alkyl group having 1 to 12 carbon atoms or an aryl group having 6 to 10 carbon atoms and l is an integer of 1 to 4, X--(CH2)p --Si(OR3)q R4 3-q ( 4) wherein X represents an organic group containing at least one group selected from the group consisting of epoxy group, amino group, carboxyl group, hydroxy group, mercapto group, ureido group, maleimido group and trialkoxysilyl groups and having 0