An experimental study designed to elucidate mechanistic details regarding the thermal polymerization of styrene between 260 and 340degreesC is reported. The data show that back-biting to the third or fifth carbon from the chain end, followed by beta scission, is the dominant chain-producing reaction in the molecular weight development. This conclusion is supported by the C-13-NMR data coupled with preparative gel permeation chromatography, which show that the predominant low-molecular-weight oligomers are 2,4-diphenyl-1-butene and 2,4,6-triphenyl-1-hexene, that is, the products of the 1:3 and 1:5 back-biting/beta-scission reactions, respectively. The presence of head-to-head or head-to-tail branching, due to chain transfer to the polymer or back-biting, is shown to be negligible through C-13-NMR analysis. Finally, the distribution of terminal unsaturations, determined by the relative rates of termination, back-biting, and chain transfer to polymer, has been measured with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This has shown that the back-biting/beta-scission reaction dominates the molecular weight development in comparison with either termination or chain transfer to the polymer. (C) 2004 Wiley Periodicals, Inc.