The degree of short chain branch (SCB) formation during ethylene polymerization catalyzed with nickel-diimine/trimethylaluminum systems is a function of the relative rates of polymerization and catalyst chain walking. The proper choice of polymerization temperature (30–50°C) and ethylene pressure (1.0–18.4 kgf cm −2 ) allows one to tailor make the SCB content of the polyethylene produced with these catalysts. Within such range, polyethylene can be prepared with 10–100 branches per 1000 carbon atoms in the backbone. The unique branch distribution, which mimics the one obtained in the copolymerization of ethylene and α-olefins, is described in detail using Monte-Carlo simulation of polymerization and chain walking. The four probability parameters used in the model were correlated to polymerization temperature, T, and ethylene concentration, [ E]. With this model, it is possible to predict polyethylene microstructure as a function of T and [ E]. The dependence of each parameter on these two polymerization conditions is helpful to clarify the mechanism of branch formation with these intriguing catalyst systems.