UNBRIDGED BIVALENT LANTHANIDOCENES FOR ISOTACTIC AND STEREOMULTIBLOCKPOLYMERIZATION OF METHYL-METHACRYLATE

Bivalent organolanthanides with unbridged substituted indenyl or fluor enyl ligands (1-SiMe(3)Ind)(2)Yb . THF (I), (9-SiMe(3)Flu)(2)Yb . THF (II), were found to efficiently catalyze the stereoregular polymerizat ion of methyl methacrylate. The microstructure of resultant polymers w as shown to be dependent of a conformation that the ligands adopt at t he polymerization temperature. The formation of isotactic rich PMMAs f rom complex II was proposed to be associated with the fluctuation of t he 9-trimethylsilylfluorenyls around a C-2 symmetric twisted-conformat ion. The formation of the multi(syndioPMMA-block-iso-PMMA) polymers fr om the mixture of rac- and meso-isomers of I was rationalized on the b asis of competing conjugate addition and inversion of the metallocene conformation. Surprisingly, both rac- and meso-isomers of I were found to operate with similar activity and stereospecifity, although the st ereospecific operation of the meso-form was not completely understanda ble. (C) 1998 John Wiley & Sons, Inc.