The cationically polymerisable material (A) may be, for example, a cyclic ether such as an oxetane or a tetrahydrofuran, a cyclic ester such as a lactone, an episulphide such as ethylene sulphide or an episulphide resin, a vinyl monomer such as a vinyl ether, a styrene or a vinyl carbazole, or a vinyl prepolymer.