The synthesis and structural X-ray diffraction studies for some benzamidinate ligations and several group 4 benzamidinate complexes are presented. The use of the cis-octahedral C(2)-symmetry compounds was studied to shed light on the conceptual applicability of these complexes as potential catalysts for the stereoregular polymerization of propylene. We demonstrate that the stereoregular polymerization of propylene catalyzed by early-transition metal octahedral benzamidinate complexes, activated with either MAO or B(C(6)F(5))(3) as cocatalysts, can be modulated by pressure (from atactic to isotactic through elastomers). The different effects in the polymerization process such as the nature of solvent or cocatalyst, temperature, pressure, molar ratio catalyst:cocatalyst, and the relationship between the symmetry of the complex and the polymer microstructure have been investigated. When the complex [4-CH(3)-C(6)H(4)C(NTMS)(2)](2)ZrMe(2) (9) was activated with MAO, it was found to be a good catalyst for the polymerization of propylene, at atmospheric pressure, producing an oily polymer resembling an atactic polypropylene. Being activated with B(C(6)F(5))(3), complex 9 produces a highly isotactic (mmmm = 98%) product. Likewise, when the polymerization of propylene was performed with complex 9 and MAO at high pressure (liquid propylene), a highly stereoregular polymer was also obtained. Larger activities and stereoregularities were achieved by performing the reaction in CH(2)Cl(2) as compared to toluene. Contrary to complex 9, at atmospheric pressure the complex [4-CH(3)-C(6)H(4)C(NTMS)(2)](2)TiMe(2) (10) is not active either in CH(2)Cl(2) or in toluene. At high pressure, complex 10 produces elastomeric polypropylene. Activities of the isolobal complexes [C(6)H(4)C(NTMS)(2)](2)ZrMe(2) (11) and [C(6)H(4)C(NTMS)(2)](2)TiMe(2) (12) were found to be larger than those of complexes 9 and 10, respectively. Contrary to the structures of the elastomeric polypropylenes described in the literature, the obtained elastomers are characterized by frequent alternation of the isotactic domains with stereodefects. The stereoregular errors are formed by the intramolecular epimerization of the growing chain at the last inserted unit. The epimerization reaction was corroborated through the isomerization of alkenes.

The new neutral nickel complexes PhC(NSiMe 3 ) 2 Ni(acac) (3), PhC(NSiMe 3 ) 2 Ni(acac)-(TMEDA) (4), [PhC(NSiMe 3 ) 2 ] 2 Ni 2 (5), [PhC(NSiMe 3 ) 2 ] 2 Ni‚Li 2 Br 2 (TMEDA) 2 ( 6), (TMEDA) 2 Ni (7), p-MePhC(NSiMe 3 ) 2 Ni(acac) (8), p-MePhC(NSiMe 3 ) 2 Ni(acac)(TMEDA) (9), [p-MePhC-(NSiMe 3 ) 2 ] 2 Ni (10), NiMe 2 Py 2 (11) and [p-MePhC(NSiMe 3 )] 2 Ni(Py) ( 14) have been synthesized and characterized. The solid-state molecular structures of all complexes have been confirmed by low-temperature X-ray diffraction analysis. The methyl complex p-MePhC(NSiMe 3 ) 2 Ni-(Me)(Py) ( 13) is not stable and rapidly decomposes, forming the complex [p-MePhC(NSiMe 3 )] 2 -Ni(Py) ( 14). Complex 3 activated with MAO has been shown to be an efficient catalytic system for the norbornene vinyl-type polymerization. The activity of the catalyst and the molecular weights of the resulting polynorbornenes were found to be dependent on the reaction time, the MAO/precatalyst ratio, and the reaction temperature. In addition, this catalytic system has been found to dimerize propylene to a mixture of hexenes with a high turnover frequency of η ) 9040 h -1 and oligomerize ethylene to either a mixture of C 4 -C 6 or/and C 4 -C 14 products, depending on the temperature and the solvent, extremely rapidly (η ) 83 500 h -1 ).

Treatment of racemic bis(acetylacetonato) group 4 complexes with MAO yields cationic complexes acting as active catalysts for the polymerization of propylene under high monomeric concentrations. By the appropriate choice of metal and solvent the properties of the polymer products can be tailored, allowing the formation of either elastomer to highly isotactic polypropylene.

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