The microstructures of polypropenes produced with several zirconocene-based catalyst systems are compared, to verify the possible correlation between the type of stereospecificity and the amount of regioirregularities. It is confirmed that, while syndiospecific and aspecific zirconocenes are highly regiospecific, isospecific systems produce substantial amounts of regioirregular monomeric units. The amount of these secondary units strongly depends on the nature of the π-ligands and on the type of the bridge connecting them. Molecular mechanics calculations are reported, indicating that the intermediates which are energetically suitable for the secondary and primary insertions, for isospecific or syndiospecific complexes, coordinate monomer enantiofaces of the opposite or the same chirality, respectively. This difference accounts for the lower regiospecificity of the isospecific catalytic complexes, assuming that the energy barrier for the rotation of the coordinated monomer around the metal−olefin bond, between the orientations suitable for the primary and secondary insertions is lower than (or comparable to) the activation energy for secondary monomer insertion.

The experimental data available in the literature concerning syndiotactic-specific polymerization of styrene and the data reported in this paper, mainly concerning the comparison of the behaviour of different catalytic systems, lead to the tentative suggestion that the true catalytic species might be complex cations similar to those involved in a-olefin polymerization.

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The stereoregularity of polystyrenes prepared in the presence of different syndiotactic specific homogeneous catalysts, at different temperatures and monomer concentrations, has been evaluated by I3C NMR. It is confirmed that the statistical model of the stereospecific propagation is first-order Markovian. The stereoregularity of the polymers decreases while increasing the polymerization temperature and is affected by the concentration of the monomer, by the ligands of the transition metal precursor of the catalyst, and is higher in the presence of titanium based catalysts. Polymerization of substituted styrenes is increasingly stereospecific in the order p-chlorostyrene < styrene < p-methylstyrene. The results strongly support the polyinsertion mechanism proposed in a previous paper by some of us.

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We recently succeeded in preparing highly syndiotactic polystyrene, similar to that described by Ishihara et al.'), in the presence of catalytic systems consisting of either tetrabenzyltitanium (TBT)Z) or tetrabenzylzirconium (TBZ),) and methylalumoxane (~~0 1 3 ) . E. g. 4,8 . lo-, mol of TBT plus 0,27 g of M A 0 dissolved in 40 ml of dry toluene and 20 ml of styrene under nitrogen atmosphere at 60°C after 1 h yield 0,9 g of highly syndiotactic polystyrene 97% of which is insoluble in boiling 2-butanone. The polymer was recovered after coagulation with methanol acidified with HCI, washed several times with methanol and dried under vacuum.

Some catalytic systems based on Ti or Zr compounds and methylalumoxane have been investigated with regard to their ability to promote styrene-ethylene copolymerization. 13C NMR analysis shows that the structure and the composition of the copolymerization products are dependent on the catalytic system and on the transition metal/methylalumoxane mole ratio. Polymers containing phenylethylene units bridging polyethylene sequences are generally obtained in the presence of Ti-based catalysts. In the presence of cyclopentadienyltitanium trichloridd methylalumoxane with high AVTi mole'ratio one can obtain copolymers of ethylene and syndiotactic polystyrene. Zr-based catalyst don't promote copolymerization at all.

Stereoblock polypropylenes have been obtained performing the polymerization of propene at different temperatures in the presence of [1-methyl-1-naphthylethyl-2-inden-1-yl]zirconium(IV) trichloride and methylaluminoxane. The stereoblock microstructure probably arises from to fact that the zirconium complex exists in equilibrium among structures having the naphthyl group coordinated to the metal, corresponding to chiral and/or pseudo-achiral forms, and a structure having the naphthyl group not coordinated, corresponding to a semimetallocene form. The chiral form produces regular isotactic sequences, whereas the pseudo-achiral and the semimetallocene forms produce atactic sequences, giving rise to isotactic-atactic stereoblocks. The length of the isotactic block depends on the polymerization temperature. A structural characterization and an analysis of the thermal behavior of these stereoblock polypropylenes have been performed. The samples crystallize from the melt in mixtures of R and γ forms of isotactic polypropylene. In the samples polymerized at low temperatures (17 and 30 °C), the amount of γ form is lower than that observed in samples prepared at higher temperatures (50 °C). This indicates that the regular isotactic sequences are longer in samples prepared at low temperatures, according with the interconversion mechanism of the catalyst among isospecific and aspecific forms. In these samples the amount of γ form is, in any case, lower than that observed in the literature for samples having higher stereoregularity and a random distribution of stereodefects, confirming the stereoblock microstructure of the samples prepared with the flexible catalyst.