Preparation and properties of fluorine-containing polysiloxanes obtained via ring-opening copolymerization of trifluoropropyltrimethylcyclotrisiloxane with cyclotetrasiloxane catalyzed by rare earth solid superacid SO2-/TiO2/Ln~(3+)

摘要

Traditional catalysts such as (CH3)4NOH, NaOH, KOH, n-BuLi and CF3SO3H can catalyze the copolymerization of trifluoropropyltrimethylcyclotrisiloxane with cyclotetrasiloxane to afford fluorine-containing polysiloxanes. However, use of these catalysts poses significant difficulties in handling and separation. In this work, fluorine-containing polysiloxanes were synthesized through a novel and environmentally friendly method: ring-opening copolymerization of trifluoropropyltrimethylcyclotrisiloxane with cyclotetrasiloxane catalyzed by rare earth solid superacid SO_4~(2-)/TiO2/ Ln~(3+). The effects of reaction conditions were examined in detail. The yield sequence of various rare earth catalysts is Nd ~ La ~ Y ~ Sm > Gd, while the number-average molecular weight sequence is Nd > La > Y > Sm > Gd. The optimum conditions for the ring-opening copolymerization of trifluoropropyltrimethylcyclotrisiloxane with cyclotetrasiloxane are as follows: [Nd~(3+)] = 0.05 mol L~(-1) and [SO_4~(2-)] = 1.85 mol L~(-1) in the immersing solution, SO_4~(2-)/TiO2/Nd~(3+) calcined at 500 °C and the copolymerization conducted at 80 °C for 40min. Structures of resulting copolymers were characterized using size exclusion chromatography, ~1H NMR spectroscopy, differential scanning calorimetry, thermogravimetric analysis and contact angle measurements. According to the copolymerization features, a cationic equilibrium reaction mechanism is proposed.