Population balances were used to model the short chain branch distribution of polyethylene made with Ni-diimine catalysts. Branches are formed according to the chain walking mechanism without addition of comonomer. The proposed model can be used to simulate the cumulative or instantaneous distributions of short chain branches formed with these catalysts. The instantaneous distribution is particularly useful, since it can be applied to interpret experimental results with minimum effort. A simplified version of the complete instantaneous model was compared with experimental results for polyethylene branching measured with C-13 NMR. The mathematical model can predict the experimental trends well and constitutes a useful data interpretation tool for the investigation of polymerization with these catalysts.