Monomers Polymerization Inhibitors

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

The polymeric products can be made to vary widely in physical properties through controlled variation in the ratios of monomers employed in thek preparation, cross-linking, and control of molecular weight. They share common quaHties of high resistance to chemical and environmental attack, excellent clarity, and attractive strength properties (see Acrylic ester polymers). In addition to acryHc acid itself, methyl, ethyl, butyl, isobutyl, and 2-ethylhexyl acrylates are manufactured on a large scale and are available in better than 98—99% purity (4). They usually contain 10—200 ppm of hydroquinone monomethyl ether as polymerization inhibitor. 

The dehydrogenation of the mixture of m- and -ethyltoluenes is similar to that of ethylbenzene, but more dilution steam is required to prevent rapid coking on the catalyst. The recovery and purification of vinyltoluene monomer is considerably more difficult than for styrene owing to the high boiling point and high rate of thermal polymerization of the former and the complexity of the reactor effluent, which contains a large number of by-products. Pressures as low as 2.7 kPa (20 mm Hg) are used to keep distillation temperatures low even in the presence of polymerization inhibitor. The finished vinyltoluene monomer typically has an assay of 99.6%. 

Polymerization inhibitors are key additives which prevent premature gelation of the adhesive. The foimulator must carefully balance shelf stability and the required cure on demand. Due to its high propagation rate, MMA is difficult to inhibit. Some comments on specific inhibitors follow. The most common inhibitor to be found in component monomers is 4-methoxyphenol, which is also called the methyl ether of hydroquinone. This inhibitor is effective only in the presence of oxygen. A mechanism has been proposed, and is illustrated in Scheme 13 [128]. 

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. 

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

The use of vinyl monomers for gel formation requires a polymerization process in the formation. This technique is used to enable a solution to gel slowly even at high temperatures. An aqueous solution of a vinyl monomer is mixed with a radical-forming initiator, and if necessary, with a dispersant. The initiator decomposes at elevated temperatures and initiates the polymerization process. In this way, a gel is formed in place. The polymerization process is sensitive to molecular oxygen. To further delay curing, polymerization inhibitors may be added to the solution in small amounts. This technique is used in the treatment of subterranean formations, especially for plugging lost... 

Certain polymerization inhibitors added to stabilize monomers require a small concentration of dissolved 02 to be effective (NFPA... 

These monomers polymerize readily in presence of light, heat or catalysts (such as benzoyl peroxide) and must always be stored or shipped with inhibitor present to avoid spontaneous and explosive polymerization. 

Polymerization inhibitors stop or slow down polymerization by reacting with the initiator or growing-chain radicals. A wide variety of substances can behave as inhibitors quinones, hydroquinones, aromatic nitro compounds, aromatic amines, and so on. In cases where the inhibitor is a hydrogen donor (symbolized here by InH), then for inhibition to occur, the radical resulting from hydrogen transfer (In-) must be too stable to add to monomer. If it does add to monomer and starts a new chain, chain transfer occurs instead of inhibition. For perfect inhibition, the In- radicals must combine with themselves (or initiator radicals) to give inert products ... 

Oxazines with an olefinic substituent were suggested as polymerizable monomers.307-309 1,3-Oxazines were claimed as plasticizers for cellulose acetate,309 tanning agents,19 and corrosion inhibitors.310 N-Oxides of 1,3-oxazines were reported to be antioxidants and polymerization inhibitors.34... 

FIGURE 5.13 Left low MWCO membrane (deionized water 2-methoxyethanol = 3.7 1). Right high MWCO membrane (deionized water 2-methoxyethanol = 0.34 1). The post diameter is 50 pm. For phase-separation polymerization, the monomer is 2-(N-3-sulfo-propyl-N,N-dimethylammonium) ethyl methacrylate, the cross-tinker is methylene bisacry-lamide, and the plotoinitiator is 2,2 -azobis(2-methylpropanimidamide dihydrochloride). To prevent unwanted polymerization that may occur by heat and molecular diffusion outside the UV-irradiated region, a polymerization inhibitor, hydroquinone, is also added. To facilitate covalent attachment of the porous membrane to the silica surface, it is first coated with 3-(trimethoxysilyl)propylacrylate [347]. Reprinted with permission from the American Chemical Society. 

Monomers may be polymerized and should be stored with a polymerization inhibitor, from which the monomer can be separated by distillation just before use. 

Viscosity Effects. The behavior of a polymerization reaction depends markedly on the viscosity and on monomer and inhibitor concentrations in a sample, and these features of polymerization processes should be kept in mind for understanding the behavior of these photosensitive systems (42) ... 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

To prevent premature polymerization of styrene monomer, special inhibitors had to be added so that it could be stored until needed. Polymerization inhibitors were also required to prevent polymer formation during distillation of styrene monomer from ethylbenzene. 

Some substances retard or suppress free-radical polymerization by reacting with primary radicals or macroradicals to yield nonradical products or radicals thal are loo stable to add further monomer. Some inhibitors and retarders are chain transfer agents others act by addition processes. 

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). 

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