Controlled Cationic Polymerization of Hexamethylcyclotrisiloxane

Abstract

Cationic polymerization of hexamethylcyclotrisiloxane (D3) initiated by HCl and antimony pentachloride (SbCl5) may give a high polymer (HP) with controlled molecular weight in high yield, and only small amounts of cyclic compounds. Theoretical Mn's were formerly obtained with D3 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 % D6/HP and up to 20% macrocycles. D6 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 D6 and macrocycles was significantly decreased and was sometimes suppressed. The polymerization was rapid with SbCl5 and HCl at −10 °C, producing about 90% high polymer of Mn ≥ 105, 10−12% small cycles (only 3% D6) and no macrocycles. The Mn'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 D6. 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., (CH3)3Si+) was formed by reaction of R3SiH and Ph3C+, B(C6F5)4-. At 20 °C, Mn > Mnth, but degradation occurred afterward, while at −20 °C, Mn ∼ Mnth, and degradation was very slow. However, cyclics formation was, in both cases, larger than with HCl and SbCl5 (25−33%), although very little D6 formed, which seems to exclude propagation involving mainly silyloxonium ions.