Isospecific Living Polymerization of 1-Hexene by a Readily Available Nonmetallocene C2-Symmetrical Zirconium Catalyst
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The search for newR-olefin polymerization catalysts based on transition metal complexes is a field of major interest involving many academic and industrial research groups. The ligands surrounding the metal play a crucial role in determining the activity as well as the stereospecifity of the catalyst, by affecting the steric and electronic properties at the metal. Over the last two decades, this field has been dominated by the metallocene complexes of group IV metals. Especially, ansa-metallocenes of C2 symmetry were found to induce isospecificity in the resulting polymers. 1 Recently, there has been a growing interest in the development ofnon-cyclopentadienyl ligands for the polymerization of R-olefins.2 Most attention was drawn to chelating di(amido) ligands, 3 some of whose group IV transition metal complexes induce polymerization in a liVing manner, 3a-c whereas chelating di(alkoxo) ligands 4 drew a more limited attention. The number of nonmetallocene systems, which were found to induce tacticity in the resulting polymer, is, however, quite small. 5 In this communication we introduce a novel family of di(alkoxo) complexes, one member of which is the first nonmetallocene C2symmetrical complex, which, upon activation, leads to a highly isospecific living polymerization of 1-hexene. Recently, we introduced the amine bis(phenolate) family of ligands to group IV transition metals. 6 We found that the presence of an extra donor group on a sidearm leads to octahedral LigMX 2type complexes, in which the two labile X groups are forced into a cis geometry. 6a Catalysts derived from these complexes (e.g. 1a) lead to highly reactive 1-hexene polymerization catalysts. 6b The Cs-symmetry of1a allows olefin approach from the two possible directions in each active position without preference, thus the polymer obtained is atactic. Therefore, we aimed at complexes of a different symmetry which may induce tactic polymerization, that incorporate ligands having similar functional groups yet having a different connectivity. Our approach is based on replacing the “branched” mode of connectivity of donor atoms with a sequential connectivity mode, namely diamine bis(phenolate) ligands. This new family of dianionic tetradentate chelating ligands is easily synthesized by a one-pot Mannich condensation between readily available di(secondary) amines, formaldehyde, and substituted phenols as demonstrated in eq 2. 2, a structural isomer