Tandem Catalyst System for Linear Low-Density Polyethylene with Short and Long Branching

A zirconium-based polymerization catalyst, Me2Si(eta(1)-C29H36)(eta(1)-N-tBu)ZrCl2 center dot OEt2 (1), and a chromium-based oligomerization catalyst, [((BuSCH2CH2)-Bu-t)(2)NH]CrCl3 (2), in combination with methylaluminoxane (MAO), constitute a novel tandem catalyst system for converting ethylene alone to linear low-density polyethylene with short and long branches. Independent reactions demonstrated that 1/MAO converts ethylene to polyethylene with ethyl and long (>= 6C) branches while 2/MAO converts ethylene to 1-hexene and 1-octene. Under certain reaction conditions (80 degrees C, 120 psi ethylene), 2/MAO produces 1-hexene and 1-octene from ethylene with 80-85% selectivity for 1-hexene and minimal production of polymer. Polymerization of ethylene by 1/MAO in the presence of 2/MAO yields polyethylene with 14-44 total branches per 1000 carbon atoms, with 4-16 of those branches arising from incorporation of 1-hexene. The fraction of branches arising from 1-hexene varies from 12 to 54%, depending on the relative amounts of 1, 2, and MAO present, as well as on the pressure of ethylene and the prepolymerization time. By adjusting the polymerization conditions, the branching characteristics of polyethylene could be varied in a controlled manner.