Thermal analysis of polysiloxanes. II. Thermal vacuum degradation of polysiloxanes with different substituents on silicon and in the main siloxane chain
Thermogravimetric (TG) investigations of various substituted polysiloxanes of the type documentclass{article}pagestyle{empty}begin{document}$ rlap{--} ({rm R}_1 {rm R}_2 {rm SiOrlap{--} )}_n $end{document} have been carried out in vacuo and the activation energies for the depolymerization processes calculated from the resulting thermograms. (R1 and R2 are methyl, ethyl, n-propyl, trifluoropropyl, or phenyl.) It is postulated that the activation energy is mainly a function of the inductive effect of the substituent group and that electron-withdrawing groups attached to silicon increase the activation energy, whereas electron-donating groups decrease it. A linear relation is found between the Taft constant σ* for the substituent on silicon and the calculated activation energy for depolymerization. Product analysis results from isothermal degradations indicate that the degradation mechanism in a silmethylene siloxane polymer and a silethylene-siloxane polymer is very similar to that in polydimethylsiloxanes (PDMS). For the documentclass{article}pagestyle{empty}begin{document}$ rlap{--} ({rm R}_1 {rm R}_2 {rm SiOrlap{--} )}_n $end{document} polymers, the amount of cyclotrisiloxane in the degradation products increases with the size of the substituent on silicon, and it is postulated that the rate of depolymerization is mainly influenced by short-range steric interactions between the substituents on the silicon atoms of the siloxane chain.
have been carried out in vacuo and the activation energies for the depolymerization processes calculated from the resulting thermograms. (R1 and R2 are methyl, ethyl, n-propyl, trifluoropropyl, or phenyl.) It is postulated that the activation energy is mainly a function of the inductive effect of the substituent group and that electron-withdrawing groups attached to silicon increase the activation energy, whereas electron-donating groups decrease it. A linear relation is found between the Taft constant σ* for the substituent on silicon and the calculated activation energy for depolymerization. Product analysis results from isothermal degradations indicate that the degradation mechanism in a silmethylene siloxane polymer and a silethylene-siloxane polymer 
polymers, the amount of cyclotrisiloxane in the degradation products increases with the size of the substituent on silicon, and it is postulated that the rate of depolymerization is mainly influenced by short-range steric interactions between the substituents on the silicon atoms of the siloxane chain.