Theoretical evaluation of the order of reactivity of transfer agents utilized in RAFT polymerization: group Z

In the present study we report theoretical calculations, by means of density functional theory (DFT), for 28 transfer agents used in reversible addition-fragmentation chain transfer (RAFT) polymerization. Functional PBE1PBE and 3–21G* theory levels with Gaussian 03 software were used to determine the order of reactivity of RAFT agents through the evaluation of reactivity parameters such as global softness, global hardness and global philicity. It was found that the global softness of the agent was more favored when it contained benzyl or phenyl groups as the Z group, than in RAFT agents with Z groups based on oxygen, nitrogen, or sulfur. On the one hand, when the Z group is based on oxygen or nitrogen, the tendency to form zwitterionic bonds with the adjacent radical center is very high, causing reactivity reduction in these kinds of compounds (e.g., dithiocarbamates) in comparison with compounds that do not experience this type of event; on the other hand, with Z groups based on sulfur, two fragmentation paths are possible, which reduces the fragmentation rate since both Z and R can function as leaving groups. With this investigation we contribute to the understanding of RAFT-mediated polymerization mechanisms by proposing an order of reactivity based on evaluating the importance of the Z group.