In copolymerization processes, molecular weight averages and copolymer composition are commonly used as property indices for reactor control purposes. In general, it is difficult to measure the molecular weight distribution of copolymer accurately by gel permeation chromatography (GPC) because there is no unique relationship between chain length and hydrodynamic volume for a copolymer with a heterogeneity in the composition distribution. In this paper, the hydrodynamic volume distribution is used as a primary control objective in a semibatch free radical copolymerization process. The hydrodynamic volume distribution of copolymer is modeled by computing the weight fraction of polymers over a number of hydrodynamic volume intervals. Then the target hydrodynamic volume distribution is obtained by designing optimal reactor operating policy. Both copolymer composition and the entire hydrodynamic volume distribution are controlled by adding reactants as a function of time. A numerical simulation example is presented for a styrene-methyl methacrylate copolymerization process to illustrate the proposed method.