Emulsion polymerization reactors usually operate under starved conditions to better control the polymer properties and keep a safe operation. The main drawback is the low productivity obtained. In this work, we optimize offline the flow rates for the semibatch emulsion polymerization of styrene under starved conditions and keeping the reaction under diffusion limitation in order to maximize the reaction rate. To achieve this, we impose the maximal concentration of monomer in the particles as a path constraint. This results in a higher reaction rate compared to starved conditions close to saturation. At the same time, it enables safe operation by avoiding accumulation of monomer that may react hazardously at the end of the reaction, once the gel effect occurs. Therefore, we show that high productivity can be obtained by operating the reactor under starved conditions. We compare the optimal recipe under diffusion limitations with the benchmark of keeping the concentration of monomer in the polymer particles close to saturation and with constant feeding flow rates. We find consistent improvement of both the average reaction rate and the amount of polymer produced in the optimized strategy.