sent a hydrogen atom or a hydrocarbon having 1 to 18 carbon atoms, either or both of R3 and R4 possessing at least one polymerizable unsaturated group, wherein when both R3 and R4 are a hydrocarbon, the hydrogen atom of the hydrocarbon may be substituted by a hydroxyl group, an ester group, an alkoxy group, or a halogen atom, and R5 and R6 may be identical with or different from each other and res