Linear-Free Energy Relationships for Modeling Structure-Reactivity Trends in Controlled Radical Polymerization

International audienceA set of 303 R X bond dissociation free energies (BDFEs) at 298.15 K in acetonitrile, along with corresponding values of polar, steric and radical stability or resonance descriptors for each R-group and X-group, has been calculated at the G3(MP2)-RAD level of theory in conjunction with CPCM solvation energies. The R-groups were chosen to cover the broad spectrum of steric, polar and radical stability properties of propagating polymeric radicals, while the X-groups included a variety of nitroxides, dithioester fragments (*SC(Z)=S) and halogens, chosen to be representative of control agents used in nitroxide mediated polymerization (NMP), reversible addition fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). The data have been used to design, parametrize and test a linear free energy relationship that can predict the BDFEs of any R and X combination based on the polar, steric and radical stability or resonance properties of the separate R and X groups. The final equation is BDFE[R-X] = -20.8 theta[R] - 9.73 IP[R] - 1.10 RSE[R] + 192 theta[X] + 57.4 EA[X] - 62.0 Resonance [X] - 250, where the steric descriptors theta[R] and theta[X] are measured as Tolman's cone angle of Cl-Rand CH(3)-X respectively, the polar descriptors IP[R] and EA[X] are the (gas-phase) ionization energy of R* and electron affinity of X* respectively, and the radical stability or resonance descriptors RSE[R] and Resonance[X] are measured as the standard radical stabilization energy for R* and the inverse HOMO-LUMO energy gap for X*. This general model was also fitted to the individual cases of ATRP, RAFT, and NMP and was used to analyze similarities and differences in structure-reactivity trends among the different types of polymerization process. We show how the equation can be used to select appropriate initial leaving groups for a given polymerization, or predict the correct sequence of monomer addition in block copolymer synthesis