r, which method of production is characterized by the fact that said cross-linking agent is a cross-linking agent possessing at least two polymerizable unsaturated groups and further possessing between said two polymerizable unsaturated groups at least one unit represented by the formula I:-(-CH2CH2OR1O-)- (I)wherein R1 is an alkylene group of 2 to 4 carbon atoms, said cross-linking agent is used