Ab initio calculation of the rate coefficient for short-chain branching infree-radical polymerizations

Short-chain branching in polyethylene, which involves a six-centre transiti on state, is studied by ab initio quantum mechanics up to the QCISD(T) leve l. The calculation gives a (low-pressure) activation energy of 73 kJ mol(-1 ) and a frequency factor of 4.8 x 10(12) s(-1). The frequency factor for th is six-centre transition state is expected to be of acceptable accuracy and also applicable to homologous systems, such as short-chain branching to po lymer in acrylates. These results overestimate the amount of ethylene short -chain branching observed in experiment, but the discrepancy is within the uncertainties of both experiment and calculation. (C) 2000 Elsevier Science Ltd. All rights reserved.