Radical Stabilization Energies Research Articles

Why Triple Bonds Protect Acenes from Oxidation and Decomposition

An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen ((1)O(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 × 10(5) s(-1) M(-1). This unexpectedly slow reaction is a result of a physical deactivation of (1)O(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.

ChemInform Abstract: Radical Stability — Thermochemical Aspects

AbstractReview: 89 refs.

Interaction between alkyl radicals and single wall carbon nanotubes

The addition of primary, secondary, and tertiary alkyl radicals to single wall carbon nanotubes (SWCNTs) was studied by means of dispersion corrected density functional theory. The PBE, B97-D, M06-L, and M06-2X functionals were used. Consideration of Van der Waals interactions is essential to obtain accurate addition energies. In effect, the enthalpy changes at 298 K, for the addition of methyl, ethyl, isopropyl, and tert-butyl radicals onto a (5,5) SWCNT are: -25.7, -25.1, -22.4, and -16.6 kcal/mol, at the M06-2X level, respectively, whereas at PBE/6-31G* level they are significantly lower: -25.0, -19.0, -16.7, and -5.0 kcal/mol respectively. Although the binding energies are small, the attached alkyl radicals are expected to be stable because of the large desorption barriers. The importance of nonbonded interactions was more noticeable as we moved from primary to tertiary alkyl radicals. Indeed, for the tert-butyl radical, physisorption onto the (11,0) SWCNT is preferred rather than chemisorption. The bond dissociation energies determined for alkyl radicals and SWCNT follow the trend suggested by the consideration of radical stabilization energies. However, they are in disagreement with some degrees of functionalization observed in recent experiments. This discrepancy would stem from the fact that for some HiPco nanotubes, nonbonded interactions with alkyl radicals are stronger than covalent bonds.

Photolysis of ortho-nitrobenzylic derivatives: the importance of the leaving group

Quantum yields for the photoinduced release of seven different commonly used leaving groups (LGs) from the o-nitroveratryl protecting group were measured. It was found that these quantum yields depend strongly on the nature of the LGs. We show that the quantum efficiency with which the LGs are released correlates with the stabilization that these LGs provide to o-nitrobenzyl-type radicals because radical stabilizing groups weaken the C-H bond that is cleaved in the photoinduced hydrogen atom transfer step, and hence lower the barrier for this process. At the same time these substituents lower the endothermicity of the thermal hydrogen atom transfer and thus increase the barrier for the reverse process, thereby enhancing the part of the initially formed aci-nitro intermediates which undergo cyclization (which ultimately leads to LG release). Radical stabilization energies computed by DFT methods are thus a useful predictor of the relative efficiency with which LGs are photoreleased from o-nitrobenzyl protecting groups.

Computational study of radicals derived from hydroxyurea and its methylated analogues

Structural and electronic properties and chemical fate of free radicals generated from hydroxyurea (HU) and its methylated analogues N-methylhydroxyurea (NMHU) and O-methylhydroxyurea (OMHU) are of utmost importance for their biological and pharmacological effects. In this work the cis/trans conformational processes, tautomerizations, and intramolecular hydrogen and methyl migrations in hydroxyurea-derived radicals have been considered. Potential energy profiles for these reactions have been calculated using two DFT functionals (BP86 and B3LYP) and two composite models (G3(MP2)RAD and G3B3). Solvation effects have been included both implicitly (CPCM) and explicitly. It has been shown that calculated energy barriers for free radical rearrangements are significantly reduced when a single water molecule is included in calculations. In the case of HU-derived open-shell species, a number of oxygen-, nitrogen-, and carbon-centered radicals have been located, but only the O-centered radicals (e1 and z1) fit to experimental isomeric hyperfine coupling constants (hfccs) from EPR spectra. The reduction of NMHU and OMHU produces O-centered and N-centered radicals, respectively, with the former being more stable by ca. 60 kJ mol(-1). The NMHU-derived radical e4 undergoes rearrangements, which can result in formation of several conceivable products. The calculated hfccs have been successfully used to interpret the experimental EPR spectra of the most probable rearranged product 10. Reduction potentials of hydroxyureas, radical stabilization energy (RSE) and bond disocciation energy (BDE) values have been calculated to compare stabilities and reactivities of different subclasses of free radicals. It has been concluded, in agreement with experiment, that reductions of biologically relevant tyrosyl radicals by HU and NMHU are thermochemically favorable processes, and that the order of reactivity of hydroxyureas follows the experimentally observed trend NMHU > HU > OMHU.

