Theoretical Studies on the Kinetics of Thermal Degradation of Hydroxylalkylpolysiloxanes
|Course||Polymer Chemistry and Physics|
|Keywords||hydroxylalkyl polysiloxane thermal degradation reaction mechanism transition state theory density functional theory|
In spite of possessing a variety of unique and superior properties, carbofounctionalized polysiloxanes play important roles in modifitions of organic polymers, such as the copolymerization of polysiloxanes and organic polymers, with the aid of the reaction activity of the carbofunctional groups, to endow the organic polymers with the excellent properties of the polysiloxanes. However, because of the decomposition or rearrangement under certain conditions, their applications are limited to some extent. Understanding their rearrangement conditions and mechanisms will be of great important values for the control and application of these compounds.In this dissertation, theoretical studies on thermal degradation or rearrangement reactions of hydroxylpropyl polydimethylsiloxanes, hydroxylethyl polydimethylsioxanes and hydroxyl-ended poly-dimethylsiloxanes and effects of methyl and phenyl attached to different atoms on the reactions were carried out by employing polydimethylsiloxane, hydroxylpropyl polydimethyl sioxane or hydroxylethyl polydimethylsioxanes oligomers containing three or four silicon atoms as model compounds. For all reactions under study, the reaction mechanisms were revealed through Density Functional Theory (DFT) and molecular orbital theory at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) level, and reaction rate constants were evaluated by using Transition State Theory (TST) with direct dynamic method. The important and valuable results are summarized as follows: 1. On the thermal rearrangement reactions of α-silylalcohols(1) Two dyotropic reactions may occur when α-silylalcohols are heated. One is via the Brook rearrangement reaction, where the silyl group migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three-membered ring transition state, forming alkyloxysilane. In the other rearrangement, the reactant undergoes a hydroxyl group migration from carbon atom to silicon atom coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom via a double three-membered ring transition state, forming alkylsilanol. The hydroxyl group migration reaction is preferred to the Brook rearrangement reaction both thermodynamically and kinetically.