Free radical processes

Hydrolyzed Polyacrylamide. HPAM (6) can be prepared by a free-radical process ia which acrylamide is copolymerized with incremental amounts of acryUc acid or through homopolymerization of acrylamide followed by hydrolysis of some of the amide groups to carboxylate units. 

In general, acryUc ester monomers copolymerize readily with each other or with most other types of vinyl monomers by free-radical processes. The relative ease of copolymerization for 1 1 mixtures of acrylate monomers with other common monomers is presented in Table 7. Values above 25 indicate that good copolymerization is expected. Low values can often be offset by a suitable adjustment in the proportion of comonomers or in the method of their introduction into the polymerization reaction (86). 

Autoxida.tlon. The autoxidation (7) of unsaturated fatty acids in phosphoHpids is similar to that of free acids. Primary products are diene hydroperoxides formed in a free-radical process. 

Density. The density (crystallinity) of catalyticaHy produced PE is primarily determined by the amount of comonomer ( a-olefin) in ethylene copolymer. This amount is easily controlled by varying the relative amounts of ethylene and the comonomer in a polymerization reactor. In contrast, the density of PE produced in free-radical processes is usually controlled by temperature. 

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

The most common reaction of methylene chloride is its reaction with chlorine to give chloroform and carbon tetrachloride. This occurs by a free-radical process initiated by heat or light in the gas or Hquid phase. Catalytic chlorination to these same products is also known (see Chlorocarbons and Cm OROHYDROCARBONS, Cm OROFORM). 

Liquid-phase chlorination of butadiene in hydroxyhc or other polar solvents can be quite compHcated in kinetics and lead to extensive formation of by-products that involve the solvent. In nonpolar solvents the reaction can be either free radical or polar in nature (20). The free-radical process results in excessive losses to tetrachlorobutanes if near-stoichiometric ratios of reactants ate used or polymer if excess of butadiene is used. The "ionic" reaction, if a small amount of air is used to inhibit free radicals, can be quite slow in a highly purified system but is accelerated by small traces of practically any polar impurity. Pyridine, dipolar aptotic solvents, and oil-soluble ammonium chlorides have been used to improve the reaction (21). As a commercial process, the use of a solvent requites that the products must be separated from solvent as well as from each other and the excess butadiene which is used, but high yields of the desired products can be obtained without formation of polymer at higher butadiene to chlorine ratio. 

CSM products may be divided into three groups depending on the type of precursor resin low density (LDPE), high density (HDPE), and linear low density (LLDPE). LDPE is made by a high pressure free-radical process, while HDPE and LLDPE are made via low pressure, metal coordination catalyst processes (12) (see Olefin polymers). 

Free-Radical Polymerization. The best method for polymerising isoprene by a free-radical process is emulsion polymerisation. Using potassium persulfate [7727-21-1] as initiator at 50°C, a 75% conversion to polyisoprene in 15 h was obtained (76). A typical emulsion polymerisation recipe is given as follows (77). 

Free Electron Molecular Orbital method colour and constitution, 1, 342 Freelingyne occurrence, 4, 706 Free radical processes in photography, 1, 387-389 Friedlander synthesis quinolines, 2, 443 thioindigo dyes, 4, 910 Fries rearrangement chroman-4-one synthesis from, 3, 850 Fructose, 1-deoxy- C NMR, 4, 575 Frusemide as diuretic, 1, 174 metabolism, 1, 245 FS-32 — see 1/f-Indazole, l-[3-... 

The emission signal corresponding to benzene confirms that it is formed by a free-radical process. As in steady-state EPR experiments, the enhanced emission and absorption are observed only as long as the reaction is proceeding. When the reaction is complete or is stopped in some way, the signals rapidly return to their normal intensity, because equilibrium population of the two spin states is rapidly reached. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Unsaturated rubbers. Unsaturated rubbers have been cured by free radical processes using heat activated initiators for many years. The pendant or vinyl double bonds are particularly reactive (see Fig. 2). 

Additions of elemental halogens to unsaturated compounds are among the most common preparations of halogenated fluoroorganics. The transformations are usually fairly clean and proceed in good yields. Besides the numerous examples of halogen addition tofluoroalkenes and fluoroalkyl-substituted alkenes, additions to perfliioropropyl vinyl ether [2] and fluormated styrenes [7, 4] have been reported. Both ionic and free-radical processes occur (equations 1 and 2)... 

However ocher pfxxhids may be formed simultaneously by a free radical process especially in the presence of catalytic amounts of C0CI2 or CuQ ... 

Den Hertog and Overhoff - observed that when pyridine in sulfuric acid is added to molten potassium sodium nitrate the 3-nitro derivative is formed at 300°C, whereas at 450°C 2-nitropyridme is the main product. The latter is probably a free-radical process. Schorigin and Toptschiew obtained 7-nitroquinoline by the action of nitrogen peroxide on quinoline at 100°C, possibly through the homolytic addition of NOa. Laville and Waters reported that during the decomposition of pernitrous acid in aqueous acetic acid, quinoline is nitrated in the 6- and 7-positions. They considered that the reaction proceeds as shown in Scheme 3. 

The intramolecular attack on the alkyl side chain is suggestive of a free-radical process. 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

Peroxodiphosphoric acid (PDPA) may also be used to convert sulphoxides to sulphones in good yields. An initial study of this reaction80 concluded that the mechanism was a free radical process, involving the reaction of a hydroxyl radical with the sulphoxide as shown in equation (26). This was later claimed to be incorrect the reaction actually occurs by the initial decomposition of PDPA to PMPA which then reacts as described above81. 

The investigated cw-stilbene derivatives, 4-methoxy, 4,4 -dimethyl, unsubstituted, and 4,4 -bis(trifluoromethyl)stilbenes, had k2 values spanning 6-7 powers of ten both in methanol and in acetic acid. Products 2, 4, 5 and 6 were formed. Table 8 reports the results of the cis-trans isomerization test in acetic acid (ref. 29). No acid catalyzed or free radical process was found to be responsible for these isomerizations. 

A free-radical process in which the carbene directly abstracts a hydrogen from the substrate to generate a pair of free radicals ... 

A free-radical process consists of at least two steps. The first step involves the formation of free radicals, usually by homolytic cleavage of bond, that is, a cleavage in which each fragment retains one electron ... 

A different method for the conversion of ketones to a-hydroxy ketones consists of treating the enolate with a 2-sulfonyloxaziridine (such as 15). This is not a free-radical process the following mechanism is likely ... 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

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