Spectroscopy and thermochemistry of a jet-cooled open-shell polyene: 1,4-pentadienyl radical

The 1,4-pentadienyl (vinylallyl) radical has been observed for the first time by optical spectroscopy. An excitation spectrum is recorded on m/z 67 by resonant two-color two-photon ionization spectroscopy. Several bands are observed with the origin transition identified at 19 449 cm(-1). The spectrum is assigned by a comparison with ab initio frequencies calculated at the CASPT2/cc-pVTZ level of theory, with an accompanying Franck-Condon calculation of the excitation spectrum, including Dushinsky mixing. The b(1) and a(2) outer C-C bond torsional modes are calculated to halve in frequency upon electronic excitation, bringing about their appearance in the excitation spectrum. This can be readily understood by considering the torsional sensitivity of the frontier molecular orbital energies. High-level quantum chemical calculations of the radical stabilization energy, resulting in a value of nearly 120 kJ mol(-1), provide quantitative confirmation that this radical is highly stabilized.

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

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 descrip- tors 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)dS) 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 nal equation is BDFE(R! X)=! 20.8! (R)! 9.73 IP(R)! 1.10 RSE(R) +192! (X) + 57.4 EA(X)! 62.0 Resonance(X)! 250, wherethe steric descriptors! (R)and! (X)aremeasuredasTolman'sconeangleofCl! RandCH3! Xrespectively,thepolardescriptorsIP(R)and EA(X) are the (gas-phase) ionization energy of Rand electron a# nity of Xrespectively, and the radical stability or resonance descriptors RSE(R) and Resonance(X) are measured as the standard radical stabilization energy for Rand the inverse HOMO! LUMO energy gap for X • . This general model was also tted to the individual cases of ATRP, RAFT, and NMP and was used to analyze similarities and di! erences in structure! reactivity trends among the di! erent 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.

NH and NCl homolytic bond dissociation energies and radical stabilization energies: An assessment of theoretical procedures through comparison with benchmark‐quality W2w data

AbstractThe performance of a large variety of contemporary density functional theory (DFT), double‐hybrid DFT, and high‐level Gaussian‐n (Gn) procedures has been evaluated for the calculation of bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with NX bonds (X = H, Cl). The chosen set of 62 NX systems (31 NH and 31 NCl) span a wide range of biologically relevant species. As reference values, we used benchmark‐quality W2w data that we recently obtained as part of a systematic thermochemical study of substituent effects in these systems. Of the Gn schemes, the modified G4 procedures (G4‐5H and G4(MP2)‐6X) perform somewhat better than the corresponding standard G4 procedures for the NX BDEs of these systems. For the NH RSEs, G3X, G3X(MP2), G3X(MP2)‐RAD, G4‐5H, and G4(MP2)‐6X emerge as excellent performers, with mean absolute deviations (MADs) from the benchmark W2w values of 0.9–1.4 kJ mol–1. However, for the NCl RSEs, G4 is the best performer, with an MAD of 1.7 kJ mol–1. The BDEs of both NH and NCl bonds represent a challenge for DFT procedures. In particular, only a handful of functionals (namely, B3P86, M05‐2X, M06‐2X, and ROB2‐PLYP) perform well, with MADs ≤ 4.5 kJ mol−1 for both bond types. Nearly all of the considered DFT procedures perform significantly better for the computation of RSEs, due to a significantly larger degree of error cancelation compared with the BDEs. For the RSEs, BH&HLYP, M05‐2X, M06, M06‐2X, BMK, PBE0, B2‐PLYP, B2GP‐PLYP, B2T‐PLYP, and ROB2‐PLYP are the best performers, with MADs ≤ 4.2 kJ mol−1. Reliable values of NH and NCl BDEs may be obtained by using the RSEs calculated by these functionals in conjunction with a thermochemical cycle involving an experimental (or high‐level theoretical) BDE for the H2NH or H2NCl bond. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Effect of Substituents on the Strength of N–X (X = H, F, and Cl) Bond Dissociation Energies: A High-Level Quantum Chemical Study

