Redox radical polymerization

Living radical polymerization Redox initiation Activation energy Cationic polymerization Living cationic polymerizations Gegen ion... 

The reversible addition of sodium bisulfite to carbonyl groups is used ia the purification of aldehydes. Sodium bisulfite also is employed ia polymer and synthetic fiber manufacture ia several ways. In free-radical polymerization of vinyl and diene monomers, sodium bisulfite or metabisulfite is frequentiy used as the reduciag component of a so-called redox initiator (see Initiators). Sodium bisulfite is also used as a color preventative and is added as such during the coagulation of crepe mbber. 

Recently, in our laboratory the following novel Ce(IV) ion redox initiation systems have been investigated for vinyl radical polymerization. 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

The initiator in radical polymerization is often regarded simply as a source of radicals. Little attention is paid to the various pathways available for radical generation or to the side reactions that may accompany initiation. The preceding discussion (see 3.2) demonstrated that in selecting initiators (whether thermal, photochemical, redox, etc.) for polymerization, they must be considered in terms of the types of radicals formed, their suitability for use with the particular monomers, solvent, and the other agents present in the polymerization medium, and for the properties they convey to the polymer produced. 

Metal complex-organic halide redox initiation is the basis of ATRP. Further discussion of systems in this context will be found in Section 9.4, The kinetics and mechanism of redox and photoredox systems involving transition metal complexes in conventional radical polymerization have been reviewed by Bam ford. 

Bamford, Eastmond and coworkers have employed metal complexpolymeric halide redox systems to initiate block and graft copolymerization. The polymeric halides can be synthesized by a variety of techniques, including radical polymerization,281 anionic polymerization (Scheme 7.28),"so... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

When R-X and NiX have high reactivities for chain transfer and/or primary radical termination (Eqs. 36 and 37), and the C-X bond at the chain end further reacts with Ni° by redox reaction (Eq. 38), the polymerization proceeds via a living radical polymerization mechanism. In this polymerization, the polymerization which has R and X groups at both chain ends is produced ... 

Similarly, it was also found that radical polymerization was induced in the Ni(CO)3(PPh3)/CBrCl3 redox system [155]. This complex is soluble in the polymerization medium, and the polymerization proceeded in a homogeneous system. This redox iniferter system has been intensively developed to the recent successful living radical polymerization using transition-metal complexes in combination with alkyl halides by several independent research groups (see Sect. 6.2). 

Case 1 appears to accurately predict the observed dependence on persulfate concentration. Furthermore, as [Q]+otal approaches [KX], the polymerization rate tends to become independent of quat salt concentration, thus qualitatively explaining the relative insensitivity to [Aliquat 336]. The major problem lies in explaining the observed dependency on [MMA]. There are a number of circumstances in free radical polymerizations under which the order in monomer concentration becomes >1 (18). This may occur, for example, if the rate of initiation is dependent upon monomer concentration. A particular case of this type occurs when the initiator efficiency varies directly with [M], leading to Rp a [M]. Such a situation may exist under our polymerization conditions. In earlier studies on the decomposition of aqueous solutions of potassium persulfate in the presence of 18-crown-6 we showed (19) that the crown entered into redox reactions with persulfate (Scheme 3). Crematy (16) has postulated similar reactions with quat salts. Competition between MMA and the quat salt thus could influence the initiation rate. In addition, increases in solution polarity with increasing [MMA] are expected to exert some, although perhaps minor, effect on Rp. Further studies are obviously necessary to fully understand these polymerization systems. 

Another system under investigation is the iron/ chromium redox flow battery (Fe/Cr RFB) developed by NASA. The performance requirements of the membrane for Fe/Cr RFB are severe. The membrane must readily permit the passage of chloride ions, but should not allow any mixing of the chromium and iron ions. An anionic permselective membrane CDIL-AA5-LC-397, developed by Ionics, Inc., performed well in this system. ° It was prepared by a free radical polymerization of vinylbenzyl chloride and dimethylaminoethyl methacrylate in a 1 1 molar ratio. One major issue with the anionic membranes was its increase in resistance during the time it was exposed to a ferric chloride solution. The resistance increase termed fouling is related to the ability of the ferric ion to form ferric chloride complexes, which are not electrically repelled by the anionic membrane. An experiment by Arnold and Assink indicated that... 

The initiation of a radical polymerization of a monomer can be achieved with practically every peroxo or azo compound. This means that in these cases the type of initiator influences only the rate and degree of polymerization, the nature of the end groups and branching but not the polymerizability of the monomer as such. This is not the case with redox systems as radical initiators. As a... 

