THE INFLUENCE OF ELECTRONIC AND STERIC EFFECTS AND THE IMPORTANCE OF POLYMERIZATION CONDITIONS IN THE ETHYLENE POLYMERIZATION WITH ZIRCONOCENE MAO CATALYSTS/

A series of Cp'(C5H5)ZrCl2 and Cp' = ZrCl2 precatalysts (Cp' = C(5)Me( 4)H, C(4)Me(4)P, C(5)Me(5)) together with (C5H5)(2)ZrCl2 has been inve stigated in terms of steric and electronic variations and their cataly tic activities in combination with methylalumoxane (MAO) towards the p olymerization of ethylene are compared. The changes in the steric envi ronment were evaluated on the basis of the structural data available a nd supplemented by theoretical structural studies on the semiempirical (ZINDO, EHMO) and density functional (DF) level. The X-ray structures of (C(5)Me(4)H)(2)ZrCl2 (3) and (C(4)Me(4)P)(C5H5)ZrCl2 (4) have been determined (3: orthorhombic, Cmcm, a = 6.714(4), b = 17.275(4), c = 1 5.643(5) Angstrom, Z = 4; 4: monoclinic, P2(1)/c, a = 8.8791(5), b = 7 .8051(8), c = 20.9215(10) Angstrom beta = 94.422(4)degrees, Z = 4. Zr- 91 NMR data for the above series has been measured and is correlated t o changes in the HOMO-LUMO gap available from electronic structure cal culations. Under mostly homogeneous polymerization conditions, at very low zirconium concentrations the order of the catalytic activity foun d for ethylene polymerizations is (C5H5)(2)ZrCl2 > (C(5)Me(4)H)(C5H5)Z rCl2 > (C(5)Me(5))(C5H5)ZrCl2 > (C(4)Me(4)P)(C5H5)ZrCl2 > (C(5)Me(4)H) (2)ZrCl2 > > (C(5)Me(4))(2)ZrCl2 > (C(4)Me(4)P)(2)ZrCl2, which for the most part is inversely proportional to the steric demand of the ring ligands in the metallocene precatalysts except for the phospholyl syst ems. The lower activities of the phospholyl vs. the tetra- and penta-m ethylcyclopentadienyl compounds might imply an electronic effect such that the electron withdrawing phosphorus substituent decreases the act ivity, although further studies are needed to clarify this situation. Emphasis is placed on the control of the polymerization conditions and evaluation of the time-activity profiles. At higher zirconium concent rations an increased precipitation of polyethylene takes place during the course of polymerization and results in a transfer to the heteroge neous phase with a diffusion controlled reaction rate thereby invalida ting any activity-comparing studies.