The effect of substituents on the strength of N-X (X = H, F, and Cl) bonds has been investigated using the high-level W2w thermochemical protocol. The substituents have been selected to be representative of the key functional groups that are likely to be of biological, synthetic, or industrial importance for these systems. We interpreted the effects through the calculation of relative N-X bond dissociation energies (BDE) or radical stabilization energies (RSE(NX)). The BDE and RSE(NX) values depend on stabilizing/destabilizing effects in both the reactant molecule and the product radical of the dissociation reactions. To assist us in the analysis of the substituent effects, a number of additional thermochemical quantities have been introduced, including molecule stabilization energies (MSE(NX)). We find that the RSE(NH) values are (a) increased by electron-donating alkyl substituents or the vinyl substituent, (b) increased in imines, and (c) decreased by electron-withdrawing substituents such as CF(3) and carbonyl moieties or through protonation. A different picture emerges when considering the RSE(NF) and RSE(NCl) values because of the electronegativities of the halogen atoms. The RSE(NX)s differ from the RSE(NH) values by an amount related to the stabilization of the N-halogenated molecules and given by MSE(NX). We find that substituents that stabilize/destabilize the radicals also tend to stabilize/destabilize the N-halogenated molecules. As a result, N-F- and N-Cl-containing molecules that include alkyl substituents or correspond to imines are generally associated with RSE(NF) and RSE(NCl) values that are less positive or more negative than the corresponding RSE(NH). In contrast, N-F- and N-Cl-containing molecules that include electron-withdrawing substituents or are protonated are generally associated with RSE(NF) and RSE(NCl) values that are more positive or less negative than the corresponding RSE(NH).

On the Advantages of Hydrocarbon Radical Stabilization Energies Based on R−H Bond Dissociation Energies

Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R-H vs CH(3)-H bond dissociation energies have inherent advantages over RSEs based on R-CH(3) vs CH(3)-CH(3), as well as R-R vs CH(3)-CH(3) comparisons, since the R-CH(3) and R-R reference systems are prone to unbalanced contaminating intramolecular interactions involving the R groups. When the effects of steric crowding, branching, protobranching, conjugation, and hyperconjugation are taken into account, R-CH(3) and R-R based RSE values are nearly identical to R-H RSEs. Corrections for electronegativity differences between H and R are not needed to achieve agreement.

O - H Bond Dissociation Energies

High-level composite, ab initio and density functional theory (DFT) procedures have been employed to study O–H bond dissociation energies (BDEs), as well as radical stabilization energies (RSEs) in the oxygen-centred radicals that are formed in the dissociation of the O–H bonds. Benchmark values are provided by Wn results up to W3.2 and W4.x. We are able to recommend revised BDE values for FO–H (415.6 ± 3 kJ mol–1), MeC(O)O–H (459.8 ± 6 kJ mol–1) and CF3CH2O–H (461.9 ± 6 kJ mol–1) on the basis of high-level calculations. We find that Gn-type procedures are generally reliable and cost-effective, and that some contemporary functionals and double-hybrid DFT procedures also provide adequate O–H BDEs/RSEs. We note that the variations in the O–H BDEs are associated with variations in the stabilities of not only the radicals but also the closed-shell precursor molecules. Most substituents destabilize both species, with σ-electron-withdrawing groups having larger destabilizing effects, while π-electron acceptors are stabilizing. Although there is little correlation between the stabilizing/destabilizing effects of the substituents and the RSEs, we present some general patterns in the RSEs that emerge from the present study.

