Initiation by free radicals

Such reactions can be initiated by free radicals, derived from compounds (initiators) such as benzoyl peroxide, ammonium persulphate or azobis-isobutyronitrile or by ionic mechanisms... 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

Grafting presents a means of modifying the cellulose molecule through the creation of branches of synthetic polymers, which impart to the cellulose certain desirable properties without destroying the properties of cellulose. The polymerization of vinyl monomers may be initiated by free radicals or by certain ions. Depending on the monomer, one or the other type of initiation may be preferred. The grafting process depends on the reactivity of the monomer used, the type of initiation, and cellulose accessibility [1,2]. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

Ethylene reacts by addition to many inexpensive reagents such as water, chlorine, hydrogen chloride, and oxygen to produce valuable chemicals. It can be initiated by free radicals or by coordination catalysts to produce polyethylene, the largest-volume thermoplastic polymer. It can also be copolymerized with other olefins producing polymers with improved properties. Eor example, when ethylene is polymerized with propylene, a thermoplastic elastomer is obtained. Eigure 7-1 illustrates the most important chemicals based on ethylene. 

There is much evidence" for this mechanism, including side products (RH, alkenes) characteristic of free-radical intermediates and the fact that electrolysis of acetate ion in the presence of styrene caused some of the styrene to polymerize to polystyrene (such polymerizations can be initiated by free radicals, see p. 978). Other side products (ROH, RCOOR) are sometimes found these stem from further oxidation of the radical R to the carbocation... 

Mixtures of two or more monomers can polymerize to form copolymers. Many copolymers have been developed to combine the best features of each monomer. For example, poly(vinyl chloride) (called a homopolymer because it is made from a single monomers) is brittle. By copolymerizing vinyl chloride with vinyl acetate, a copolymer is obtained that is flexible. Arrangement of the monomer units in a copolymer depends on the rates at which the monomers react with each other. Graft copolymers are formed when a monomer is initiated by free radical sites created on an already-formed polymer chain. 

In summary, therefore, the evidence seems convincing that exercise modifies circulating and tissue concentrations of antioxidants and enzyme activities. It is much less certain that the fatigue or damage to skeletal muscle associated with various forms of excessive or unaccustomed exercise is initiated by free radical-mediated degradation. Considerably more work is required in this area to clarify the underlying pathogenic mechanisms. 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

As mentioned earlier, oxidation of LDL is initiated by free radical attack at the diallylic positions of unsaturated fatty acids. For example, copper- or endothelial cell-initiated LDL oxidation resulted in a large formation of monohydroxy derivatives of linoleic and arachi-donic acids at the early stage of the reaction [175], During the reaction, the amount of these products is diminished, and monohydroxy derivatives of oleic acid appeared. Thus, monohydroxy derivatives of unsaturated acids are the major products of the oxidation of human LDL. Breuer et al. [176] measured cholesterol oxidation products (oxysterols) formed during copper- or soybean lipoxygenase-initiated LDL oxidation. They identified chlolcst-5-cnc-3(3, 4a-diol, cholest-5-ene-3(3, 4(3-diol, and cholestane-3 3, 5a, 6a-triol, which are present in human atherosclerotic plaques. 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

Traditional polymerizations usually involve AB-type monomers based on substituted ethylenes, strained small ring compounds using chain reactions that may be initiated by free radical, anionic or cationic initiators [20]. Alternatively, AB-type monomers may be used in polycondensation reactions. 

The standard gel-forming reaction is shown in Figure 8.2. Acrylamide and the cross-linker N, A-methylenebisacrylamide (bis) are mixed in aqueous solution and then copolymerized by means of a vinyl addition reaction initiated by free radicals.1317 Gel formation occurs as acrylamide monomer polymerizes into long chains cross-linked by bis molecules. The resultant interconnected meshwork of fiberlike structures has both solid and liquid components. It can be thought of as a mass of relatively rigid fibers that create a network of open spaces (the pores) all immersed in liquid (the buffer). The liquid in a gel maintains the gel s three-dimensional shape. Without the liquid, the gel would dry to a thin film. At the same time, the gel fibers retain the liquid and prevent it from flowing away. 

Many polymerizations are initiated by free radicals, especially alkoxy radicals formed by thermal decomposition of peroxides. A general mechanism for olefin free radical polymerization with initiation, propagation, and termination is given in Fig. 14.1. 

The role of gas phase initiation processes was further explored by Tibbitt et al. . These authors proposed that the polymerization of unsaturated hydrocarbons in a 13.56 MHz plasma is initiated by free radicals formed in the gas by electron-monomer collisions, the initiation reactions listed in Table 6. Moreover, it was assumed that the formation of free radicals on the polymer surface due to the impact of charged particles could be neglected. This assumption is supported by the fact that at 13.56 MHz and pressures near one torr the discharge frequency is significantly greater than either f, or f and that as a result the fluxes of charged particles to the electrode surfaces are quite small. 

