Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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 (DPn) of the linear polymer formed with hexamethylcyclotrisiloxane (D3) in CH2Cl2 at 20°C is inversely proportional to the ester concentration and near to the theoretical value. The polymer contains one PhCH2(CH3)2 Si group per macromolecule, in agreement with a propagation occurring on the monoester PhCH2(CH3)2 SiD3x 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 D9 and D12. But the amount of D6 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 D3 polymerization is internally 1st order in [D3] 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 D6 formation by a ring expansion involving the polymer chain.
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