Effect of substituents on the stabilities of multiply-substituted carbon-centered radicals

The bond dissociation energies (BDEs) and radical stabilization energies (RSEs) which result from 166 reactions that lead to carbon-centered radicals of the type ˙CH(2)X, ˙CHXY and ˙CXYZ, where X, Y and Z are any of the fourteen substituents H, F, Cl, NH(2), OH, SH, CH[double bond, length as m-dash]CH(2), C[triple bond, length as m-dash]CH, BH(2), CHO, COOH, CN, CH(3), and CF(3), were calculated using spin-restricted and -unrestricted variants of the double-hybrid B2-PLYP method with the 6-311+G(3df,2p) basis set. The interactions of substituents X, Y, and Z in both the radicals (˙CXYZ) and in the precursor closed-shell molecules (CHXYZ), as well as the extent of additivity of such interactions, were investigated by calculating radical interaction energies (RIEs), molecule interaction energies (MIEs), and deviations from additivity of RSEs (DARSEs) for a set of 152 reactions that lead to di- (˙CHXY) and tri- (˙CXYZ) substituted carbon-centered radicals. The pairwise quantities describing the effects of pairs of substituents in trisubstituted systems, namely pairwise MIEs (PMIEs), pairwise RIEs (PRIEs) and deviations from pairwise additivity of RSEs (DPARSEs), were also calculated for the set of 61 reactions that lead to trisubstituted radicals (˙CXYZ). Both ROB2-PLYP and UB2-PLYP were found to perform quite well in predicting the quantities related to the stabilities of carbon-centered radicals when compared with available experimental data and with the results obtained from the high-level composite method G3X(MP2)-RAD. Particular selections of substituents or combinations of substituents from the current test set were found to lead to specially stable radicals, increasing the RSEs to a maximum of +68.2 kJ mol(-1) for monosubstituted radicals ˙CH(2)X (X = CH[double bond, length as m-dash]CH(2)), +131.7 kJ mol(-1) for disubstituted radicals ˙CHXY (X = NH(2), Y = CHO), and +177.1 kJ mol(-1) for trisubstituted radicals ˙CXYZ (X = NH2, Y = Z = CHO).

The spectroscopy and thermochemistry of phenylallyl radical chromophores

The resonant two-color two-photon ionization and laser induced fluorescence excitation spectra of the 1-phenylallyl (cinnamyl) and inden-2-ylmethyl radicals are reported. The 1-phenylallyl radical is found to fluoresce with low yield, permitting only a coarse dispersed fluorescence spectrum, while the inden-2-ylmethyl radical yields sufficient fluorescence to obtain ground-state vibrational frequencies and two-dimensional fluorescence spectra. Computed ionization energies and thermochemical properties including radical stabilization energies are reported for a range of resonance-stabilized radicals, including the phenylpropargyl, vinylpropargyl and phenylallyl radicals.

The Radical Stabilization Energy of a Substituted Carbon-Centered Free Radical Depends on Both the Functionality of the Substituent and the Ordinality of the Radical

Chemical intuition suggests that the stabilization of a carbon-centered free radical by a substituent X would be the greatest for a prim and least for a more stable tert radical because of "saturation". However, analysis of a comprehensive recent set of bond dissociation energies computed by Coote and co-workers (Phys. Chem. Chem. Phys. 2010, 12, 9597) and transformed into radical stabilization energies (RSE) suggests that this supposition is often violated. The RSE for a given X depends not only on the nature of X but also on the ordinality (i.e., prim, sec, or tert) of the radical onto which it is substituted. For substituents that stabilize by electron delocalization but also contain electron-withdrawing centers, such as the carbonyl function, the stabilization of XCMe(2)(•) compared with HCMe(2)• is greater than that for XCH(2)• compared with HCH(2)•. However, for substituents that stabilize by lone-pair electron donation, such as N or O centers, the order is strongly reversed. This contrast can be qualitatively rationalized by considering charge-separated VB contributors to the radical structure (R(2)C(+)-X(-•) and R(2)C(-)-X(+•)) and the contrasting effects of methyl substituents on them. This conclusion is not dependent on the particular definition used for RSE.

