groups of the following general formulae (4) and (5), tert-alkyl groups of 4 to 20 carbon atoms, trialkylsilyl groups whose alkyl groups each have 1 to 6 carbon atoms, and oxoalkyl groups of 4 to 20 carbon atoms. [0059] In the formulae, R6 and R7 each are a hydrogen atom or normal, branched or cyclic alkyl group having 1 to 18 carbon atoms, preferably 1 to 6 carbon atoms, more preferably 1 to 5 c