Imidazolium triflimide-based Brønsted acidic ionic liquid as organocatalyst to trigger the cationic ring-opening polymerization of cyclotrisiloxanes

Abstract

The use of imidazolium-based ionic liquids as organocatalysts for the ring opening polymerization of hexamethylcyclotrisiloxane (D₃) is reported for the first time. Among various tested candidates, 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIMSO₃H]NTf₂) as a Brønsted acidic ionic liquid (BAIL) efficiently catalyzes the polymerization of (D₃) providing polydimethylsiloxane with molar mass higher than 200 kg mol⁻¹ with low cycles content (less than 8 wt%) within minutes. To regulate the molar mass of the resulting polymers, different types of chain-regulators (hydroxyl-containing molecules (trialkylsilanols and alcohols) and disiloxanes) were tested. Disiloxanes turn out to be more effective, making it possible to prepare PDMS with predictable molecular weight and controlled chain ends. Optimized conditions were extended to the use of functional molecules, such as vinyl-containing disiloxane and cycotrisiloxanes (1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (D^Vi3₃), and vinyl-pentamethyl cyclotrisiloxane (D^Vi₃)). It is shown that vinyl-functional groups can be introduced at the chain end or along the chain, and with using D^Vi₃ in combination with D₃, copolymers with tunable vinyl content and molar mass can be prepared. Screening of various catalysts highlighted that the polymerization proceeds through an activated monomer mechanism by the acidic proton from sulfonic acid moiety that should be covalently bonded to the imidazolium ring.