Kinetics of the polycondensation of dimethylsilanediol

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Abstract
Dimethylsilanediol is easily condensed, giving a mixture of linear and cyclic polysiloxanes. Because of the technical importance of polysiloxanes, the chemicals processes of synthesis are of the greatest importance. Most fundamental researches on the mechanism of these processes use the methods expounded by Adrianov. The polymerization reactions followed by formation of high molecular weight compounds have been studied in detail and their mechanism and kinetics have been determined. Nevertheless, there were no data concerning the rate of polycondensation of dimethylsilanediol, partly because of some difficulties encountered in the purification of the monomer and its conservation in the pure state. In this paper, the data of polycondensation rates were given for dimethylsilanediol in dioxane and methanol. Acid and basic catalysts were used. The determination at given time intervals of the silanol and water was carried out following the methods of titration of Fischer. In both solvents two steps of reaction could be distinguished on the basis of rate measurements. The first, very rapid step corresponds principally to the reaction of silanediol monomer molecules. After reaction of about half of the number of the silanol groups initially present, the rate of condensation decreases; this is due to the low chemical reactivity of the OH groups in the polysiloxane molecules. Indeed, the tetramethyldisiloxanediol at the same molar concentration as the monomer condenses at a noticeably slower rate in the presence of the same quantity of catalyst. The polycondensation in dioxane was practically complete. The inverse reactions—the hydrolysis of siloxane bands—are absent in dioxane; in methanol, concentrations equivalent to those in the preceding reaction, are obtained. The rate of the first step of the polycondensation in dioxane presents approximately a second order with respect to the silanol. Some divergences may be attributed to the influence of changes in the reaction medium on account of the catalytic activity of the acid. The influence affects the kinetics of the reaction; it is still more pronounced in the second step of the condensation, which is slower. The rates of polycondensation of tetramethyldisiloxanediol show an induction period. The activity of the catalyst increases progressively with an increase of water content in the system. This is the reason why there is no simple kinetic scheme in this case. The polycondensation products have been studied during some particular assays. The main products were siloxane ring compounds with 8, 10, and 12 members; a small amount of linear compounds was also present. Among the polycondensates of dimeric diol, only rings with an even number of members are found. In the presence of acid catalyst in methanol solutions, the initial rate was higher than in dioxane. It was established that the equilibrium was analogous to that obtained by Grubb during the polycondensation of monosilanol in alcohol. The rates of the first step also indicate a reaction course analogous to that of monosilanols; this means that a preponderant importance must be attributed to methanol and methoxysilanes during these reactions. The condensation of the methoxysilane group with the silanol group is the rate‐determining step. Consequently, the kinetics of polycondensation correspond to the third order, typical of most of the concentrations studied. It was found that bases are the most, active catalysts of polycondensation of dimethylsilanediol in methanol. The equilibrium was approximately the same whether acid or basic catalysts were used. Nevertheless, the mechanism in the case of silanol obeyed a first order over a broad range of concentrations. The experimental details, the numerical data (rate constants and equilibrium constants), and the proposed mechanisms are presented.

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