Cationic polymerization of hexamethylcyclotrisiloxane (D-3) 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 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 SbCl5 and HCl at -10 degreesC, producing about 90% high polymer of M-n greater than or equal to 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., (CH3)(3)Si+) was formed by reaction of R3SiH and Ph3C+, B(C6F5)(4)(-). At 20 degreesC, M-n > M-nth, but degradation occurred afterward, while at -20 degreesC, M-n similar to M-nth, and degradation was very slow. However, cyclics formation was, in both cases, larger than with HCl and SbCl5 (25-33%), although very little D-6 formed, which seems to exclude propagation involving mainly silyloxonium ions.