Dependence of Thermal Stability on Molecular Structure of RAFT/MADIX Agents: A Kinetic and Mechanistic Study
The thermal decomposition of different classes of RAFT/MADIX agents, namely dithioesters, trithiocarbonates, xanthates, and dithiocarbamates, were investigated through heating in solution. It was found that the decomposition behavior is complicated interplay of the effects of stabilizing Z-group and leaving R-group. The mechanism of the decomposition is mainly through three pathways, i.e., β-elimination, α-elimination, and homolysis of dithiocarbamate (particularly for universal RAFT agent). The most important pathway is the β-elimination of thiocarbonylthio compounds possessing β-hydrogen, leading to the formation unsaturated species. For the leaving group containing solely α-hydrogen, such as benzyl, α-elimination takes place, resulting in the formation of (E)-stilbene through a carbene intermediate. Homolysis occurs specifically in the case of a universal RAFT agent, in which a thiocarbonyl radical and an alkylthio radical are generated, finally forming thiolactone through a radical process. The stabilities of the RAFT/MADIX agents are investigated by measuring the apparent kinetics and activation energy of the thermal decomposition reactions. Both Z-group and R-group influence the stability of the agents through electronic and steric effects. Lone pair electron donating heteroatoms of Z-group show a remarkable stabilizing effect while electron withdrawing substituents, either in Z- or R-group, tends to destabilize the agent. In addition, bulkier or more β-hydrogens result in faster decomposition rate or lower decomposition temperature. Thus, the stability of the RAFT/MAIDX agents decreases in the order where R is (with identical Z = phenyl) −CH2Ph (5) > −PS (PS-RAFT 15) > −C(Me)HPh (2) > −C(Me)2C(═O)OC2H5 (7) > −C(Me)2Ph(1) > −PMMA (PMMA-RAFT 16) > −C(Me)2CN (6). For those possessing identical leaving group such as 1-phenylethyl, the stability decreases in the order of O-ethyl (11) > –N(CH2CH3)2 (13) > –SCH(CH3)Ph (8) > −Ph (2) > −CH2Ph (4) > −PhNO2 (3). These results consort with the chain transfer acitivities measured by the CSIRO group and agree well with the ab initio theoretical results by Coote. In addition, the difference between thermal stabilities of the universal RAFT agents at neutral and protonated states has also been demonstrated.
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