. . "y hydrocarbon groups being bonded to two carbon atoms on a cyclopentadienyl ring, Q is a group capable of cross-linking (C5H4???mR1 m) and (C5H4???nR2 n), and is a divalent hydrocarbon group, unsubstituted silylene group or silylene group substituted with a hydrocarbon, and X1 and X2 may be the same or different and are hydrogen atoms, halogen atoms or hydrocarbon groups, can be preferably used." . . .