Polymerization of hexamethylcyclotrisiloxane in at 30ÄC, initiated by triýic (D 3 ) CH 2 Cl 2 acid (TfOH) was studied in the presence of siloxanediols or which polycondense (HD 2 OH HD x OH) 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 and of linear high polymer (D 6 , D 9 ) (HP) vary strongly with the molar ratio When r increases from 1 to 100 (in homo-r = [ H 2 O] / [ TfOH] . geneous phase), increases from 0.2 to 2 and decreases from 1 to 0.3. The large decrease of D 9 /D 6 D 6 /HP amount and the increase in are attributed to the suppression of their formation through D 6 D 9 /D 6 oxonium ions and to their exclusive formation, for r > 100, by cyclization of silanol-esters, which is more rapid for than for HD 9 OTf HD 6 OTf. For polymerization of in the presence of there is a fast and limited ring-opening of D 3 HD 2 OH, D 3 (about 10% conversion over 6 min) at the beginning, without formation of cyclics etc). Then (D 6 , D 9 , reaction with stops. Polymerization of takes place simultaneously, at the same rate as in D 3 HD 2 OH the absence of with slow formation of high polymer. At the end of the polycondensation (after, eg, D 3 , 4 h) polymerization starts again, giving etc, and HP. The inhibition period for is attrib-D 3 D 6 , D 9 , D 3 uted to the complexation of triýic acid hydrates by silanol groups. These activated silanol groups do not react with D 3 . A comparison of polymerization with the addition of water, or leads to the D 3 HD 2 OH HD 15
OH 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.