Suitable as the monomer which provides such constituent unit are hydroxyalkyl acrylates (corresponding to the case where, in the above general formula (3), p is 1 and R3, R4 and R6 each is a hydrogen atom) such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate and 2-hydroxybutyl acrylate; various polyalkylene glycol monoacrylates (corresponding to the case where, in the above general formula (3