A DFT study on the cyclization-degradation mechanism for phenylmethylsiloxanes in thermal vacuum

•Whether for polyphenylmethylsiloxanes or for polydimethylsiloxanes, the cyclization degradation in thermal vaccum undergoes the four-memberred cyclic transition state, which is bound to determine that the flexibility of backbone plays a decisive role in the activation energy of degradation.•The effect of the end group on the activation energy of depolymerization is little, and vanishes as the growth of the main chain. If the end group has not a great influence on the flexibility of the backbone, it is less vulnerable to the activation energy of backbone depolymerization.•Based on the above two points, present result confirms that the hydroxyl terminal structure confers less stability than trimethylsiloxyl terminal one in the polymethylphenylsiloxanes. The principal effect of hydroxyl end-groups is to assist in the cleavage of Si-Ph or Si-C bonds in the way of the hydrogen abstraction.