The influence of the Ti3+ species on the microstructure of ethylene/1-hexene copolymers

Abstract

Copolymerization of ethylene and 1-hexene was carried out with catalysts having isolated Ti 3+ and multinuclear Ti 3+ species. Carbon-13 nuclear magnetic resonance spectroscopy ( 13 C NMR), crystallization analysis fractionation (CRYSTAF), and gel permeation chromatography (GPC) studies showed that the microstructure of ethylene and 1-hexene copolymers strongly depends upon the structure of the Ti 3+ species. Isolated Ti 3+ species increase the relative reactivity of ethylene in copolymerizations and produce copolymers with high molecular weight and broad short chain branching distribution (SCBD), with a large ethylene-rich fraction. Multinuclear Ti 3+ species increase the relative reactivity of 1-hexene and produce copolymers with low molecular weight and broad SCBD, with a large rubbery ethylene/1-hexene fraction. Comparative studies of the copolymer microstructure from isolated Ti 3+ and multinuclear Ti 3+ in combination with different cocatalysts, Al(CH 3 ) 3 , Al(C 2 H 5 ) 3 , and methylaluminoxane (MAO) were also carried out. Isolated Ti 3+ species in combination with MAO cause remarkable changes in the 1-hexene incorporation rate and SCBD in comparison with Al(CH 3 ) 3 and Al(C 2 H 5 ) 3 , while multinuclear Ti 3+ species in combination with MAO do not affect as much the 1-hexene incorporation rate. This difference may be related to the mechanism of active site formation between the different Ti 3+ structures and MAO.