This investigation studied the solution polymerization of ethylene in Isopar E in a semibatch reactor using CGC-Ti as catalyst and methylalumoxane (MAO) and tris(pentaflourophenyl)borane [B(C6F5)(3)] as cocatalysts. The effects of cocatalyst type and amount on the chain microstructure were investigated. C-13 NMR and gel permeation chromatography were used to determine the long-chain branching (LCB) content and molecular weight distribution (MWD), respectively, of the samples. It was observed that higher concentrations of MAO increased the LCB content and decreased the molecular weight of the polymer. On the other hand, increasing the amount of B(C,F,), lowered the LCB content, increased the molecular weight, and broadened MWD significantly. We believe that this approach can be used as an efficient way to control the microstructure of polyolefins made with these catalytic systems. (C) 2004 Wiley Periodicals, Inc.