On the mechanism of olefin polymerisation with titanium β-diketonato complexes. A model density functional study

According to density functional calculations (BP86/AE1+ZPE level) for a model system with L=HC(O)CHC(O)H, cationic titanium complexes of the type L2TiR+ (R=growing alkyl chain) can be viable intermediates in the homogeneous olefin polymerisation catalysed by titanium β-diketonato complexes. As with the related cationic metallocenes, olefin insertion in β- and α-agostic intermediates is a facile process. Little activation for chain propagation is computed on the potential energy surface, 2.9 kcal mol−1 (incl. ZPE), but this value is raised to 15.8 kcal mol−1 on the free energy surface at 298 K. The main portion of this total barrier stems from the entropic destabilisation of the intermediate olefin π-complexes. Chain termination via β-H transfer requires significantly higher activation, consistent with the observation of olefin polymerisation, rather than oligomerisation. Electron-withdrawing substituents at the diketonate ligand are predicted to lower the barrier for chain propagation by as much as 4.5 or 5.4 kcal mol−1 for L=F3CC(O)CHC(O)CF3 (hfac) or NCC(O)CHC(O)CN, respectively. Thus, hfac complexes should produce highly active catalysts.