The ternary system composed of Cp* 2 ZrBu 2 (Cp* ¼ Me 5 Cp), [CPh 3 ][B(C 6 F 5 ) 4 ], and tetraisobutyldialuminoxane (TIBAO) catalyzed the polymerization of ethylene in a controlled fashion at temperatures up to 60 8C. The consumption of ethylene remained constant during the polymerization process, the molecular weight of the polyethylene increased linearly with time, and polydispersity indexes down to 1.3 were obtained. Characterization of the polyethylene by 13 C-NMR and FT-IR spectroscopy did not indicate any branching or C¼C linkages, even for polymer produced at 40 8C or above. The linear and saturated polymer structure is due to the absence of b-hydride transfer, b-hydride elimination and chain walking during the polymerization. The absence of termination reactions is consistent with the systems demonstrated controlled polymerization.
Introduction. -Mechanism-based catalyst engineering seeks to design catalytic systems that give access to products with desirable properties. In a living polymerization, monomers are enchained without chain transfer or termination, and such systems are used to produce polymers with controlled molecular weight and molecular structures [1 -3]. A particularly interesting design target is a living or controlled polymerization of ethylene catalyzed with metallocene at temperatures significantly higher than room temperature. This would have the advantage that cooling of the exothermic reaction could be achieved with room-temperature water, an important consideration in the translation of laboratory results into a practical industrial process. Controlled polymerization of ethylene above room temperature was reported in a few instances as recently reviewed in [4] [5].
We report herein the polymerization of ethylene catalyzed by Cp* 2 ZrR 2 (R ¼ Me, Bu) activated with [CPh 3 ][B(C 6 F 5 ) 4 ] (TB) and an alkylaluminum species (tetraisobutyldialuminoxane (TIBAO) or triisobutylaluminum (TIBA)). The bulk polymer molecular structure is discussed on the basis of GPC, IR, and NMR spectroscopy. We demonstrate that, despite the high activity of the catalyst, controlled polymerization of ethylene is feasible, and narrow molecular-weight distributions of polymer can be obtained at temperatures up to 60 8C.
Recently, our group reported an electrospray ionization mass spectrometry (ESI-MS) technique [6] [7] that allow quick quantification of b-hydride (b-H) elimination and b-H transfer for a given catalytic system. By this method, it was demonstrated that neither significant b-H elimination to metal nor b-H transfer to monomer occurs at temperatures up to 60 8C for polymerizations catalyzed by Cp* 2 ZrCl 2 activated with methylaluminoxane (MAO) [8]. We concluded that, under these conditions, the