Linear-free energy relationships for modeling structure-reactivity trends in controlled radical polymerization

Abstract

A 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 θ[R] - 9.73 IP[R] - 1.10 RSE[R] + 192 θ[X] + 57.4 EA[X] - 62.0 Resonance[X] - 250, where the steric descriptors θ[R] and θ[X] are measured as Tolman's cone angle of Cl-R and CH3-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. Show more