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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