A DFT study of R-X bond dissociation enthalpies of relevance to the initiation process of atom transfer radical polymerization

DFT calculations at the B3P86/6-31G** level have been carried out to derive the bond dissociation energies (BDE) and free energies for a number of R-X systems (X = Cl, Br, I, N-3, and S2CNMe2) that have been or can potentially be used as initiators for atom transfer radical polymerization (ATRP). For selected systems, a conformational search was carried out for R-X and R by using semiempirical (PM3) and molecular mechanics (MM+ augmented with appropriately optimized parameters for the radical systems) methods. The MM+ technique is more suited to search for the most stable conformations. The computed energies are in good agreement with the experimentally available BDEs and reveal a small weakening effect caused by the substitution of an alpha-H atom with a CH3 group. The free energies are used to derive the relative equilibrium constant for the ATRP activation/deactivation process. These are compared with the equilibrium constants that have been determined from ATRP polymerization rates and from model studies of activation-deactivation-termination processes in the absence of monomer. These comparisons reveal the effectiveness of the DFT-computed BDEs for predicting polymerization rates for new monomers in ATRP processes.