Unbridged bivalent lanthanidocenes for isotactic and stereomultiblock polymerization of methyl methacrylate

Abstract

Bivalent organolanthanides with unbridged substituted indenyl or fluorenyl ligands (1-SiMe 3 Ind) 2 Yb-THF (I), (9-SiMe 3 Flu) 2 Yb-THF (II), were found to efficiently catalyze the stereoregular polymerization of methyl methacrylate. The microstructure of resultant polymers was shown to be dependent of a conformation that the ligands adopt at the polymerization temperature. The formation of isotactic rich PMMAs from complex II was proposed to be associated with the fluctuation of the 9-trimethylsilylfluorenyls around a C 2 symmetric twisted-conformation. The formation of the multi(syndioPMMA-block-iso-PMMA) polymers from the mixture of rac- and meso-isomers of I was rationalized on the basis 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 stereospecific operation of the meso-form was not completely understandable.