Table 3.5. Some common oxidizing and reducing agents that are suitable for initiating radical polymerization by redox reactions...

These superabsorbents are synthesized via free radical polymerization of acrylic acid or its salts in presence of a crosslinker (crosslinking copolymerization). Initiators are commonly used, water-soluble compounds (e.g., peroxodi-sulfates, redox systems). As crosslinking comonomers bis-methacrylates or N,hT-methylenebis-(acrylamide) are mostly applied. The copolymerization can be carried out in aqueous solution (see Example 5-11 or as dispersion of aqueous drops in a hydrocarbon (inverse emulsion polymerization, see Sect. 2.2.4.2). 

Reversible redox reactions can initiate radical chemistry without a follow-up reduction or oxidation reaction. In successful reactions of this type, the redox step that produces the radical is thermodynamically disfavored. For example, Cu(I) complexes react reversibly with alkyl hahdes to give Cu(II) hahde complexes and an alkyl radical. The alkyl radical can react in, for example, an addition reaction, and the product radical will react with the Cu(II) hahde to give a new alkyl halide. This type of reaction sequence, which has been apphed in living radical polymerizations, is in the general family of nonchain radical reactions discussed earlier. ... 

The main problem is how to generate free radicals at low temperatures. It was discovered this can be done by using the trialkylborane-oxygen redox couple. Prior to the studies on thiocarbonyl compounds, Furukawa and Tsuruta (68) had used a mixture of trialkylboranes and oxygen for vinyl polymerizations, and studies by Fordham and Sturm (69) and Zutty and Welch (70) had confirmed them as free-radical polymerizations. For the fluorothiocarbonyl work (39), it was shown that at - 78° C the reaction of a trialkylborane and oxygen proceeds cleanly to an alkylperoxydialkylborane, V. 

A number of metal salts photosensitize radical polymerization (36). Most of them concuct photo redox reactions to produce radicals by processes similar to reactions (23), (24) and (25). Besides spontaneous photodecomposition of metal complexes, metal complexes may react directly with the monomer. 

The low redox potential of Af,AT-dialkyl-4,4 -bipyridinium salts (viologens) has prompted their incorporation into various polymeric systems. Addition polymerizable viologens have been prepared, for example, by reaction of monoalkylated bipyridyl with vinylbenzyl chloride (Scheme 30). Aqueous free radical polymerization and copolymerization of... 

The synthesis of mixed peroxides formed from /-butyl hydroperoxide and carbon-centred radicals has been studied. The reactions were strongly effected by solvents as well as catalytic amounts of Cun/Fem. The kinetic data suggest that the conditions for the Ingold-Fischer persistent radical effect are fulfilled in these cases.191 The use of Cu /Cu" redox couples in mediating living radical polymerization continues to be of interest. The kinetics of atom-transfer radical polymerization (ATRP) of styrene with CuBr and bipyridine have been investigated. The polymer reactions were found to be first order with respect to monomer, initiator and CuBr concentration, with the optimum CuBr Bipy ratio found to be 2 1.192 In related work using CuBr-A-pentyl-2-... 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

Comonomer l-vinyl-2-pyrrolidone (VP) comonomer can be purified by distillation at reduced pressure prior to use. Potassium persulfate (KPS) can be purified in a mixture of water and methanol. NIPAM-co-VP copolymers with different amounts of VP can be prepared at temperatures lower or higher than the LCST of PNIPAM by free radical polymerization in water with an initiator of KPS/N,N,N/,N/-tetramethylethylenediamine (TEMED) redox. The resultant copolymer can be harvested by precipitation, i.e., pouring the reaction mixture into an equal volume of methanol. Each resultant copolymer can be further purified by several cycles of re-dissolution in water and precipitation in methanol to ensure a complete removal of residual monomers. The final product can be dried under reduced pressure at 40 °C. 

In due course, radicals 011 were the subject of great attention however, no practical results were obtained. For example, the main factories producing 0H radicals, which are Fenton and similar redox systems, manifested themselves as initiating systems, i.e. operates as the triggers of radical polymerization. 

The Ru(II) catalysts currently used for olefin hydroarylation reactions are limited in terms of incorporation of substituents into the olefin substrate. For example, attempted hydrophenylation of isobutylene with TpRu(CO)(NCMe)(Ph) as catalyst does not yield new organic products. In addition, extension of catalysis to hetero-functionalized olefins using the TpRu(CO)(NCMe)(aryl) systems has not been successful. For electron-deficient olefins (e.g. styrene, methyl methacrylate, acrylonitrile) the TpRu(II) complexes initiate radical polymerization of the olefin in transformations that probably involve a Ru(III/II) redox change [4]. The... 

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