Conformational analysis, RIS models and single-chain properties of structurally modified polycarbonates. 2. Effect of substitutional rigidity of the isopropylidene linkage

Abstract

Continuing our study, conformational analysis and chain properties of another set of different polycarbonates (PC's) having substitutions on the isopropylidene linkage, with no change in the linkage itself, have been investigated in detail. Conformational analysis, rotational isometric state (RIS) models and chain properties of three PC's are reported here for the first time, while new insights are presented for two additional PC's. Replacement of one or both methyl groups in bisphenol A polycarbonate by a H atom leads to lower energy barriers for phenyl ring rotations. Replacement of the methyl group by relatively bulkier substituents such as phenyl, isopropyl, or cyclohexyl groups results in substantially increased hindrance to phenyl ring mobility and differences in the number and location of torsional minima within the bisphenol moiety. The number of energy minima are reduced by one half when non-symmetric substituents such as the isopropyl and cyclohexyl groups are linked to the C_α atom. The rotations about the Ph-O bond remain unhindered and the locations of the stable energy minima remain unaffected, irrespective of the type of substitutions at the C_α atom. With a cyclohexyl substituent, the domain of the lower-energy locations about the Ph-O bond becomes narrow. All the modified polycarbonates showed higher chain dimensions than bisphenol A polycarbonate. The chain dimensions decreased with temperature for all the polycarbonates. A considerable amount of information relating local molecular structure to the conformational behavior for five PC's is presented here.