Inhibitor of free radical polymerization

Quinones are formed by the reaction of the peroxyl radical with phenoxyls (see Chapter 15). They are known as inhibitors of free radical polymerization of monomers where they retard the reaction terminating chains by the reaction with macroradicals [9]. Quinones do not react with peroxyl radicals and react with alkyl radicals by a few orders magnitude [5-7] more slowly than dioxygen does. It was a surprising phenomena that quinones appeared to... 

A number of a-aryl-A-alkyl nitrones and contrast enhancement compositions, which can be used to make contrast enhancement layer photoresist composites (230, 231), and inhibitors of free radical polymerization of monomers in nonexposed regions of the photoresist layer at selective actinic radiation (232). Histidine was used as a catalyst in the synthesis of a, A-diaryl nitrones in situ (233). To study diphenylborate chelates with mono- and bidentate ligands, a series of hydroxyl-containing nitrones have been synthesized (Fig. 2.7) (234-237). 

Both stable nitroxyls and nitroxyl biradicals are known as inhibitors of free radical polymerization they intercept reactive free radicals. Scheme 12.13 presents processes occurring upon an encounter of r and N ... 

In connection with a study of a number of anticancer compounds which, presumably also act as inhibitors of free-radical polymerization, eight classes of compounds were studied as to their inhibitory properties. The classes studied were unsaturated hydrocarbons, phenolic compounds, quinones, amines, stable free-radicals, sulfiir compounds, carbonyl compounds, and metallic salts. The most effective inhibitors, of those evaluated, were cupric acetate and cupric resinate, followed by /runs-1,3,5-hexatriene, hydroquinone, benzoquinone, and diphenylamine as modest inhibitors. Among the low-activity inhibitors were 2,2-diphenyl-1-picrylhydrazyl, benzene thiol, and crotonaldehyde [70]. 

Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can, however, be greatly influenced by impurities that can act as ion scavengers. These... 

Chemical considerations indicate that the more diffuse the charges the more stable are the ions. Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can, however be greatly influenced by impurities that can act as ion scavengers. These can be water, ammonia, amines, or any other compounds that can be basic in character, affecting rates and molecular weights of the products. Typical cationic polymerizations proceed at high rates even at low temperatures, as low as — 100°C [8]. In the literature one can find many reports of cationic polymerizations of many different monomers with many different initiators. Often, however, such initiators are quite specific for individual monomers and their activities are strongly influenced by the solvents. 

Hydroquinone -kwi- non [ISV] (ca. 1872) (p-dihydroxybenzene, hydroquinol, p-hydroxyphenol, qui-nol) n. C6H4(OH)2. A white crystalline material derived from aniline, used, as are many of its derivatives, as an inhibitor of free-radical polymerization in unsaturated polyester resins and in monomers such as vinyl acetate. 

U.S. 4,168,982. Photopolymerizable Compositions Containing Nitroso Dimers to Selectively Inhibit Thermal Polymerization. Pazos, Jose F. (E. I. Du Pont de Nemours and Company). September 25, 1979. Cl. 430/281.1 430/917 522/16 522/18 522/28 522/63 522/65 522/76 522/121 522/167 Appl. December 7, 1977. Thermally stable photo-polymerizable compositions comprise (i) at least one nongaseous ethylenicaUy unsaturated compound, (ii) a nitroso dimer which is a noninhibitor of free-radical polymerization but thermally dissociates to nitroso monomer which is an inhibitor of free-radical polymerization, and (iii) an organic, radiation-sensitive free-radical generating system. 

Another common additive found in inkjet ink formulas are in-can stabilizers to extend the shelf life of the product. Typically, the shelf life must be at least 9 months although 12 months is preferred. Frequently, radical scavengers are used to prevent polymerization from occurring in the container. Also, it is important to leave adequate headspace in the bottle, as oxygen is also a very effective inhibitor of free radical initiated polymerization. 

Figure 6-2 compares the effects of inhibitors and retarders on the rate of free-radical polymerization. Polymerization at normal rales is resumed when and if the inhibitor or retarder has been consumed. 

