Steric and Polar Effects of the Cyclic Nitroxyl Fragment on the C−ON Bond Homolysis Rate Constant

Fischer, Hanns; Krämer, Andreas; Marque, Sylvain R. A.; Nesvadba, Peter

doi:10.1021/ma0512612

Cited by 62 publications

(73 citation statements)

References 77 publications

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“…On the other hand, the plot of log k c vs. E s (Fig. 3, Table 1) is linear for 3, 5, 8 -10, 13, and 16, as already reported [31]. The downward deviations for nitroxide (Fig.…”

supporting

confidence: 83%

“…As already observed, k c increases only very little with the temperature [22] [23] [30]. For 11, 12, and 14, for which the steric effect is expected to be constant, k c decreases weakly along the series in parallel with the decreasing electron-withdrawing capacity [31] of the substituent (s I;MeCHCðOÞCH 2 = 0.10, s I;MeCHCHðOAcÞCH 2 = 0.04, s I;MeCHCHðOHÞCH 2 = 0.02), in line with the observations done for 17 and 18.…”

supporting

confidence: 51%

“…k ) Measured in MeCN, k c = 15.0 · 10 7 M À1 s À1 . l ) It was assumed [31] that s I;ðPhCH2 ÞðMeÞNCO % s I;Me2NCO = 0.28. m ) It was assumed that the spiro-cyclohexane rings in positions a,a' were not more sterically demanding than the Me group due to the peculiar conformation of the ring generated by the presence of two sp 2 C-atoms [31]. n ) Measured in MeCN, k c = 9.8 · 10…”

mentioning

confidence: 99%

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Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

Fischer

Marque²,

Nesvadba

2006

Helvetica Chimica Acta

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Dedicated to Professor Hanns Fischer for his invaluable contribution in the fields of nitroxide-mediated polymerization and radical chemistryIn nitroxide-mediated radical polymerization, the polymerization times decrease with the increasing re-formation rate constant of the CÀON bond (! alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re-formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re-formation rate constant k c increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re-formation than for the homolysis of the CÀON bond, and may be mainly attributed to the relocation of the odd electron onto the O-atom of the NÀO moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of k c by combining both the polar/stabilization (s I ) and steric effects (E , nitroxide-mediated radical polymerization (NMP) has often been used for the synthesis of living polymers and copolymers with narrow polydispersities and controlled molecular masses and architecture [4 -6]. The mechanism of NMP outlined in Scheme 1 involves the cleavage of capped chain molecules (dormant species) into propagating alkyl and persistent nitroxide radicals (k d ). The propagating radicals grow by monomer addition (k p ) and form longer dormant chains by cross-coupling with the nitroxide radicals (k c ). Simultaneously, the usual self-termination (k t ) of the propagating radicals into unreactive polymer products takes place. This removes the propagating radicals and increases the concentration of nitroxide radicals over time; the cross-coupling reaction (k c ), therefore, dominates over self-termination [7] [8]. For an extended time range, a quasi-equilibrium of the reversible cleavage exists which is characterized by weakly time-dependent radical concentrations and a large excess of the persistent radicals over the propagating radicals.For the well-controlled and living radical polymerization of a given monomer with propagation and termination rate constants k p and k t , the rate constants of the reversible cleavage k d and k c must fall into proper ranges [7 -9]. In general, one aims at rela-

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“…On the other hand, the plot of log k c vs. E s (Fig. 3, Table 1) is linear for 3, 5, 8 -10, 13, and 16, as already reported [31]. The downward deviations for nitroxide (Fig.…”

supporting

confidence: 83%

supporting

confidence: 51%

mentioning

confidence: 99%

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Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

Fischer

Marque²,

Nesvadba

2006

Helvetica Chimica Acta

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“…Replacement of two Me groups by two Et groups (! 10) results in a decrease of the activation energy by 10 kJ/mol [23]. As reported in the present paper, replacement of all four Me groups by Et groups leads to a further decrease of the activation energy by 3 kJ/mol.…”

supporting

confidence: 82%

A New Sterically Highly Hindered 7‐Membered Cyclic Nitroxide for the Controlled Living Radical Polymerization