Direct oxidation of alkenes with molecular oxygen11,255,256 initiated by free radicals to yield epoxides occurs through addition of peroxy radicals to produce the more stable P-peroxy alkyl radicals (28) 257... 

The radical polymerization of ethylene, in practice, is initiated by free-radical initiators, although radiation-induced,155 210 photoinduced,211-213 and thermal213,214 initiations are also possible. The temperature of high-pressure polymerization should not exceed 350°C since above this temperature a rapid exothermic (AH = —30.4kcal/mol) thermal decomposition of ethylene can take place leading to a runaway reaction ... 

It is believed that polymerization of hydrophobic monomers is initiated by free radicals in the aqueous phase and that the surface-active oligomers produced migrate to the interior of the emulsifier micelles where propagation continues. Monomer molecules dispersed in the water phase also solubilize by diffusing —to the expanding lamellar micelles. These micelles disappear as the polymerization continues and the rate may be measured by noting the increase in surface tension of llie system. 

Three different synthetic methods have been developed for the copolymerization of CO with olefins. The copolymerization may be initiated by free radicals, or induced by y-rays in addition, a number of transition metal compounds are effective catalysts. These copolymerization procedures are described below. 

When the size of the particles is very small, the last term dominates, AG is positive and the embryos are unstable, i.e. they will disaggregate. When r reaches a critical size, r, the slope of dAG /dr becomes negative, so that further growth is favored. Thus chains, initiated by free radicals generated in the continuous phase, must grow until they reach a critical size... 

UV light induced grafting onto wood cellulose is reported for several vinyl monomers. The reaction is initiated by free radical initiators such as phenylace-tophenone and benzophenone derivatives. Percent grafting-time conversion curves are determined as a function of the initiators, monomers, pulps and additives. Additional typical results obtained in IR spectroscopy, GPC and thermal analysis are reported. A discussion about the use of a photochemical procedure in obtaining cellulose graft copolymers is presented. 

With most systems of this nature it is important that the two parts be mixed in the correct proportions (that is stoichiometrically) so that the reactions are completed and an excess of neither component is left in the hardened product. The most usual ratio of mixing is equal proportions of both parts (1 1) but differing ratios are by no means uncommon—especially for systems in which chain polymerization initiated by free radicals takes place (when relatively small amounts of free radical initiators will start the setting). 

Sulfoxides form complexes with hydroperoxides and can inhibit a partially oxidized substrate, but their inhibiting activity is destroyed by the simultaneous addition of an acidic substance such as stearic acid. The "activity of sulfur compounds cannot be wholly accounted for by their peroxide-decomposing action, and although they suppress peroxide-initiated autoxidation, they do not suppress oxidations initiated by free radical sources such as azobisisobutyronitrile (10). 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

In the absence of a free radical catalyst, Reaction 25 is dominant, particularly when precomplexed acrylonitrile is present, accompanied by Reaction 27. In the presence of both a free radical catalyst and zinc chloride, Reactions 25, 26, and 27 occur simultaneously. In addition, Reactions 26 and 27 are initiated by free radicals. 

The fundamental chemistry of the structural adhesives described here can change very little. Vinyl and acrylic monomers polymerize by chain growth polymerization initiated by free radicals or ions. Isocyanate and epoxy compounds react with compounds containing active hydrogen in step growth polymeriza-... 

Polymerization of the vinyl monomer in the wood may be done with either radiation or free radical catalysts. The polymerization of the vinyl monomers in both processes depends upon the same mechanism, that is, initiation by free radicals. In the radiation process, the gamma rays passing through the monomer and the woody tissue create a large number of excited and ionized molecules, many of which break into fragments,... 

By 1951 (29) it had been concluded that polyacetaldehyde is an amorphous polyacetal of very high molecular weight. The polymerization of acetaldehyde is initiated by free radicals assisted by the crystallization of the monomer (crystallization polymerization). 

Vinyl monomers, such as styrene, methyl methacrylate, vinyl acetate, vinyl chloride or acrylonitrile are preferably polymerized by chain polymerization techniques initiated by free radicals. Suitable free radicals can be handily achieved from unstable chemicals like peroxides (benzoyl peroxide, dicumil peroxide) or di-azo reagents (e.g. 2,2 -azo-bis-isobutyronitrile, AIBN) which are dissolved in monomer and usually thermally decompose at temperature range of 40-120 °C. Alternatively, suitable radicals for polymerization can also be activated without addition of external initiators, by just applying ultraviolet light (wave length 200-350 nm) or ultrasound (15,33,34) onto monomer. 

Fig. 3. The mechanism of polyunsaturated fatty acid oxidation initiated by free radicals.

Cellulose can also be modified by introducing long-chain polymer(s) onto its main chain. The preparation of a graft copolymer requires the formation of a reactive site on cellulose in the presence of a polymerizable monomer. The principal techniques frequently used are (1) grafting initiated by free radical polymerization, (2) grafting initiated by ionic polymerization (3)... 

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