Shortcomings of Basing Radical Stabilization Energies on Bond Dissociation Energies of Alkyl Groups to Hydrogen

Stabilization energies (SE(H)) of carbon radicals (R(*)) are traditionally defined as the difference between the bond dissociation energy (BDE) of CH(3)-H, as a reference point, and of R-H. The term "stabilization energy" implies that it is an intrinsic property of the radical and a quantitative measure of stability. Applicable only to carbon-centered radicals, SE(H) stabilization energies are not transferable and cannot be used to estimate carbon-carbon BDE[R-R'], symmetrical BDE[R-R], or any other BDE[R-X]. SE(H) values by themselves are neither an intrinsic property nor a quantitative measure of stability. There is available an alternative that is not limited only to carbon-carbon and carbon-hydrogen bonds, does not depend on any one particular molecule or BDE as a reference point, and is accurate with several hundred different types of bonds.

Borane–Lewis Base Complexes as Homolytic Hydrogen Atom Donors

Radical stabilization energies (RSE)s have been calculated for a variety of boryl radicals complexed to Lewis bases at the G3(MP2)-RAD level of theory. These are referenced to the B-H bond dissociation energy (BDE) in BH(3) determined at W4.3 level. High RSE values (and thus low BDE(B-H) values) have been found for borane complexes of a variety of five- and six-membered ring heterocycles. Variations of RSE values have been correlated with the strength of Lewis acid-Lewis base complex formation at the boryl radical stage. The analysis of charge- and spin-density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.

ChemInform Abstract: The EPR D Parameter as a Measure of Spin Delocalization in Cyclopentane-1,3-diyl Triplet Diradicals with Extended Conjugation π-Type Substituents.

π-Substituted cyclopentane-1,3-diyl triplet diradicals 2 were readily prepared from the corresponding azoalkanes 1 by photodenitrogenation in a 2-methyltetrahydrofuran matrix at 77 K. The D values of these triplet diradicals, obtained from the EPR spectra under matrix isolation, depend on the π substituent in the order butadienyl > 2-anthryl > vinyl > 2-naphthyl > phenyl. Good linear correlations of the D values have been obtained with the reported hyperfine coupling constants (α-hfc), with the ab initio (B3LYP) spin densities for the corresponding monoradicals 3, and with the radical stabilization energies (RSE). The present results for these extended π systems are compared with those for previously reported heteroaryl-substituted triplet diradicals.

Linear Free-Energy Relationships for the Alkyl Radical Affinities of Nitroxides: A Theoretical Study

High-level ab initio calculations have been used to construct linear free-energy relationships describing the kinetics and thermodynamics of the combination and dissociation reactions between alkyl radicals and nitroxides in terms of easily accessible parameters that quantify the electronic, steric and radical stabilization characteristics of the coreactants. For the gas-phase equilibrium constant (Keq = kc/kd) of the combination reaction at 298 K, the following equation was obtained: log (Keq) = −0.10IP − 0.177RSE − 0.130RSEnxd + 38.3. In this equation, IP is the vertical ionization potential of the alkyl radical, RSE is the standard radical stabilization energy the alkyl radical, while RSEnxd is a new descriptor for the nitroxide radical, related to the standard radical stabilization energy, but measuring in this case the flexibility of the nitroxide to the geometric changes associated with formation of an alkoxyamine. The equation was successful for combinations of substituents not included in the original fitting and can thus be used to predict the behavior for larger systems for which direct calculation is impractical. Similar equations were also fitted to available experimental data for kc, at 298 K and kd, at 393 K, both in tert-butyl benzene, to allow the prediction of rate constants. The equation-determined rate constants, kc,eq and kd,eq are given by log (kc,eq) = −0.408IP − 0.0597RSE − 0.103RSEnxd + 14.5 and log (kd,eq) = 0.794IP + 5.68θ + 0.0873RSE + 0.0821RSEnxd − 27.7. For the decomposition rate, an additional parameter, Tolman’s cone angle θ, which measures the steric bulk of the attacking alkyl radical, was found to improve the fit to the data. The equations could in principle be fitted to experimental or calculated rate and equilibrium constants under a variety of reaction conditions. On the basis of our analysis, it appears that the stability of the alkyl radical has the largest effect on the kinetics and thermodynamics of the combination and dissociation reactions, with smaller but significant contributions from the remaining parameters.

