Three diethers, 2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane, differing in the bulkiness of the alkyl substituents in position 2, have been used as internal donors in MgCl2-TiCl4-diether/AlR3 catalysts for the polymerization of propylene. The performance of the three diether-based catalysts is compared with that of a "traditional" MgCl2-supported catalyst based on the phthalate/silane donor pair and with that of a "donor-free" catalyst by coupling the temperature rising elution fractionation (TREF) analysis with a study of the stereoregularity of the first monomeric unit insertion. On the basis of the experimental data obtained, the action mechanisms of the different donors are discussed taking into account the various models presented in the literature.

A best-fitting procedure for the quantitative determination of the molar fractions of the stereosequences that define the microstructure of an ethylene−norbornene (E−N) copolymer from 13C NMR spectra has been set up. The quantitative determination of copolymer microstructure will allow one to clarify the E−N copolymerization mechanism. This method utilizes the observed peak areas of the 13C signals and takes into account the consistency between peak areas and the stoichiometry of the copolymer chain. Thus, a further extension of signal assignments is made possible by guessing assignments of unknown signals and by discarding inconsistent hypotheses. This procedure has been applied to the analysis of the 13C NMR spectra of a large number of E−N copolymers, prepared with catalyst precursors rac-Et(indenyl)2ZrCl2 (1), rac-Me2Si(2-Me-benz[e]indenyl)2ZrCl2 (2), Me2Si(Me4Cp)(N t Bu)TiCl2 (3), and Me2C(Flu)(Cp)ZrCl2 (4). An estimate of the molar fractions of the various stereosequences with a standard deviation on the order of 1−2% has been obtained. The comparison between controversial assignments existing in the literature for a number of ethylene signals has confirmed our previous assignments. New signals such as those of the C2/C3 carbons of EENNEE meso sequences (M) and of the external carbons C5 of MM and MR triads in ENNNE sequences have been assigned.

Ethylene-norbornene (E-N) copolymers were synthesized by catalytic systems composed of racemic isospecific metallocene or a constrained geometry catalyst (CGC) and methylaluminoxane. The following metallocenes were used: rac-Et(indenyl) 2ZrCl2 (1), rac-Me2Si(indenyl)2ZrCl2 (2), rac-Me2Si(2-Me-[e]-benzindenyl)2ZrCl2 (3), and Me2Si(Me4Cp)(N t Bu)TiCl2 (4). The copolymers were characterized by 13 C NMR and the copolymer microstructures were analyzed in detail. A procedure for computing the molar fractions of the stereosequences that completely define the microstructure of an E-N copolymer at tetrad level, distinguishing between meso and racemic contributions to alternating and block sequences, was utilized. The information was converted into the complete tetrad distribution, which allowed us to determine the reactivity ratios, testing the first-order and the second-order Markov statistics. Here, examples of such an use of tetrad description of copolymers to test possible statistical models of copolymerization are given. The first-order r 1 and r2 reactivity ratios of copolymers prepared with all catalysts depend on the monomer concentration. The products r1r2 were found in the range between 0 and 0.177. The tendency to alternate ethylene and norbornene is 4 > 3 > 1 > 2. The root-mean-square deviations between experimental and calculated tetrads demonstrate that penultimate (second-order Markov) effects play a decisive role in E-N copolymerizations. Our first results show clues for more complex effects depending on the catalyst geometry in copolymers obtained at high N/E feed ratios.

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Cp2Zr(13CH3)2 (1) has been used as a probe for the reactivity of metallocene−methylaluminoxane catalysts for olefin polymerization. A 1H and 13C NMR study of the reaction equilibria between Cp2Zr(13CH3)2 and Lewis acids such as AlMe3 (2), B(C6F5)3 (3), and methylaluminoxane (MAO) (4) has been performed. AlMe3 is always present in MAO solutions, and B(C6F5)3 is a relatively strong Lewis acid, which has a capacity to form and stabilize ion pairs comparable to that of MAO. The use of isotopically 13C-enriched dimethylzirconocene has permitted the study of these systems by 13C NMR in conditions as close as possible to usual polymerization conditions, which require large excesses of MAO for reaching high activities. The comparisons of the reactivity of Cp2Zr(13CH3)2 with B(C6F5)3 and with MAO have provided the first direct evidence of the formation in solution of monomeric [Cp2Zr(13CH3)]+[Me·MAO]- (8), of dimeric [Cp2Zr(13CH3)]2(μ-13CH3)+[Me·MAO]- (7), and of the [(Cp2Zr(μ-Me)2AlMe2]+[Me·MAO]- (9) cationic species, having MeMAO- counterions. The influence of temperature, Al/Zr mole ratio, and zirconium concentration on the equilibria of ion pair formation has been elucidated.

Ethylene−norbornene (E−N) copolymers were synthesized with the C 2 metallocene rac-Me2Si(2-Me-[e]-benzindenyl)2ZrCl2 (3) and with the constrained geometry Me2Si(Me4Cp)(NtBu)TiCl2 (4) in the presence of methylaluminoxane. The E−N copolymerizations were carried out using a variety of monomer feed compositions. Copolymers were fully characterized by 13C NMR spectroscopy, gel permeation chromatography, and differential scanning calorimetry. Copolymer microstructures were analyzed in detail, through a procedure which accounts for the stoichiometric requirements of the copolymer chain as well as for the measured areas of 13C NMR signals. This analysis, which quantifies the differences in sequence distribution and tacticity of the polymers, evidenced that mainly alternating stereoregular and stereoirregular copolymers were prepared with 3 and 4, respectively. The copolymer prepared with 4 contains both meso and racemic NEN sequences and small amounts of meso and racemic NN diads, while the alternating copolymer prepared with 3 contains only meso NEN sequences and small amounts of meso NN diads. The formation of NN diads is disfavored with both catalysts. Surprisingly, a significant amount of norbornene (up to ∼10%) belonging to NNN triads (T) is obtained with the C 2 catalyst.

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