C1-8 alkylene group, R6 is a C1-18 alkyl group, and X2 is a halogen atom such as a chlorine atom, a bromine atom or an iodine atom, with a water-soluble monomer of e.g. a hydroxyalkyl(meth)acrylate such as hydroxymethyl(meth)acrylate, hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate or hydroxybutyl(meth)acrylate.