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Density functional theory, DFT, PBE1PBE functional and 6-31+G(d,p) basis set in Gaussian 03 software were used in order to determine the reactivity order of the R group in RAFT agents used in the radical addition-fragmentation transfer polymerization, through the evaluation of reactivity parameters such as: global and local electronegativity, hardness, softness, and philicity. It was found that the reactivity order is governed by both the number and the composition of the substituent group (primary, secondary or tertiary); that is, the larger those parameters are the larger factors like steric hindrance, polar effects and electronic interchanges are, which favors the breaking of the C-S bond from the adduct radical, permitting the exit of the leaving radical and allowing, as a consequence, the fragmentation step in RAFT polymerization. Trisubstituted dithioesters with structure S = C(Z)S-R, where Z = Phenyl and R = C(CH(3))(2)CONH(2), C(CH3)2Ph or (CH(3))(2)C(6)H(8)OCH(3), in accordance with the previously exposed, presented the most favorable reactivity parameters.

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.

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