Controlled Cationic Polymerization of Hexamethylcyclotrisiloxane

Cationic polymerization of hexamethylcyclotrisiloxane (D 3 ) initiated by HCl and antimony pentachloride (SbCl 5 ) may give a high polymer (HP) with controlled molecular weight in high yield, and only small amounts of cyclic compounds. Theoretical M n 's were formerly obtained with D 3 using as initiator either triflic acid or a combination of a silyltriflate activated by triflic acid. However, considerable amounts of cyclic compounds were also formed, e.g. 50 wt % D 6 /HP and up to 20% macrocycles. D 6 was mainly formed by a reaction involving transitory oxonium ions and macrocycles by cyclization of silanol esters. Using two other initiating systems, the formation of D 6 and macrocycles was significantly decreased and was sometimes suppressed. The polymerization was rapid with SbCl 5 and HCl at −10 °C, producing about 90% high polymer of M n ≥ 10 5, 10−12% small cycles (only 3% D 6 ) and no macrocycles. The M n 's agreed with an initiation by HCl and a propagation involving the silanol end groups, which explains the absence of macrocycles and the low amount of D 6 . However, the polymer degraded somewhat after the completion of the polymerization. A comparison was done with the initiation described by G. Olah et al., in which a silyl cation (e.g., (CH 3 ) 3 Si + ) was formed by reaction of R 3 SiH and Ph 3 C +, B(C 6 F 5 ) 4 - . At 20 °C, M n > M n th, but degradation occurred afterward, while at −20 °C, M n ∼ M n th, and degradation was very slow. However, cyclics formation was, in both cases, larger than with HCl and SbCl 5 (25−33%), although very little D 6 formed, which seems to exclude propagation involving mainly silyloxonium ions.