Control of both molecular weight and comonomer response of ethylene polymerization via utilization of alkyl branches at the para-xylene bridge of new dinuclear constrained geometry catalysts

Abstract

We synthesized a series of four dinuclear constrained geometry catalysts (DCGCs) containing alkyl-substituted para-xylene bridges [TiCl2{N(tBu)Si(Me)2}C9H5]2[(CH2){(R)2C6H2}(CH2)]: 13 (R = hydrogen), 14 (R = isopropyl), 15 (R = n-hexyl), and 16 (R = n-octyl). The structures and compositions of the synthesized complexes were conveniently identified by 1H NMR, 13C NMR and elemental analysis (EA). In order to determine the effect of steric and electronic properties of various alkyl branches on the xylene group, ethylene homo and copolymerization experiments by use of these metallocenes have been conducted. Dow CGC ([Me2Si (μ5-Me4Cp)NtBu]TiCl2) has been used as the control catalyst for comparison. It was found that the activity of Catalyst 13 was highest and the activities of the new DCGCs were much higher than that of Dow CGC. Polyethylene having more than 1,000,000 g/mol of molecular weight that may be classified as ultra high molecular weight polyethylene (UHMWPE) has been able to be successfully produced from the new DCGCs. Most importantly it was demonstrated that the control of polymerization properties of DCGCs was determined by the nature of the alkyl substituent at para-xylene bridge. Catalyst 14 having isopropyl substituent at the bridge produced the longest polymer with the lowest catalytic activity. On the other hand, DCGCs 15 and 16 exhibited the greatest comonomer reactivity to make ethylene/styrene copolymers with the highest styrene contents.