The persistent radical effect: a principle for selective radical reactions and living radical polymerizations.

3.10 - Nitroxide-Mediated Polymerization

Kinetics of Living Radical Polymerization

The special of Chemical Review informs about studies conducted on single-electron transfer and single-electron transfer degenerative chain transfer living radical polymerization. Researchers have demonstrated that living radical polymerization (LRP) can be significantly effective for the synthesis of tailored polymers. The special issue aims at covering the latest progress in two related polymerization methodologies that rely on single-electron transfer (SET), single-electron transfer degenerative chain transfer living radical polymerization (SET-DTLRP), and single-electron transfer living radical polymerization (SET-LRP). It is demonstrated that SET-DTLRP proceeds through SET initiation and competition of SET activation, deactivation, and degenerative transfer (DT). SET-LRP proceeds exclusively through a SET initiation, activation, and deactivation. It is also revealed that the two techniques have arose from investigations into Cu-catalyzed LRP initiated with sulfonyl halides.

Single-Electron Transfer and Single-Electron Transfer Degenerative Chain Transfer Living Radical Polymerization

The successful exploitation of syntheses involving the generation of new carbon-carbon bonds by radical reactions rests on some prior knowledge of the rate constants for the addition of carbon-centered radicals to alkenes and other unsaturated molecules, and of the factors controlling them. Two former classical reviews in Angewandte Chemie by Tedder (1982) and by Giese (1983) provided mechanistic insight and led to various qualitative rules on the complex interplay of enthalpic, polar, and steric effects. In the meantime, the field has experienced very rapid progress: many more experimental absolute rate constants have become available, and there have been major advances in the efficiency and reliability of quantum-chemical methods for the accurate calculation of transition structures, reaction barriers, and reaction enthalpies. Herein we review this progress, recommend suitable experimental and theoretical procedures, and display representative data series for radical additions to alkenes. On this basis, and guided by the pictorial tool of the state-correlation diagram for radical additions, we then offer a new and more stringent quantification of the controlling factors. Our analysis leads to a partial revision of the previous qualitative rules, and it more clearly exhibits the interplay of the reaction enthalpy effects, polar charge-transfer contributions, and steric substituent effects on the reaction energy barrier. The various contributions are cast into the form of new, simple, and physically meaningful but non-linear, predictive equations for the preestimation of rate constants. These equations prove successful in several tests but call for additional theoretical and experimental foundation. The kinetics of related reactions such as polymer propagation, copolymerization, and the addition of radicals to alkynes and aromatic compounds is shown to follow the same principles.

Factors Controlling the Addition of Carbon‐Centered Radicals to Alkenes—An Experimental and Theoretical Perspective

Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from <2 × 105 M-1 s-1 to 2.3 × 109 M-1 s-1 and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.

Entropy control of the cross-reaction between carbon-centered and nitroxide radicals.

N-Alkoxyamines cleave into transient alkyl and persistent aminoxyl radicals which then combine and regenerate the parent compounds. Simultaneously, the alkyl species self-terminate, and this causes a continuous build-up of excess aminoxyl. Hence, the back-reaction to the alkoxyamine accelerates and the self-termination slows down in time. An analysis of this self-regulating Persistent Radical Effect shows that in the absence of scavengers the alkoxyamines decay and the aminoxyls appear according to unusual t1/3 rate laws governed by a rate constant combination. This is verified by studying the aminoxyl release from 2-phenyl-2-(2′,2′,6′,6′-tetramethylpiperidine-1′-oxyl)propane (cumyl–TEMPO). The rate constants of the individual reactions are also determined under isolation conditions and explain the behaviour of the combined reaction system. Mechanistic details of the reactions are given, and their relevance for ‘living’ radical polymerizations is pointed out.

Radical reaction kinetics during homolysis of N-alkoxyamines: verification of the persistent radical effect

Absolute rate constants for the addition of the highly nucleophilic 2-hydroxy-2-propyl radical to eight fast-reacting 1- and 1,1-disubstituted alkenes in MeOH at room temperature have been determined by laser flash photolysis. Also the absorption spectra of the 2-hydroxy-2-propyl and the benzylic and alkyl-type adduct radicals are presented. The rate constants were obtained using various methods for the analysis of the kinetic traces and support earlier findings.

Rate Constants for the Addition of 2-Hydroxy-2-propyl Radicals to Alkenes in Solution studied by laser flash photolysis

Use of Phosphonylated Nitroxides and Alkoxyamines in Controlled/"Living" Radical Polymerization

EPR and kinetic absorption spectroscopy experiments reveal the simultaneous formation of transient radicals and a persistent reduced tungsten radical species during the photoreaction of W10O324– with organic substrates in solution. It is shown that a special kinetic effect operates which causes the observed selective product formation. Theoretical predictions on the ratio of radical concentrations and on their time developments are confirmed. Moreover, some mechanistic aspects of the reactions are clarified.

Photoreactions of the decatungstate anion W10O324– with organic substrates in solution studied by EPR and kinetic absorption spectroscopy: an example for the persistent radical effect

Rainer Martschke

Papers

Entropy control of the cross-reaction between carbon-centered and nitroxide radicals.

Radical reaction kinetics during homolysis of N-alkoxyamines: verification of the persistent radical effect

Rate Constants for the Addition of 2-Hydroxy-2-propyl Radicals to Alkenes in Solution studied by laser flash photolysis

Use of Phosphonylated Nitroxides and Alkoxyamines in Controlled/"Living" Radical Polymerization

Photoreactions of the decatungstate anion W10O324– with organic substrates in solution studied by EPR and kinetic absorption spectroscopy: an example for the persistent radical effect