Designing polymer architectures through synthesis, characterization and kinetic analysis

Gradient copolymers with well-defined sequence are increasingly desired in a wide range of applications. Although sequence along the copolymer chain plays an important role in the potential applications, few efforts have been directed at the investigation of the explicit sequence along the chain. We developed a simulation framework based on kinetic Monte Carlo written in house, which can predict the explicit sequence. This comprehensive kinetic modeling approach was validated extensively against data for styrene (S)/methyl methacrylate (MMA) gradient copolymer synthesized via nitroxide-mediated controlled radical polymerization (NM-CRP) in a semi-batch reactor. The most striking feature that our research revealed is that copolymers that have been categorized as typical gradient copolymers according to current characterization criteria may have sequences resembling those of random copolymers. This finding suggests that the current synthesis strategies for making gradient copolymer may not necessarily generate a gradient in sequence, and thus it is necessary to optimize current synthesis strategies. To that end, we studied the factors affecting the formation of the individual segments, arrangement of these segments along the chain, as well as the uniformity of monomer sequences. Our research suggested new synthesis strategies to achieve a better structural gradient. It was also demonstrated that the uniformity of sequence patterns can be affected by the entire growth history of the chains, and thus strong control over the uniformity of chain growth is required throughout the reaction. In addition, the kinetic details of segment formation in living radical polymerization (LRP) were also investigated. It was demonstrated for the first time that the classic theory describing the distribution of instantaneous segment lengths developed for conventional free radical polymerization (FRP) does not always hold in LRP. Due to the complexity in the formation of monomer sequence, it is necessary to involve programmed synthesis for making gradient copolymers with well-defined sequence. Based on the understanding of sequence formation, we have developed a design tool to generate synthesis recipes according to targeted copolymers. With the aid of this technique, it is possible to tune monomer sequence along the chain more precisely.