A DFT Study on the Cyclization-Mechanism during Process of Thermal Vacuum Degradation for Poly(dimethylsiloxanes)

Abstract

A DFT investigation was performed on the cyclization mechanism during the process of thermal degradation in vacuum for trimethylsilyl-terminated polydimethylsiloxane, methylsiloxane-α,ω-diol and methylsiloxane-1-ol. The formation of an intramolecular, four-centre cyclic transition state results from the flexibility of a main chain and the ability to rearrange siloxane bonds. The energy barrier of the reaction depends mostly on the four-centre cyclic structure, independent of molecular weight, the position of nucleophilic attacking, and products. The replacement of trimethylsilyl by hydroxyl leads to a great decrease of activation energy of degradation reaction, because the energy barrier of the Si-O bond interchange is much higher than that of hydrogen abstraction. The large cyclosiloxane tend to higher degree of cyclization degradation than linear siloxane with the same number Si atoms. From energetic viewpoint, cyclosiloxane products of various sizes have almost equal chance to form. Due to the continuous degradation of large ring, the D3 accounts for the predominating proportion, and steadily decreasing amounts of D4 and D5.