Herein, R1 is a monovalent hydrocarbon group free of aliphatic unsaturation, X is an alkenyl-containing organic group, a is an integer of 0 to 3, preferably 1, m is a number of at least 0, n is a number of at least 100, a and m are not 0 at the same time, p is a number of at least 1, and q is a number of at least 100.