Side Reactions of Nitroxide-Mediated Polymerization: Nâ‹TMO versus Oâ‹TMC Cleavage of Alkoxyamines

Free energies for the homolysis of the NO−C and N−OC bonds were compared for a large number of alkoxyamines at 298 and 393 K, both in the gas phase and in toluene solution. On this basis, the scope of the N−OC homolysis side reaction in nitroxide-mediated polymerization was determined. It was found that the free energies of NO−C and N−OC homolysis are not correlated, with NO−C homolysis being more dependent upon the properties of the alkyl fragment and N−OC homolysis being more dependent upon the structure of the aminyl fragment. Acyclic alkoxyamines and those bearing the indoline functionality have lower free energies of N−OC homolysis than other cyclic alkoxyamines, with the five-membered pyrrolidine and isoindoline derivatives showing lower free energies than the six-membered piperidine derivatives. For most nitroxides, N−OC homolysis is normally favored above NO−C homolysis only when a heteroatom that is α to the NOC carbon center stabilizes the NO−C bond and/or the released alkyl radical is not sufficiently stabilized. As part of this work, accurate methods for the calculation of free energies for the homolysis of alkoxyamines were determined. Accurate thermodynamic parameters to within 4.5 kJ mol−1 of experimental values were found using an ONIOM approximation to G3(MP2)-RAD combined with PCM solvation energies at the B3-LYP/6-31G(d) level. Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Physical Chemistry A 114 (2010): 10458, doi:10.1021/ jp1064165. Copyright 2010 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/532 Side Reactions of Nitroxide-Mediated Polymerization: N-O versus O-C Cleavage of Alkoxyamines Jennifer L. Hodgson,† Luke B. Roskop,‡ Mark S. Gordon,‡ Ching Yeh Lin,† and Michelle L. Coote*,† Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National UniVersity, Canberra, ACT 0200, Australia, and Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed: July 12, 2010; ReVised Manuscript ReceiVed: August 18, 2010 Free energies for the homolysis of the NO-C and N-OC bonds were compared for a large number of alkoxyamines at 298 and 393 K, both in the gas phase and in toluene solution. On this basis, the scope of the N-OC homolysis side reaction in nitroxide-mediated polymerization was determined. It was found that the free energies of NO-C and N-OC homolysis are not correlated, with NO-C homolysis being more dependent upon the properties of the alkyl fragment and N-OC homolysis being more dependent upon the structure of the aminyl fragment. Acyclic alkoxyamines and those bearing the indoline functionality have lower free energies of N-OC homolysis than other cyclic alkoxyamines, with the five-membered pyrrolidine and isoindoline derivatives showing lower free energies than the six-membered piperidine derivatives. For most nitroxides, N-OC homolysis is normally favored above NO-C homolysis only when a heteroatom that is R to the NOC carbon center stabilizes the NO-C bond and/or the released alkyl radical is not sufficiently stabilized. As part of this work, accurate methods for the calculation of free energies for the homolysis of alkoxyamines were determined. Accurate thermodynamic parameters to within 4.5 kJ mol-1 of experimental values were found using an ONIOM approximation to G3(MP2)-RAD combined with PCM solvation energies at the B3-LYP/6-31G(d) level.