AbstractComplexes of antimony pentachloride with acyl chlorides such as acetyl chloride or 1‐naphthoyl chloride polymerize various completely methylated cyclosiloxanes such as hexamethylcyclotrisiloxane (D3) or dodecamethylcyclohexasiloxane (D6) with high rates of initiation and propagation, their activity being unchanged in the presence of the proton‐trap 2,6‐di‐tert‐butyl‐4‐methylpyridine. For polymerization of D3conducted at −10°C in methylene dichloride, there is a simultaneous and proportional formation of high‐molecular‐weight polymer (HP), of small cycles of the D3xtype (concentrations [D6]>[D9]>[D12]>…) and of macrocycles. A linear increase of the molecular weight (M) of HP is observed up to at least 50% conversion, in agreement with the presence of a constant number of macromolecules of high molecular weight, mainly consisting of a living polymer population. For higher conversions,Mmay grow more rapidly which may be explained by an increasing copolymerization of D6and D9with D3. Homopolymerization of D6occurs under similar conditions with a rate smaller than that for D3, but also with linearly increasingMof HP with conversion. The simultaneous formation of the various products is attributed to the presence of two populations of macromolecules. One of these would bear either two active centres or one active centre and one non‐reacting end‐group, and the other population would bear one active centre and one reacting end‐group, giving by end‐to‐end ring closure, macrocycl