Abstract
Ziegler-Natta polymerizations with group 4 catalyst systems usually occur through primary propene insertion. With heterogeneous catalytic systems as well as with tetrahedral bis(cyclopentadienyl)-based catalysts, the amount of regiomistakes (secondary propene insertion) is usually well below 1%. The amount of secondary insertions raises to about 5% with mono-Cp amido catalysts, or to 20% with a particular design of the metallocene skeleton. Catalysts based on octahedral complexes of Zr with tetradentate amine bis(phenolate) ligands also polymerize propene via primary insertion. The recent discovery that bis-salycilamidinate based Ti catalysts syndiospecifically polymerize propene according to a secondary mechanism, instead, was rather unexpected. The origin of this behavior is still unclear, and the molecular modeling of propene insertion with these systems could contribute to shed light on this unsolved question. For these reasons, we performed a systematic study of the regioselective behavior of metallocene based systems of the type [Me sub 2 Si(Cp) sub 2 Mt-R] exp n+ (Mt = Sc, Y, Ti and Zr; n = 0, 1). Group 3 and 4 metals (both formally d deg ) have been considered to investigate the role of the total charge of the catalyst (neutral and cationic for group 3 and 4, respectively) as well as to investigate about differences as we move down the triad (i.e. from Ti to Zr). Full quantum mechanics as well as combined QM/MM (quantum mechanics/molecular mechanics) techniques were used to partitionate between steric and electronic effects. Moreover, we also considered models of the recently discovered catalysts based on octahedral complexes of Ti and Zr.