The influence of electronic and steric effects and the importance of polymerization conditions in the ethylene polymerization with zirconocene/MAO catalysts

Abstract

A series of Cp′(C 5H 5)ZrCl 12 and Cp 2 ′ZrCl 12 precatalysts (Cp′ = C 5Me4H, C 4Me 4P, C 5Me 5) together with (C 5H 5) 2ZrCl 12 has been investigated in terms of steric and electronic variations and their catalytic activities in combination with methylalumoxane (MAO) towards the polymerization of ethylene are compared. The changes in the steric environment were evaluated on the basis of the structural data available and supplemented by theoretical structural studies on the semiempirical (ZINDO, EHMO) and density functional (DF) level. The X-ray structures of (C 5Me 4H) 2ZrCl 2 ( 3) and (C 4Me 4P)(C 5H 5)ZrCl 2 ( 4) have been determined ( 3: orthorhombic, Cmcm, a = 6.714(4), b = 17.275(4) c = 15.643(5) A ̊ , Z = 4 ; 4: monoclinic, P2 1/ c, a = 8.8791(5), b = 7.8051(8), c = 20.9215(10) A ̊ β = 94.422(4)° , Z = 4. 91Zr NMR data for the above series has been measured and is correlated to changes in the HOMO-LUMO gap available from electronic structure calculations. Under mostly homogeneous polymerization conditions, at very low zirconium concentrations the order of the catalytic activity found for ethylene polymerizations is (C 5H 5) 2ZrCl 2 > (C 5Me 4H)(C 5H 5)ZrCl 2 > (C 5Me 5)(C 5H 5)ZrCl 2 > (C 4Me 4P)(C 5H 5)ZrCl 2 > (C 5Me 4H) 2ZrCl 2 > (C 5Me 5) 2ZrCl 2 > (C 4Me 4P) 2ZrCl 2, which for the most part is inversely proportional to the steric demand of the ring ligands in the metallocene precatalysts except for the phospholyl systems. The lower activities of the phospholyl vs. the tetra- and penta-methylcyclopentadienyl compounds might imply an electronic effect such that the electron withdrawing phosphorus substituent decreases the activity, 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 concentrations an increased precipitation of polyethylene takes place during the course of polymerization and results in a transfer to the heterogeneous phase with a diffusion controlled reaction rate thereby invalidating any activity-comparing studies.