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

Polymerization of hexamethylcyclotrisiloxane (D 3 ) 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 [D 3 ] and the constant active sites concentration [P*] varies as [P*] = k · [TfOH]   0 3· [D 3 ]   0 −1,8 . The two main products of the reaction, both formed in similar amount from the beginning, are the dimer D 6 and a high‐molecular‐weight polymer (HP). The other cyclic products also formed are the multiples of D 3 (D 9 , D 12 , …), macrocycles MC (number‐average molecular weight M̄ n ≈ 10 4 ) and small amounts of D 4 , D 5 … (with [D 5 ] > [D 4 ]). The concentration of HP, MC, D 6 , D 9 , D 12 , … and the M̄ n of the high polymer grow proportionally to conversion. The main reaction giving the high polymer and D 6 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. D 6 is mainly formed by a special type of back‐biting reaction involving transitory tertiary siloxonium ions. The other cycles D 3 x 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.