Mechanism of Octamethylcyclotetrasiloxane Polymerization in the Presence of Siloxanediols

Abstract

Polymerization of hexamethylcyclotrisiloxane (D 3 ) in CH 2 Cl 2 at 30°C, initiated by triflic acid (TfOH) was studied in the presence of siloxanediols (HD 2 OH or HD x OH) which polycondense giving water, or in the presence of water alone for comparison. D represents a siloxane unit OSiMe 2 . In the second case, the relative amounts of cyclic oligomers (D 6 , D 9 ) and of linear high polymer (HP) vary strongly with the molar ratio r = [H 2 O]/[TfOH]. When r increases from 1 to 100 (in homogeneous phase), D 9 /D 6 increases from 0.2 to 2 and D 6 /HP decreases from 1 to 0.3. The large decrease of D 6 amount and the increase in D 9 /D 6 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 HD 9 OTf than for HD 6 OTf. For polymerization of D 3 in the presence of HD 2 OH, there is a fast and limited ring-opening of D 3 (about 10% conversion over 6min) at the beginning, without formation of cyclics (D 6 , D 9 , etc). Then reaction with D 3 stops. Polymerization of HD 2 OH takes place simultaneously, at the same rate as in the absence of D 3 , with slow formation of high polymer. At the end of the polycondensation (after, eg, 4 h) D 3 polymerization starts again, giving D 6 , D 9 , 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 D 3 . A comparison of D 3 polymerization with the addition of water, HD 2 OH or HD 15 OH 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.