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The synthesis of polyolefins with two metallocene catalysts is explored via a simple Monte Carlo simulation in the two cases of a batch and a semibatch reactor. The algorithm we develop can be applied, in principle, to an arbitrary number of catalysts with different propensities toward the formation of long chain branches. We compare our model to an experimental two-catalyst system, in which one of the catalysts makes only linear chains and the ratio of the two catalysts is varied, to synthesize a series of different molar mass resins. Results from our simulations show good agreement with experimentally measured molecular weight distributions, the ratio of unsaturated to saturated chain ends (found by 13C NMR) and the number of branches per 10000C. We also examine the effect on the amount of branching when molecular hydrogen gas is added to the system to act as a chain transfer agent.