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

According to density functional calculations (BP86/AEI + 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 beta-diketonato complexes. As with the related cationic metallocenes. olefin insertion in beta- and alpha-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 pi-complexes. Chain termination via P-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. (C) 2002 Elsevier Science B.V. All rights reserved