When R1 to R4 in the above-mentioned formula (Ia) represent a hydrocarbon group, it is preferably an alkyl group having 1 to 100 carbon atoms (more preferably having 1 to 50 carbon atoms), a cycloalkyl group having 3 to 100 carbon atoms (more preferably having 3 to 50 carbon atoms), an aralkyl group having 7 to 30 carbon atoms (more preferably having 7 to 20 carbon atoms, particularly preferably h