Indenyl-amido titanium and zirconium dimethyl complexes: improved synthesis and use in propylene polymerization

The synthesis of a series of indenyl amido titanium dimethyl complexes, by means of the direct synthesis from the ligand, a 2-fold excess of MeLi, and TiCl4 is reported. The 1H NMR spectra of the complexes show a quartet structure for the metal-bound methyl groups, due to through-metal proton–proton coupling. Coupling of Ti-methyl protons with protons on the Cp ring is also revealed by COSY 2D-NMR. The performance of the Ti complexes in propylene polymerization, including [Me2Si(Me4C5)(t-BuN)]TiMe2 (1-TiMe2), [Me2Si(Ind)(t-BuN)]TiMe2 (2-TiMe2) and six other methyl titanium complexes bearing substituted indenyl ligands, has been investigated with different cocatalysts and at different polymerization temperatures and propylene concentrations. All complexes produce amorphous polypropylene (am-PP). The catalytic activity and molecular weight strongly depend on the substitution of the Cp ring: 2-TiMe2 gives polymers of lower molecular weight, while the presence of a methyl group in position 2 (as in 3-TiMe2) determines up to 4-fold increase in molecular weight. The type of cocatalyst influences mainly the catalytic activity, the borates being better activators than MAO, but also molecular weight, with again the borates giving higher molecular weights than MAO. 5-TiMe2–Ph3CB(C6F5)4 shows an overall activation energy of polymerization of 7.35 kcal mol−1. The rate of chain release is first order in monomer. The following activation energies for overall chain release have been calculated: ΔΔE‡2-TiMe2=3.4 kcal mol−1, ΔΔE‡5-TiMe2=3.8 kcal mol−1, ΔΔE‡3-TiMe2=6.3 kcal mol−1. Even if all the polymers produced are amorphous, 2-TiMe2 and 5-TiMe2 show a microstructure unbalanced towards isotacticity, while 3-TiMe2, 6-TiMe2 and 8-TiMe2 are syndiotactic-enriched. Chiral induction comes mainly from a weak enantiomorphic site control.