Initiator group, free radical

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. 

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. 

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

N-Vinylpyrrole-2-carbaldehydes 157 were selectively thiylated with ethanethiol either at the aldehyde (acid catalysis) or vinyl group (free-radical initiation) to afford in high to excellent yields N-vinylpyrrole-2-carbaldehyde thioacetals 167 (88-99%) and l-(2-ethylthioethyl) pyrrole-2-carbaldehydes 168 (68-89%), respectively (Equation (52)) (07S452). 

A high-molecular-weight, insoluble polymer is obtained when perfluoro-2-butyne is subjected to various initiators for free-radical polymerisation (Figure 7.87). The off-white colour of this material is remarkable for a polyacetylene [307, 308]. Indeed, it is largely ignored in discussions on polyacetylenes because, of course, the fact that it is not coloured also means that the system is not conjugated the trifluoromethyl groups keep the TT-systems out of plane relative to each other. 

Radical anions of geminal nitro compounds in DMF cleave rapidly to nitrite and a radical which initiate a free radical chain in which a v/c-dinitro compound is formed [43]. Radical anions of other a-substituted nitroalkanes may cleave either the nitro group or the other substituent 1-cyano-l-nitrocyclohexane thus splits off nitrite whereas 1-nitrocyclo-hexyl p-tolyl sulfone loses p-tolylsulfinate [44]. 

A few other methods have been used to prepare polypeptide hybrid copolymers. Inoue polymerized Bn-Glu NCA off of amine-functionalized styrene derivatives, and then copolymerized these end-functionalized polypeptides with either styrene or methyl methacrylate using free radical initiators to yield hybrid comb architecture copolymers [38]. Although unreacted polypeptide was identified and removed by fractionation, copolymers were obtained with polypeptide content that increased with feed ratio. There was no mention if the polypeptide interfered with the radical chemistry. In similar work, Imanishi and coworkers converted the amine-ends of polypeptides to haloacetyl groups that were used to initiate the free radical polymerization of either styrene or methylmethacrylate to yield hybrid block copolymers [39]. Studies using CPC showed that the crude product contained mixtures of copolymers and homopolymers, and so removal of the homopolymers by extraction was necessary. 

Despite the utility of radical dehalogenation, development of more stable and milder initiators than AIBN, EtsB, and Et2Zn is still required. It has recently been reported that InCh, which is easy to handle because of its stability in the air, can serve well as an initiator of free-radical dehalogenation with BusSnH at room temperature, enabling carbonyl groups to remain intact (Scheme 12.127) [230]. 

Thus the reaction appears to proceed via elimination of the two NO groups accompanied by C-C bond scission. However, in the liquid phase the products of decomposition of butane-2,3-dinitrite were mainly RC(0)—C(0)R and (RCH0H)2 as well as NO. This suggests that the initially formed free radical is stable long enough to abstract an H atom om the parent compound. This sequence would be repeated by both reactants, so that the initially formed radical would become the dialcohol, and the secondary radical, which had lost an H atom, would become the diketone. 

There is a large group of metal-based compounds capable of initiating the free radical photopolymerization of unsaturated compounds (see Table 10.4) [23, 24]. 

Basic acrylate chemistry. The basic acrylic monomers or oligomers contain unsaturated double bonds (vinyl groups), and consequently cure by addition polymerization involving a free-radical reaction. Free-radical-producing compounds such as peroxides, peracetic acids, and sulfones are added to acrylic resins to initiate polymerization. Free-radical polymerization of acrylics may also be induced by exposure to U V or visible light. These UV-curing adhesives, most of which are based on acrylic or modified acrylic... 

In monomers with two or more polymerizable sites the structure of the resulting polymer depends on the initiator. Vinyl isocyanate, CH2=CHNCO, polymerizes via the vinyl group free radically, but via the nitrogen/ oxygen double bond in anionic polymerization. Diketenes polymerize to polyesters, polyketones, or poly(vinyl esters) according to what initiator is used ... 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

Thus, the initiator free radical becomes an end group of the polymer. Consequently, initiators in free radical polymerizations are not catalysts since they are consumed in the start reaction. 

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