Some substances suppress free-radical polymerization of monomers by reacting with primary radicals or polymer radicals to yield either nonradical products or radicals that are of too low reactivity to undergo propagation. Such polymerization suppressors are called inhibitors or retarders depending on their effectiveness. Inhibitors stop every radical and polymerization is completely halted until they are consumed. Retarders are less efficient and stop only a fraction of the radicals. In this case, polymerization is not halted but it occurs at a slower rate. Figure 6.12 compares these effects on the rate of free-radical polymerization. 

In order to explain how inhibitors work, we need to quickly describe the mechanism of free radical polymerization encountered for the ethylenically unsaturated monomers. For more details, several reviews (refs. 1-7) also described free radical polymerization. 

The inhibition of radical polymerization is important in the chemical industry to prevent unwanted polymerization of vinyl monomers during processing, storage and transportation. Having discussed some of the principles of free-radical polymerization, a brief overview of possible inhibition mechanisms is presented. A short summary of the major classes of radical inhibitors gives an idea of the wealth of choice of chemical compounds available for use in this application. Owing to specifications for industrial inhibitors and the attractive intrinsic properties of... 

In another study, the monoacrylate 35 was grafted onto PP in the presence of tris(acryIoyhnethyl)propane (36) as coagent.Again close to 100% grafting yield was obtained. This was so despite the fact that 35 was anticipated to be an inhibitor of free radical reactions (in fact, phenols are poor inhibitors of (meth)acrylate polymerization - Section 5.3.4). The tris-acrylatc 36 and related species have previously been used for producing crosslinked/branched xhe structure... 

The formation of polymer can be followed directly by isolation of the polymer as it is produced. This method has the advantage that the physical and chemical structure of the polymer can be studied as a function of conversion. In this method, the polymerization is terminated by adding suitable substances (inhibitors in free radical polymerization) or by quenching the reaction by cooling rapidly. The monomer and/or solvent can be distilled from the polymer, although often not all the monomer can be removed when the polymerizate viscosity is high. In addition, neither the initiator nor the catalyst is removed. A distillation must in any case be carried out at very low temperatures, since otherwise the polymer may be degraded or the polymerization may continue. 

The generation of free radicals usually does not immediately start polymerization in commercial adhesives. These contain small amounts of inhibitors, which are chemical compounds that prevent free radical polymerization. Inhibitors are purposely added to acrylic adhesives to obtain practical shelf life. Inhibitors stop polymerization by reacting with active free radicals to form a less reactive species... 

Because they are acrylic monomers, alkyl cyanoacrylate esters still require the addition of radical polymerization inhibitors, such as hydroquinone or hindered phenols, to prevent radically induced polymerization over time [3j. Since basic initiation of alkyl cyanoacrylate monomers is the predominant polymerization mechanism, large quantities of free radical inhibitors can be added, with little or no effect on adhesive performance. 

Vinyl Acetate CH3COOCH=CH2 OH compds, HCN, Halides, Halogens, Mer-cap tans, Amine, Silanes Oxygen Vap in Air 2.6 to 13.4% > Ambient > Ambient Inhibitor—Methyl Ether of Hydroquinone or 3-5ppm Diphenylamine. Store in a dry, cool place shield from light impurities 20.9-21.5 402 427 Free-radical polymerization initiated by Benzoyl Peroxide... 

At the conclusion of the induction period due to oxygen, polymerization sets in at a rate exceeding that for pure monomer under the same conditions. The polymeric peroxides apparently furnish a source of free radicals. Oxygen therefore combines the roles of inhibitor, comonomer, and (indirectly) of initiator. 

The hazards of a rigid classification of substances which may modify the course of a free radical polymerization are well illustrated by the examples of inhibitors and retarders which have been cited. The distinction between an inhibitor or retarder, on the one hand, and a co-monomer or a transfer agent, on the other, is not sharply defined. Moreover, if the substance is a free radical, it is potentially either an initiator or an inhibitor, and it may perform both functions as in the case of triphenylmethyl. If the substance with which the chain radicals react is a molecule rather than a radical, three possibilities may arise (i) The adduct radicals may be completely unreactive toward monomer. They must then disappear ultimately through mutual interaction, and we have a clear-cut case of either inhibition or retarda-... 

Free radical polymerization processes [41] are carried out in bulk, solution, suspension, emulsion, or by precipitation techniques. In all cases the monomer used should be free of solvent and inhibitor or else a long induction period will result. In some cases this may be overcome by adding excess initiator. 

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