The compound (b) need not necessarily be used.Examples of the compound (a) of the above formula (I) in which X is a hydroxyl group, include hydroxyalkyl acylates such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, etc.; hydroxyalkyl methacrylates such as2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, etc.; alkyl(2-hydroxyalkyl