Polycondensation of tetramethyldisiloxane 1,3‐diol catalyzed by trifluoromethanesulfonic acid

Abstract The kinetics of polycondensation of tetramethyldisiloxane 1,3‐diol (HD 2 OH) have been followed by stopping the reaction at increasing times (by neutralization of the acid and trimethylsilylation of the silanol end‐groups). Analyses of the linear and cyclic oligomers and of the higher polymer have been made by gas–liquid chromatography and by steric exclusion chromatography. At the beginning of the reaction (e. g. between 1 min and 1 h) the main products are linear oligomer H(D) 2 n OH and D 4 (formed by cyclization of HD 4 OH). There are only very small amounts of large cycles. Linear high polymer is then formed slowly together with increasing amounts of cycles D 5 , D 6 , D 7 , …, formed by backbiting reactions. After 24h, the yield of D 4 is 40–50% and that of linear polymer 40–30% ( M peak × 10 4 –10 5 ). With disilanol concentration 0.45–0.9 mol liter −1 and CF 3 SO 3 H concentration about 10 −3 mol liter −1 , the reaction is very rapid during 1–2 min and the medium becomes heterogenous with the formation of an inverse water emulsion. The reaction then slows down considerably, most of the hydrated acid being trapped in the water droplets. Silanol consumption is second order in [SiOH] and first order [TfOH] o . Polymerizations made in polar and non‐polar solvents have similar rates, which however increase moderately in the order toluene<1,2‐dichloro‐ethane<dichloromethane, this corresponding to an increasing solubility of the hydrated acid in these solvents. Polycondensation occurs in the organic phase near the interface, either by esterification of silanol followed by heterccondensation or by homocondensation catalyzed by the hydrated acid (or both).