Monomer radical cation

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

Some electroinitiated polymerizations proceed via monomer radical-cations (VII) formed by electron transfer... 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

The polymerizations involved as a first step the formation of the monomer radical cations which underwent rapidly radical dimerization reactions to produce dicationic protonated TTF derivatives. The dicationic intermediates deprotonated slowly to stable vinylogous TTF polymers. This new syn-... 

Photo-CIDNP experiments using anthraquinones as photosensitizers for oxidation of a variety of uracil- and thymine-derived cyclobutane dimers, e.g. c,s-l, t,s-l, c,a-1, t,a-l, and c,s-2, 4, and 5, demonstrated the existence of both Pyr +oPyr and its dissociation product, the monomer radical cation Pyr + [6, 7]. 

A relatively sharp absorption in the UV region due to alkyl radicals is observed in electron-pulse irradiated alkanes [93]. It has an absorption maximum at 240 nm in n-dodecane and cyclohexane. (Mehnart et al. did not see this absorption maximum but found another short-lived absorption band peaking at 270 nm in n-hexane, n-heptane, and n-hexadecane containing 10 mmol dm-3 CC14. This absorption band was assigned to olefin monomer radical-cation... 

We propose that this trimer forms from coupling of the 3,3 -dimer radical cation and a monomer radical cation. (The electron density in the monomer radical cation is largest at the 3-position. The electron density in the dimer radical cation is largest in the 2-position.) The trimer is initially formed near the electrode surface, where it deposits onto the electrode. Further growth then occurs by adsorption and cyclisation on the surface... 

Funt and collaborators " conducted similar studies a few years later. In the first paper of this series, Funt and Verigin showed that the radical cations of 9,10-di-phenylanthracene were fairly stable in rigorously dried acetonitrile. Addition of styrene or substituted styrenes resulted in relatively fast initiation processes which were attributed to an electron transfer process giving the monomer radical cation in an equilibrium reaction. The latter species added one monomer molecule irreversibly to give the dimer radical cation and thereafter polymerisation proceeded. In a second paper the polymerisation of styrene was studied in methylene chloride with the same initiator. A preliminary kinetic study of this process was carried out in the presence of tetrabutylammonium perchlorate as background electrolyte. A value for the propagation rate con-... 

The above investigations clearly show that deqiite their relative stabilities, electro-chemically prepared radical-cations give rise to fairly complicated phenomenologies when they are used as initiators for cationic polymerisations. While the initiation rate constants reported are probably correct, the chemistry of these processes both with reject to the initial step and to the ensuing reactions of the monomer radical cations, is not fully understood. As for the nature and relative concentration of the chain carriers in these systems, more work would need to be done before any firm conclusion can be attained. All these problems stem at least in part from the fact that the radical cations described are only relatively stable and do suffer s)me annihilation reactions. 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

The key reaction of these three is the dimerization (reaction 2) in which the short-lived intermediate monomer radical cation is stabilized. The equilibrium constant (Kx) of this reaction determines not only the stability of the reactive intermediates but also the kinetics of crystal growth and the composition of the crystals. 

The electrochemical oxidation of thiophene and its derivatives forms part of a recent exhaustive review of such oxidations of several heteroaromatic compounds <86CJC76>. The mechanism has been explained. The initially formed radical cation is sufficiently stable to dimerize, but not so stable as to diffuse away from the electrode surface. Chain propagation takes place by coupling of the monomer radical cation with the dimer radical cation and so on. Cyclic voltammetric data for thiophene and its derivatives have been provided. 

Several pulse radiolysis studies have provided evidence that the 450-500-nm transients assigned to 1,4-acyclic radical cations react with the parent styrenes in nonpolar solvents. The rate constants for these reactions are generally in the 10 -10 M" s range, several orders of magnitude slower than the intial addition of the monomer radical cation. The reactions have been attributed to the trimerization reaction that is the first step in the chain growth in cationic polymerizations (Eq, 27). 

The measured rate constant for decay of the initial monomer radical cation from probe 1 of 1.2 x 10 s" agrees reasonably well with Bauld s earlier estimate, although the agreement appears to be fortuitous. By contrast, the rate constant for the cyclization of the monomer radical cation of probe 2 is almost 3 orders of magnitude faster than both the earlier estimate from product studies for this system (>3 x 10 s ) and the measured rate constant for 1. It is possible that some of the discrepancy may be due to cleavage of the initial adduct radical cation to regenerate the monomer radical cation, which would mean that the rate constant measured in the laser experiments does not reflect only the initial cyclization rate. The apparently slower... 

Fig. 3.7 ESR spectra observed at room temperature for 1,1,1,3.3,3-hexafluoropropen-2-ol (HFP) solution containing ca 5 mmol dm coronene (C) and thallium trifluoroacetate (TIIII). (a) Spectrum attributed to the monomer radical cation, (C) from a solution containing -10 mmol dm TIIII Simulated spectmm calculated with an isotropic H-hfs of 0.156 mT (12 H) and a peak-peak line width (ABpp) of 0.035 mT (b) Spectrum attributed to a mixture of (C) and the dimeric radical, (Ci), from a solution containing -2 mmoi dm TIIII Simulated spectrum calculated with the concentration ratio [(C) ] [ (C)2 ] = 20 1. The line shape of (C2) was calculated with an isotropic H-hfs of 0.0766 mT (12 H) and ABpp of 0.(X)9 mT. The satellite lines denoted by an asterisk, flanking the sharp (Ca) features, are attributed to the trimeric cation, (C3) (c) Spectrum attributed to a mixture of (C2) and (Cs) formed in a solution containing < 1 mmol dm TIIII. The figure is reproduced from [K. Komaguchi et al. Spectrochim. Acta A 63, 76 (2(X)6)] with permission from Elsevier...

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