Mechanistic analysis and thermochemical kinetic simulation of the products from pyrolysis of poly(α-methylstyrene), especially the unrecognized role of phenyl shift

Mechanisms for pyrolysis of poly(α-methylstyrene) must rationalize high selectivity for monomer formation, negligible formation of volatile oligomers, and notably slow decrease in molecular weight compared with the rate of weight loss, i.e., unzipping dominates both back-biting and transfer. Backbone homolysis should form both a tert-benzylic radical Rtb and a prim radical Rp, with formation of the latter potentially supplemented in chain propagation steps emanating from the former. Hence product-forming pathways characteristic of each are expected to compete. Simulations of initial product distributions based on assigned rate constants for chain propagation steps indicate that Rtb is indeed predicted to efficiently unzip with minimal transfer or back-biting. However, Rp is predicted to give comparable amounts of transfer and back-biting with minimal unzipping, behavior inconsistent with experimental data. The proposed escape from this impasse is a previously unrecognized pathway, 1,2-phenyl shift in Rp to form a tert radical. If it undergoes β-scission, the net result is an inter-conversion of Rp to Rtb. Quantitative simulations suggest that this sequence is indeed highly competitive with other reactions of Rp and thus efficiently subverts the otherwise expected propagation of chains emanating from Rp.