Free radicals radical anions

Radical (or free radical) Radical anion Radical cation Rate-determining step Reducing agent Reduction reaction... 

Cationic Polymerization. For decades cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition (see Elastomers, synthetic-BUTYLRUBBEr). More recently, development has led to the point where living cationic chains can be made, with many of the advantages described above for anionic polymerization (27,28). 

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation). 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

Not susceptible to free radical or anionic polymerization, but cationic polymerization induced by BFs, AICI3, etc., is extremely rapid even at — 100°C... 

Polystyrene Styrene Free radical Cationic Anionic Coordination... 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

It has already been stressed that the discovery of superoxide as the enzymatically produced diffusion-free dioxygen radical anion [1-3] was a pivotal event in the study of free radical processes in biology. It is not of course that the McCord and Fridovich works were the first ones in free radical biology, but the previous works were more of hypothetical character, and only after the identification of superoxide by physicochemical, spectral, and biochemical analytical methods the enzymatic superoxide production became a proven fact. 

Chapters 5 through 7 deal with polymers formed from chain-growth polymerization. Chain-growth polymerization is also called addition polymerization and is based on free radical, cationic, anionic, and coordination reactions where a single initiating species causes the growth of a polymer chain. 

It has now been more than a decade since Beckman and his collaborators first disclosed their observations that the combination of two relatively unreactive, yet biologically relevant free radicals, superoxide anion and nitric oxide, would produce a new highly reactive physiologically important reagent. The interaction of these two presumably innocuous species appears to be diffusion controlled and produces a thermally stable peroxy anion, peroxynitrite (equation 1). ... 

It would be useful to classify reactions as to their type, heterolytic (ionic) or homolytic (free radical), radical cation, or radical anion. Below are a number of examples of these types of reaction. 

Yoshino, T., J. Komiyama, and M. Shinomiya On the Propagation Steps of Free-Radical and Anionic Pol5Tncrization. J. Amer. chem. Soc. 86. 4482 (1964). 

Kinins, neuropeptides, and histamine are also released at the site of tissue injury, as are complement components, cytokines, and other products of leukocytes and platelets. Stimulation of the neutrophil membranes produces oxygen-derived free radicals. Superoxide anion is formed by the reduction of molecular oxygen, which may stimulate the production of other reactive molecules such as hydrogen peroxide and hydroxyl radicals. The interaction of these substances with arachidonic acid results in the generation of chemotactic substances, thus perpetuating the inflammatory process. 

The maleimide group can undergo a variety of chemical reactions. The reactivity of the double bond is a consequence of the electron withdrawing nature of the two adjacent carbonyl groups which create a very electron-deficient double bond, and therefore is susceptible to homo- and copolymerizations. Such polymerizations may be induced by free radicals or anions. Nucleophiles such as primary and secondary amines, phenates, thiophenates, carboxylates, etc. may react via the classical Michael addition mechanism. The maleimide group furthermore is a very reactive dienophile and can therefore be employed in a variety of Diels Alder reactions. Bisdienes such as divinylbenzene, bis(vinylbenzyl) compounds, bis(propenylphenoxy) compounds and bis(benzocyclobutenes) are very attractive Diels Alder comonomers and therefore some are used as constituents for BMI resin formulations. An important chemical reaction of the maleimide group is the ENE reaction with allylphenyl compounds. The most attractive comonomer of this family is DABA particularly when tough bismaleimide resins are desired. 

In some instances of electrolytic polymerization studies, it is conceivable that the polymerization may proceed simultaneously by a free-radical, an anionic, or a cationic mechanism in the same reaction mixture. To discriminate among the various propagation mechanisms, the analysis of copolymer compositions is often used. 

Trifonov and Panayotov (41) attempted to carry out anionic polymerizations of vinyl monomers with semiquinones generated at a cathode. Since semiquinones inhibit free-radical polymerization, anionic polymerization alone should take place in the system. When electrolysis of quinones was conducted in a solution of LiCl or N(CaH6)4I in DMF with mercury cathode, the catholyte turned to red or purple red in accordance with the semiquinones. The presence of free-radical produced on the quinone molecule was proved from the ESR spectrum. When each of the monomers, styrene, acrylonitrile and methyl methacrylate were added to the colored solutions, polymers were obtained. 

Cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition. See also Elastomers and Rubber (Synthetic). 

Free-radical initiators, Anionic initiators, Cationic initiators,... 

Considerable effort in the 1970s by Pittman, George, Hayes, Korshak, and others was applied to exploring the addition polymerization of vinylferrocene 6.1 to give organic polymers with pendent ferrocenyl side groups (6.2 in reaction (l)).1 6 This type of polymerization reaction has been attempted with the use of free radical, cationic, anionic, and Ziegler-Natta methods. 

In chain-growth polymerization, propagation is caused by the direct reaction of a species bearing a suitably generated active center with a monomer molecule. The active center (a free radical, an anion, a cation, etc.) is generated chainwise by each act of growth the monomer itself constitutes the feed (reactive solvent) and is progressively converted into the polymer. 

The active center I, which first initiates and then propagates the chain, can be a free radical, an anion, a cation or a transition-metal based initiator (Ziegler-Natta systems or metathesis reactions). 

The electron spin resonance (E.S.R.) spectra of a paramagnetic organic molecule, e.g. free radical, radical cation or radical anion, is directly related to its unpaired electron distribution (spin density). In the region of a magnetic nucleus the hyperfine interaction between the magnetic moments of the nucleus and the electron is a function of the spin density. It has been shown that, for an atom N, a direct correlation exists between its observed hyperfine coupling constant, and [pa—pP), the unpaired electron population of its atomic orbitals 1). 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

J. V. Crivello and K. Dietliker in Photoinitiators for Free Radical Cationic Anionic Photopolymerisation Volume III , 2nd edn., G. Bradley, Ed. , J. Wiley and Sons, (Chichester), 1998. 

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