Mechanistic Study of Tetrahydrofuran Polymerization Photoinitiated by a Sulfonium Salt/Thioxanthone System

The cationic photopolymerization of tetrahydrofuran was initiated by irradiation at 366 nm of a solution containing a sulfonium salt and thioxanthone in dichloromethane. The dependence of the polymerization rates on the photoinitiator system concentration was studied dilatometrically and presents an S-shaped curve, suggesting an induction period caused by the existence of two simultaneous initiation processes. Rate and equilibrium constants for the elemental reactions in the ground and excited states were determined and used to propose a mechanism for the photoinitiation. This mechanism involves photosensitization followed by an electron transfer from the thioxanthone to the triphenylsulfonium salt forming a cation radical. An expression for the apparent yield of cation radicals was deduced from this mechanism. When using the experimental constants determined for the elemental reactions involved in the photopolymerization, the behavior of the plot is similar to that of the experimentally measured overall polymerization process.