Bis(1,3-di-tert-butylimidazolin-2-iminato) titanium complexes as effective catalysts for the monodisperse polymerization of propylene

The use of bis(1,3-di-tert-butylimidazolin-2-iminato) titanium dichloride (1) and dimethyl (2) complexes in the polymerization of propylene is presented. The complexes were activated using different amounts of methylalumoxane (MAO), giving in each case a very active catalytic mixture and producing polymers with a narrow molecular weight distribution (polydispersity = 1.10). The use of the cocatalyst triphenylcarbenium (trityl) tetra(pentafluorophenyl)borate totally inhibits the reaction, producing the corresponding bis(1,3-di-tert- butylimidazolin-2-iminato) titanium(III) methyl complex, the trityl radical (CPh₃), the anionic MeB(C₆F₅)₄ ^-, B(C₆F₅)₃, and the bis(1,3-di-tert-butylimidazolin-2-iminato) titanium(IV) dimethyl·B(C ₆F₅)₃ complex. The use of a combination of physical methods such as NMR, ESR-C₆₀, and MALDI-TOF analyses enabled us to propose a plausible mechanism for the polymerization of propylene, presenting that the polymerization is mainly carried out in a living fashion. In addition, we present a slow equilibrium toward a small amount of a dormant species responsible for 2,1-misinsertions and chain transfer processes.