The solution polymerization of ethylene was studied in a semibatch reactor using a constrained geometry catalyst (CGC) and methylaluminumoxane (MAO). The influence of ethylene concentration, temperature, MAO, and catalyst concentrations on ethylene polymerization kinetics was investigated systematically. The deactivation of the CGC/MAO system during ethylene polymerization was described with a first order thermal deactivation mechanism that included reversible activation and deactivation with MAO. Interestingly, the polymerization order with respect to ethylene varied with ethylene concentration from first to second order. The trigger mechanism was shown to describe well the effect of ethylene concentration on polymer yield and polymerization kinetics. Low MAO concentration favored formation of polymer chains with unsaturated chain ends that in turn led to the production of polymers with higher long chain branch densities. It was also observed that CGC did not behave as a true single site catalyst at low MAO concentrations.