Mechanism of hexamethylcyclotrisiloxane polymerization in the presence of siloxanediols

Polymerization of hexamethylcyclotrisiloxane (D 3 ) in CH 2 Cl 2 at 30°C, initiated bytriflic acid (TfOH) was studied in the presence ofsiloxanediols (HD 2 OH or HD x OH)which polycondense giving water, or in the presence of water alonefor 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 areattributed to the suppression of their formation through oxonium ionsand to their exclusive formation, for r >100, by cyclizationof silanol‐esters, which is more rapid for HD 9 OTfthan 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 6 min) at thebeginning, without formation of cyclics (D 6 , D 9 , etc). Then reaction with D 3 stops.Polymerization of HD 2 OH takes place simultaneously, at thesame rate as in the absence of D 3 , with slow formation ofhigh polymer. At the end of the polycondensation (after, eg, 4h) 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 acidhydrates by silanol groups. These activated silanol groups do notreact with D 3 . A comparison of D 3 polymerization with the addition ofwater, HD 2 OH or HD 15 OH leads to the conclusionthat when r is not too large, propagation involves silylesterend‐groups, but that for large r ratios (100 orhigher), it probably occurs mainly on silanol groups reactingwith an activated monomer. © 1999 Society of Chemical Industry