Free-radical polymerization inhibitors

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... 

Free radical polymerization inhibitors have been prepared by reacting tetramethylpi-peridive oxyl derivatives with selected glycidyl intermediates. When used as a styrene polymerization regulator in 1.0mol% and 0.1mol% at 120°C, molecular weights of 2400 and 43,400 daltons, respectively, were obtained. 

The decomposition inhibitors are free radical polymerization inhibitors. 

Materials. VTMSK (III) was synthesized in a four-step reaction sequence, as reported previously (ii). The bright-yellow monomer was stored in a sealed glass ampoule over a free-radical-polymerization inhibitor at -15 °C. IPTMSK (IV) was obtained by a procedure reported by Danheiser et al. (13) and stored as just described. All the spectroscopic properties of VTMSK and IPTMSK were in agreement with the literature data (12, 13). 

THE USE OF PHENOLIC COMPOUNDS AS FREE-RADICAL POLYMERIZATION INHIBITORS... 

MAJOR USES Insecticide herbicide fungicide free radical polymerization inhibitor used in emulsifiable concentrate formulations used in the dyestuff industry. 

OTHER COMMENTS used in industry as a selective herbicide and insecticide used in the dyestuff industry used as a free-radical polymerization inhibitor. 

This problem has been solved by careful choice of anionic polymerization inhibitors. The materials employed are acidic compounds present at levels between 5 and 100 ppm. Table 1 shows a list of typical additives. Free radical polymerization inhibitors are also added. These are phenolic compounds such as hydroquinone or hindered phenols. 

It should not interfere with the photoreaction, e.g. as a triplet-state quencher of a photoinitiator or photosensitizer, or as a free-radical polymerization inhibitor. 

Free radical polymerization inhibitors such as hydroquinone usually are added to the resin mixture to prolong storage life. The presence of these inhibitors causes an induction period in the final curing step. The system will gel after about 1 percent cross-linking takes place, and then the rate of curing rapidly... 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

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. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reactions Stability During Transport Stable Neutralizing Agemsfor Acids and Caustics Not pertinent Polymerization Polymerizes when exposed to heat, ultraviolet light, or free-radical catalysts Inhibitor of Polymerization 200 ppm Hydroquinone. 

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... 

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-... 

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). 

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. 

Also important is that the induction period, which was about 3 min in the ESR experiment, was found to increase to about 30 min in isothermal DSC scans performed at the same cure temperature (Tollens and Lee, 1993). This is possibly due to the presence of dissolved oxygen (coming from air) in the DSC samples. Oxygen is a known inhibitor of the UP-S free-radical polymerization. This is a very important fact rate equations determined... 

The present observations could well be explained on the same basis since hexamethylcyclotrisiloxane polymerizes via both anionic and cationic mechanisms (5). (No evidence exists to date for free radical polymerizations.) The greater activity of H2O as an inhibitor, as compared with NH3, is explained on the basis of a more rapid ion transfer. Since this would be a hydrogen ion rather than a hydride ion, it further suggests an anion as the primary polymerization species in the present polymerization. The authors feel, however, that further work is required before definite conclusions can be drawn regarding the polymerization mechanism or mechanisms. 

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 ... 

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