A semibatch emulsion copolymerization of acrylonitrile and butadiene is theoretically investigated, with the aim of controlling the molecular structure of the produced NBR. An open-loop estimator based on calorimetric measurements is proposed for monitoring the chemical composition, the average molecular weights, and the average degree of branching. With little effect on the other quality variables, the intermediate addition of acrylonitrile allows to produce a polymer with a constant chemical composition. Similarly, the intermediate addition of the chain transfer agent produces a polymer with either a fixed R. or with a prespecified linear variation of the average branching. The required feed profiles are obtained from a numerical inversion of a discrete process model. By increasing the initiator loads, the semibatch strategies also allow to increase the final conversion between 3% and 6%, without altering the reaction time nor deteriorating the polymer quality with respect to the batch. The numerical procedures were tested by inputing (relatively noisy) heat measurements from an industrial batch reactor.