Mechanism of hexamethylcyclotrisiloxane polymerization in the presence of siloxanediols

Abstract

Polymerization of hexamethylcyclotrisiloxane (D3 ) in CH2Cl2 at 30°C, initiated by triflic acid (TfOH) was studied in the presence of siloxanediols (HD2OH or HDxOH) which polycondense giving water, or in the presence of water alone for comparison. D represents a siloxane unit OSiMe2. In the second case, the relative amounts of cyclic oligomers (D6, D9) and of linear high polymer (HP) vary strongly with the molar ratio r =[H2O]/[TfOH]. When r increases from 1 to 100 (in homogeneous phase), D9 /D6 increases from 0.2 to 2 and D6 /HP decreases from 1 to 0.3. The large decrease of D6 amount and the increase in D9/D6 are attributed to the suppression of their formation through oxonium ions and to their exclusive formation, for r>100, by cyclization of silanol‐esters, which is more rapid for HD9OTf than for HD6OTf. For polymerization of D3 in the presence of HD2 OH, there is a fast and limited ring‐opening of D3 (about 10% conversion over 6 min) at the beginning, without formation of cyclics (D6, D9 , etc). Then reaction with D3 stops. Polymerization of HD2OH takes place simultaneously, at the same rate as in the absence of D3, with slow formation of high polymer. At the end of the polycondensation (after, eg, 4 h) D3 polymerization starts again, giving D6 , D9, etc, and HP. The inhibition period for D 3 is attributed to the complexation of triflic acid hydrates by silanol groups. These activated silanol groups do not react with D3. A comparison of D3 polymerization with the addition of water, HD2OH or HD15OH leads to the conclusion that when r is not too large, propagation involves silylester end‐groups, but that for large r ratios (100 or higher), it probably occurs mainly on silanol groups reacting with an activated monomer. © 1999 Society of Chemical Industry