CNDO CALCULATIONS FOR PHENYLENE ROTATIONS IN POLYCARBONATES

Rotational motions of phenylene rings in tetramethyl-Bisphenol A polycarbonate (TMPC) were investigated by means of CNDO/2 (complete neglect of differential overlap) molecular orbital calculations on several model compounds. In drastic contrast to the unsubstituted counterpart (Bisphenol A polycarbonate, PC) studied previously, the mobility of phenylene rings in TMPC was found very much restricted. This is in good agreement with earlier solution NMR results. The repulsion between ortho hydrogens in neighboring phenylene rings connected to the same isopropylidene unit, previously identified as contributing most significantly to the hindrance of phenylene rotations in PC, remained important here; a more significant contribution, however, comes from the repulsion between the aromatic methyl and the carbonyl oxygen in the case of TMPC. This results in strong coupling of the carbonate linkage to the two neighboring rings. The model proposed earlier by Jones was adopted as an example to demonstrate differences in detailed molecular motions in PC and TMPC.