As mentioned above, it is possible to use a mixture of two or more kinds of polymers each of which is represented by the general formula (2) or (3). ##STR6## where R4 is a monocyclic or bridged cyclic hydrocarbon group, R5 is methyl group, ethyl group, propyl group, tert-butyl group, tetrahydropyran-2-yl group, tetrahydrofuran-2-yl group or 3-oxocyclohexyl group, R6, R7 and R8 are independently hy