Cationic polymerization of hexamethylcyclotrisiloxane by trifluoromethanesulfonic acid and its derivatives, 2. Reaction involving activated trifluoromethylsulfonates

Silyl trifluoromethanesulfonates are inactive for the polymerization of cyclosiloxanes in the complete absence of trifluoromethanesulfonic (triflic) acid, but polymerization occurs in its presence. Using a triflic ester as tagged silyl triflate (benzyldimethylsilyl triflate) the degree of polymerization ( DP n ) of the linear polymer formed with hexamethylcyclotrisiloxane (D 3 ) in CH 2 Cl 2 at 20°C is inversely proportional to the ester concentration and near to the theoretical value. The polymer contains one PhCH 2 (CH 3 ) 2 Si group per macromolecule, in agreement with a propagation occurring on the monoester PhCH 2 (CH 3 ) 2 SiD 3 x OTf. By comparison with experiments using the acid alone, polymerizations made in the presence of this ester lead to the suppression of the formation of macrocycles and to a strong decrease of that of D 9 and D 12 . But the amount of D 6 is only slightly reduced, which agrees with the hypothesis of its main direct formation on the active sites and not by ring closure of a silanol ester. The rate of D 3 polymerization is internally 1 st order in [D 3 ] and externally proportional to both acid and ester initial concentrations, in agreement with a propagation involving silyl triflates activated by the acid, interconverting with transitory siloxonium ions leading to D 6 formation by a ring expansion involving the polymer chain.