A comparison of methods for measuring relative radical stabilities of carbon-centred radicals

This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on corrected carbon-carbon bond energies, and more direct measures of the extent of radical delocalisation. As part of this work, a large set of R-H, R-CH(3), R-Cl and R-R BDEs (R = CH(2)X, CH(CH(3))X, C(CH(3))(2)X and X = H, BH(2), CH(3), NH(2), OH, F, SiH(3), PH(2), SH, Cl, Br, N(CH(3))(2), NHCH(3), NHCHO, NHCOCH(3), NO(2), OCF(3), OCH(2)CH(3), OCH(3), OCHO, OCOCH(3), Si(CH(3))(3), P(CH(3))(2), SC(CH(3))(2)CN, SCH(2)COOCH(3), SCH(2)COOCH(3), SCH(2)Ph, SCH(3), SO(2)CH(3), S(O)CH(3), Ph, C(6)H(4)-pCN, C(6)H(4)-pNO(2), C(6)H(4)-pOCH(3), C(6)H(4)-pOH, CF(2)CF(3), CF(2)H, CF(3), CCl(2)H, CCl(3), CH(2)Cl, CH(2)F, CH(2)OH, CH(2)Ph, cyclo-CH(CH(2))(2), CH(2)CH[double bond, length as m-dash]CH(2), CH(2)CH(3), CH(CH(3))(2), C(CH(3))(3), C[triple bond, length as m-dash]CH, CH[double bond, length as m-dash]CH(2), CH[double bond, length as m-dash]CHCH(3), CHO, CN, COCH(3), CON(CH(2)CH(3))(2), CONH(2), CONHCH(3), COOC(CH(3))(3), COOCH(2)CH(3), COOCH(3), COOH, COPh), and associated radical stability values are calculated using the high-level ab initio molecular orbital theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure-reactivity trends.

Formation and Interconversion of Organo-Cobalt Complexes in Reactions of Cobalt(II) Porphyrins with Cyanoalkyl Radicals and Vinyl Olefins

Observation of the formation and interconversion of organo-cobalt complexes ((TMP)Co-R) is used to reveal mechanistic features in the living radical polymerization (LRP) of methyl acrylate (MA) mediated by cobalt porphyrins. Both dissociative and associative exchange of radicals in solution with organo-cobalt complexes contribute to controlling the radical polymerization. The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion of organo-cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes. The dissociation equilibrium constant (K(d(333 K))) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO(2)CH(3))CH(2)P) was estimated as 1.15 x 10(-10) from analysis of the polymerization kinetics and (1)H NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (*C(CH(3))(2)CN) adding with monomer (k(1)) to the process of transferring a hydrogen atom to (TMP)Co(II)* (k(2)) was evaluated for the methyl acrylate system as 2 x 10(-3) which is larger than that for vinyl acetate LRP (9 x 10(-5)). Kinetic analysis places the rate constant for associative radical interchange (333 K) at approximately 7 x 10(5) M(-1) s(-1). The larger radical stabilization energy and lower energy of the singly occupied molecular orbital (SOMO) for methyl acrylate based radicals (*CH(CO(2)CH(3))CH(2)P) compared to vinyl acetate contribute to the observed prominence of organo-cobalt homolytic dissociation and much smaller chain transfer which result in substantially better control for living radical polymerization of methyl acrylate than that observed for vinyl acetate.