Damien Reardon - Vanadium-Catalyzed Ethylene−Propylene Copolymerization:  The Question of the Metal Oxidation State in Ziegler−Natta Polymerization Promoted by (β-diketonate)3V

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  Ma, Yinlin, Reardon, Damien, Gambarotta, Sandro, Yap, Glenn,   Zahalka, Hayder, Lemay, Catherine Organometallics  1999 18 (15) 2773-2781

  Abstract: Four different (?-diketonate)3V   complexes [?-diketonate = 2,4-pentanedione (acac),   2-acetylcyclohexanone (Cy-acac),   2,2,6,6-tetramethyl-3,5-heptanedione (t-Bu-acac), and   1,1,1,5,5,5-hexafluoro-2,4-pentanedione (F-acac)] have been   prepared and tested as catalysts for propylene?ethylene   copolymerization with the aim of gaining insights into the   structure of the active species. Data on polymer composition and   catalyst activity indicate that one of the roles of the Al   cocatalyst is to trigger a major ligand scrambling around the   transition metal. Attempts to isolate the catalytically active   species afforded a compound formulated as   {[(?-diketonate)AlCl2][VCl2][(?-diketonate)2AlCl]}. This species,   which displayed only a minor catalytic activity, arises from a   parasite process (catalyst deactivation). The formulation was   supported by chemical degradation experiments with THF, which   afforded a mixture of [V2Cl3(THF)6][AlCl4] and   [(acac)2Al(THF)2][Al Cl4] (5). Two unprecedented V(II) complexes   (R-acac)2V(TMEDA) [R = H(7a), Cy(7b), t-Bu (7c)] have been   prepared and reacted with halocarbons to model the reactivation   process. The results indicated that the primary role of   reactivating substances, commonly employed in the industrial   processes, is to reoxidize V(II) to the trivalent state. The   reaction formed the catalyst precursor (R-acac)2VCl(TMEDA), which   was characterized on the basis of analytical and spectroscopic   data. In agreement with this proposal, a trapping experiment   carried out with ZnCl2 or oxidation with CuCl allowed the   isolation and characterization of (t-Bu-acac)2V(TMEDA)][X] [X =   ZnCl4- (8), CuCl2- (9)]. The structures of 5, 7a, 8, and 9 have   been elucidated by X-ray diffraction. Crystal data are as   follows. 5: triclinic space group P1?, a = 8.916(3) Å, b =   9.802(3) Å, c = 15.390(5) Å, α = 88.156(4)°, ? = 86.041(4)°, ? =   82.710(4)°, Z = 2. 7a: triclinic space group P1?, a = 7.896(1) Å,   b = 10.032(2) Å, c = 13.134(2) Å, α = 75.265(2)°, ? = 88.520(2)°,   ? = 71.445(2)°, Z = 2. 8: orthorhombic space group Pbcn, a =   18.723(7) Å, b = 19.675(7) Å, c = 18.836(7) Å, Z = 4. 9:   monoclinic space group C2/c, a = 13.676(5) Å, b = 19.521(3) Å, c   = 13.206(3) Å, ? = 98.47(3)°, Z = 4.

  Address (URL): http://dx.doi.org/10.1021/om9808763