Thermal degradation of plastics. I. The kinetics of polymer chain degradation

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Abstract
Kinetic equations were derived for polymer chain degradation. The usual type of initiation‐depropagation‐termination mechanism was postulated. Chain transfer was ignored. Random and terminal initiation were considered as well as unimolecular, bimolecular, and no‐termination (complete unzipping). It was assumed that a steady state of radicals was achieved, and that monomer product rapidly diffused out of the polymer bulk as soon as it formed. Only average molecular weights were considered. Polymer and radical molecule concentrations were employed in the kinetic equations, while in all the previous work, polymer and radical molecule numbers were used. The same kinetic equations resulted for unimolecular termination and complete unzipping for both treatments, but for bimolecular termination, the results were different. Where the use of number units predicted that the rate of loss of polymer weight should be half‐order in polymer weight, concentration units gave a first‐order relationship. The equations that were derived in this work were found to be similar to those that were previously derived in treatments where number units were employed together with the assumption of exponential molecular weight distribution. A comparison of the various treatments showed that the similarities were not necessarily due to the use of the distribution assumption, but may have been a consequence of the forms of the basic kinetic equations that were utilized in the earlier work.

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