Controlled/living radical polymerization (CRP) allows a high degree of control over polymeric molecular architecture with narrow molecular weight distributions and desired polymer properties. Although the control of copolymer composition for RAFT copolymerization systems in a semibatch reactor has been studied, the control of sequence length receives little attention. In this work, using moment equations, a chain model and a sequence model were developed to describe both chain and sequence properties such as chain and sequence lengths and their distributions, for RAFT copolymerization in semibatch operations. It is seen that a copolymer with constant composition or linear gradient composition can be obtained through an optimized feeding policy. With the sequence model, it was possible to show the sequence length and distributions while controlling constant composition or linear gradient composition. More importantly, it was demonstrated that sequence length distribution can be designed and controlled using the sequence model. The sequence model may be used as a theoretical tool to understand the importance of sequence length and distributions in CRP.