Homogeneous Initiation by Radical-Anions

Radical-anions, Ar, act as useful electron donors converting monomers, M, into their radical-ions  

Since the radical-anions derived from monomers undergo rapid and virtually irreversible dimerization, the electron-transfer is quantitative even for an unfavorable equilibrium. Addition of the monomer to the resulting dimeric-dianions is then the first step of anionic propagation. 

The dimerization competes with monomer addition to monomeric radical-anions, Mr + M — M MT, Jfc,.  

Most likely ka is smaller than the propagation constant, i.e. presumably smaller than 250 M-1s-1. However, since the concentration of the monomer is at least 104 times larger than that of the monomeric radical-anions, the addition competes efficiently with the dimerizaton. Nevertheless, the resulting dimeric radical-anions, -M-M , play no role in the polymerization because their diffusion controlled disproportionation (rate constant 1010 M-1s-1) destroys them as soon as formed. Hence, radical propagation is imperceptible in such systems. 

It is appropriate to describe at this junction a technique leading to the determination of the dimerization rate of radical-anions derived from the monomers96. Flash-photolysis of 1(T6 M THF solution of the potassium salt of the dimeric-dianions of a-methyl styrene, K+, aa, K+, results in their photodissociation into a-methyl styrene radical- 

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Anionic polymerization of methacrolein was investigated by several authors [244-251]. Homogeneous initiators are the anion radicals of naphthalene [242,246], 2,4-dimethyl-benzophenone [247], 4-methylbenzophenone [247], 1-benzoylnaphthalene [247], benzo-phenone [247], 4-benzoylbiphenyl [247], diphenyl ketone [249], dihydronaphthalene [249], and also BuLi [250], NaCN [245], and BU3P [245]. Also heterogeneous initiators such as the radical anions of graphite [251] formed from graphite inclusion compounds are used. 

The mechanism of the electroreductive cyclization reaction has been studied in some detail [22], The initial thought was that it occurred via the cyclization of the radical anion derived, for example, from 25 in the first reduction step. A moment s reflection, however, reveals that there are many more mechanistically viable pathways, especially when one realizes that the transformation involves five steps - two electron transfers (symbolized below by e and d , the latter corresponding to a homogeneous process), two protonations ( p ), and cyclization ( c ). In principle, these could occur in any order, and with any one of the steps being rate-determining. 

If an electron acceptor is available in homogeneous solution, photochemical reaction can be observed. For example, when 2 is excited (X > 350 nm) in anhydrous dimethylsulfoxide (DMSO), methylation occurs, ultimately giving rise to 9,9-dimethyl-fluorene in >80% yield. By analogy with Tolbert s mechanism for photomethylation in DMSO (4), such a process may be initiated by electron transfer to DMSO to form a caged radical-radical anion pair from which subsequent C-S cleavage occurs (eqn 4). 

In an EC2j process, the initial ET step is followed by a second-order irreversible homogeneous reaction. For example, the feedback mode of SECM was employed to study the reductive hydrodimerization of the dimethyl fumarate (DF) radical anion [22]. The experiments were carried out in solutions containing either 5.15 or 11.5 mM DF and 0.1 M tetrabutylammonium tetrafluoroborate in A,A,-dimethyl form amide (DMF). The increase in the feedback current with increasing concentration of DF indicated that the homogeneous step involved in this process is not a first-order reaction. The analysis of the data based on the EC2 theory yielded the fc2 values of 180M-1 s-1 and 160M-1 s-1 for two different concentrations. Another second order reaction studied by the TG/SC mode was oxidative dimerization of 4-nitrophenolate (ArO-) in acetonitrile [23]. In this experiment, the tip was placed at a fixed distance from the substrate. The d value was determined from the positive feedback current of benzoquinone, which did not interfere with the reaction of interest. The dimerization rate constant of (1.2 0.3) x 108 M x s-1 was obtained for different concentrations of ArO-. 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

The rate of electron transfer and its potential dependence can be described by the Butler-Volmer equation (20) (see Section 2). An electron transfer often initiates a cascade of homogeneous chemical reactions by producing a reactive radical anion/cation. The mechanism can be described mathematically by a rate equation for each species these form part of the electrochemical model. The rate law of the overall sequence is probed by the voltammetric experiment. 

Volume 15 deals with those polymerization processes which do not involve free radicals as intermediates. Chapters 1 and 2 cover homogeneous anionic and cationic polymerization, respectively, and Chapter 3 polymerizations initiated by Zeigler-Natta and related organometallic catalysts. Chapters 4, 5 and 6 deal with the polymerization of cyclic ethers and sulphides, of aldehydes and of lactams, respectively. Finally, in Chapter 7 polycondensation reactions, and in Chapter 8 the polymerization of AT-carboxy-a-amino acid anhydrides, are discussed. 

Polymerization of substituted acetylenes has been carried out by a wide range of catalysts and condi-tions. Polymerization conditions include a homogeneous and heterogeneous Ziegler—Natta catalyst, transition metal complexes (Pd. Pt. Ru. W. Mo. Ni. etc.), free radical initiators such as 2.2 -azobis(isobu-tyronitrile) (AIBN). benzoyl peroxide (BPO). and di-tert-butylperoxide (DTBP). thermal polymerization, y-irradiation. cationic initiation with BF3. and anionic initiation by butyllithium. triethylamine. and sodium amide. 

The true nature of homogeneous anionic polymerization only became apparent through studies of the soluble aromatic complexes of alkali metals, such as sodium naphthalene. These species are known to be radical anions [154-158], with one unpaired electron stabilized by resonance and a high solvation energy, and are therefore chemically equivalent to a soluble sodium. They initiate polymerization by an electron transfer process [145,148], just as in the case of the metal itself, except that the reaction is homogeneous and therefore involves a much higher concentration of initiator. The mechanism... 

Homogenous polymerization of AAm is usually performed in aqueous solution. The radical polymerization leads to a linear polymer of the general structure [379,380] whereby n varies between 20,000 and 300,000. Polymer made by using anionic initiators shows a totally different structure, called nylon-3 or poly(P-alanine) [347] (Figures 1 and 2). 

By analysis of the data of the dependence of signal on time during electrolysis it is possible by means of equations for homogeneous chemical kinetics to calculate the reaction order, the reaction rate constant, and the half-life for the primary process involved in cleavage of the radical ions [76]. It is thus possible to determine kinetic parameters for the second-order cleavage of radical anions with half-lives not shorter than 1 sec at an initial concentration of Co = 540 mole/liter. 

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