = 8), the MC result depends on the ring-closure distance considered and is slightly larger than the result obtained by direct calculations on all 16 conformations allowed by the geometrical restrictions imposed by ring closure (43%). Several mechanisms for excimer dissociation have also been considered. For large cycles, excimer dissociation through rotation of a backbone bond requires 4.8 kcal/mol for racemic diads, and it is not allowed for meso diads. For the smallest cycle considered here (N = 8), such rotational excimer dissociation is not possible, but 15% excimers may dissociate through the interconversion of two isoenergetic conformations. The conformational energy map of two neighboring phenyl rings has been calculated. The results show that these side groups may freely rotate around the Si-phenyl bond by as much as 60-degrees with respect to the parallel sandwichlike conformation and that such motions require only the excimer binding enthalpy (3.2 kcal/mol) to dissociate the excimer. In consequence, this last mechanism may compete with others to give excimer dissociation.