Chang

Siegenthaler

Studer

2006

Helvetica Chimica Acta

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In memoriam Professor Hanns FischerThe synthesis of a new sterically highly hindered 7-membered alkoxyamine, 2,2,7,7-tetraethyl-1-(1-phenylethoxy)-1,4-diazepan-5-one (4), starting from known 2,2,6,6-tetraethyl-1-(1-phenylethoxy)piperidin-4-one (3) via a Beckmann-type rearrangement is presented. It is shown that ring-enlargement by insertion of an NH moiety in going from 3 to 4 leads to a more efficient regulator for nitroxide-mediated controlled living radical styrene (= ethenylbenzene) and butyl acrylate (= butyl prop-2-enoate) polymerization. In addition to the polymerization experiments, kinetic data on the reversible CÀO bond homolysis of alkoxyamines 3 and 4 are presented. . In NMP, chain-growing polymer radicals and the corresponding nitroxides are reversibly formed via clean thermal CÀO bond homolysis. As example, a 2,2,6,6-tetramethylpiperidin-1-yloxy radicalmediated (TEMPO-mediated) styrene (= ethenylbenzene) polymerization is presented in Scheme 1. Importantly, the equilibrium between the dormant polymeric alkoxyamine and the polymeric radical and nitroxide, respectively, lies far on the left side. Hence, a low concentration of radicals is ensured during the entire polymerization process. Consequently, irreversible terminations via polymer radical dimerization and disproportionation processes are suppressed due to low radical concentrations [4a].In NMP, alkoxyamines are applied as initiators/regulators [1]. In the early work, TEMPO was used as nitroxide component to regulate radical polymerization of styrene and styrene derivatives. Unfortunately, controlled NMP of acrylates (= prop-2-enoates) could not be achieved with TEMPO as a regulator [1c]. However, in the meantime, a few reports on successful acrylate polymerizations in the presence of cyclic and noncyclic alkoxyamines have appeared [5 -8]. Hawker and Braslau showed that readily pre-

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“…[5-8, 14, 16, 17] The homolysis activation energy (E a ) was found to be a good approximation of the CÀON bond dissociation energy (BDE) of alkoxyamines. [18,19] The CÀON bond strength depends on various effects (anomeric, [20,21] anchimeric, [22] polar, [23,25] stabilization, [12,18,[22][23][24][25] steric, H-bonding [21,26] ), which are related to the structure of both the alkyl and the nitroxyl fragments. [27] The application of NMP in aqueous media (polymerization in emulsions and miniemulsions) requires the design of new alkoxyamines to be used as initiators/controllers in order to reduce environmental toxicity.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Carboxylate Salt on the CON Bond Homolysis of SG1‐Based Alkoxyamines

et al. 2008

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Alkoxyamines and persistent nitroxides are important regulators of the nitroxide mediated radical polymerization (NMP). Since the polymerization time decreases with increasing values of the homolysis rate constant (kd) for the C--ON bond between the polymer chain and the nitroxide moiety, the factors influencing kd are of considerable interest. Environmentally friendly polymerization methods, such as NMP in emulsion medium, have now been developed. The success of the polymerization depends not only on the strength of the C--ON bond but also on the solubility of the initiator/controller alkoxyamines, which control the nucleation. Various salts of SG1-based alkoxyamines (S+-OOCCMeH-SG1) labelled 1 a-f, where S+=Li+ (a), Na+ (b), K+ (c), nBu3HN+ (d), NH4+ (e), and nBu4N+ (f) are prepared, all of which are soluble in most organic solvents (e.g. pentane, dichlomethane, benzene, ethanol, etc.) as well as in water. Their kd values, which are measured in tert-butylbenzene, reveal unexpected behaviors depending on the diastereoisomers. For the RR/SS diatereoisomers (i.e. slowly homolyzing isomers), kd was not found to be sensitive to any of the salts, whereas for the RS/SR diastereoisomers (i.e. quickly homolyzing isomers), kd is observed to decrease to a greater or lesser extent depending on the type of salt. The conformations of the diastereoisomers of several salts are determined by means of DFT calculations, and the orbital interactions are studied by natural bond orbital (NBO) analysis.

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Steric and Polar Effects of the Cyclic Nitroxyl Fragment on the C−ON Bond Homolysis Rate Constant

Cited by 62 publications

References 77 publications

Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

A New Sterically Highly Hindered 7‐Membered Cyclic Nitroxide for the Controlled Living Radical Polymerization

Effect of the Carboxylate Salt on the CON Bond Homolysis of SG1‐Based Alkoxyamines

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