Cationic polymerization of hexamethylcyclotrisiloxane by trifluoromethanesulfonic acid and its derivatives, 1. Initiation by trifluoromethanesulfonic acid

Polymerization of hexamethylcyclotrisiloxane (D3) initiated by trifluoromethanesulfonic acid (triflic acid TfOH) was carried out under vacuum, in methylene chloride solution at 20°C. The reaction rate is first order in [D3] and the constant active sites concentration [P*] varies as [P*] = k · [TfOH] · [D3] . The two main products of the reaction, both formed in similar amount from the beginning, are the dimer D6 and a high‐molecular‐weight polymer (HP). The other cyclic products also formed are the multiples of D3 (D9, D12, …), macrocycles MC (number‐average molecular weight M̄n ≈ 104) and small amounts of D4, D5 … (with [D5] > [D4]). The concentration of HP, MC, D6, D9, D12, … and the M̄n of the high polymer grow proportionally to conversion. The main reaction giving the high polymer and D6 is interpreted as a chain reaction, the growing HP bearing at each end potentially reactive silyl triflates which may be activated by the residual acid. D6 is mainly formed by a special type of back‐biting reaction involving transitory tertiary siloxonium ions. The other cycles D3x and the macrocycles mainly result from cyclization reactions of a second population of growing macromolecules which bear a silyl ester at one end and a nucleophilic silanol group at the other end.