ISSN:
0887-624X
Keywords:
dinuclear metallocene
;
polysiloxane bridge
;
heterogenation
;
ethylene polymerization
;
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The polymerization of ethylene was studied by using a series of polysiloxane-bridged dinuclear zirconocenes [(SiMe2O)nSiMe2(C5H4)2][(C9H7)ZrCl2]2 (7, n = 1; 8, n = 2; 9, n = 3), the corresponding mononuclear zirconocene (C5H5)(C9H7)ZrCl2, 10, and the pentamethylene-bridged dinuclear zirconocene [(CH2)5(C5H4)2][(C9H7)-ZrCl2]2, 13. From the polymerization studies using these catalysts it was found that (i) activities of the polysiloxane dinuclear zirconocenes 7-9 wre lower than that of the corresponding mononuclear zirconocene 10, (ii) molecular weights of polyethylenes produced by the dinuclear metallocenes are greater than that of polyethylene produced by the mononuclear metallocene, (iii) the complex 9 holding the longest bridging ligand exhibited the highest activity but produced a polymer having the smallest molecular weight among the polysiloxane-bridged dinuclear zirconocenes, and (iv) the pentamethylene-bridged dinuclear metallocene 13 showed higher activity than the complexes 7-9 and the mononuclear zirconocene 10. The formation of the lowest molecular weight of polyethylene by 9 was attributed to the influence of electron withdrawal caused by the Lewis acid-base interaction between the acidic aluminum of the cocatalyst and the basic oxygen at the polysiloxane linkage as well as the lack of a steric problem. An increase in steric congestion around the metal center led to not only a decrease in catalytic activity due to preventing facile monomer access to the active site but also an increase in the molecular weight of polyethylenes due to supressing β-H elimination. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3717-3728, 1997
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
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