Cationic polymerization of hexamethylcyclotrisiloxane by trifluoromethanesulfonic acid and its derivatives, 2. Reaction involving activated trifluoromethylsulfonates
✍ Scribed by Georgios Toskas; Gyula Besztercey; Michel Moreau; Michèle Masure; Pierre Sigwalt
- Publisher
- John Wiley and Sons
- Year
- 1995
- Tongue
- English
- Weight
- 969 KB
- Volume
- 196
- Category
- Article
- ISSN
- 1022-1352
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✦ Synopsis
Abstract
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.