in which R1 is a monovalent hydrocarbon group free from a cationically polymerizable group, R2 is a monovalent hydrocarbon group substituted by a cationically polymerizable group, the subscript b is 0 or a positive number not exceeding 10000 and the subscripts p, q, r and s are each 0 or a positive integer not exceeding 3 with the proviso that p+q and r+s are each 0, 1, 2 or 3, all of p, q, r and