Monomer stabilization inhibit polymerization

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. 

Inhibition of spontaneous polymerization of (meth) acrylates is necessary not only at their storage but also in the conditions of their synthesis proceeding in the presence of sulfuric acid. In this case, monomer stabilization is more urgent, since sulfuric acid not only deactivates mat r inhibitors but also is capable of intensifying polymer formation. The concentration dependence of induction periods in these conditions has a brightly expressed nonlinear character. And, unlike polymerization in bulk, decomposition of polymeric peroxides is observed at relatively low temperatures in the presence of sulfuric acid, and the values [X] of the amines studied are by ca. 10 times lower than [HQ]. ... 

Inhibition of spontaneous polymerization of (meth) acrylates is necessary not only at their storage but also in the conditions of their synthesis proceeding in the presence of suffirric acid. In this case, monomer stabilization is more urgent, since sulfuric... 

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]. 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

Polymerization is inhibited or retarded by the presence of certain substances, e.g., p-benzoquinone, 4-t-butylcatechol, 1,3,5-trinitrobenzene, and oxygen. These substances react with radicals to yield species which are no longer capable of chain propagation. The deliberate addition of such substances is useful for stabilizing monomers during transportation and storage. 

However, the initial absence of unstable groups is no guarantee for long-term stability of the compound. For example, some aldehydes and ethers are easily converted to peroxides by reaction with oxygen from air [35,37,38]. Organic peroxides represent a class of unstable materials while monomers represent a class of substances that can self-react by polymerization if not properly inhibited and if the temperature is not properly maintained. Runaway reactions can result in both of these examples. 

In normal practice, inhibitors such as hydroquinoue (HQ) [123-31-9] or the monometliyl ether of hydroquinoue (MEHQ) [150-76-5] are added to acrylic monomers to stabilize them during shipment and storage. Uninhibited acrylic monomers should be used promptly or stored at 10 °C or below for no longer than a few weeks. Improperly inhibited monomers have the potential for violent polymerizations. HQ and MEHQ require the presence of oxygen to be effective inhibitors therefore, these monomers should be stored in contact with air and not under inert atmosphere. Because of the low concentration of inhibitors present in most commercial grades of acrylic monomers (generally less than 100 ppm), removal before use is not normally required. However, procedures for removal of inhibitors are available (67). 

The polymerization starts after an inhibition time C which results from the presence of alkali used to stabilize the monomer. The values of the induction period depend on the inhibitor concentration and at constant inhibitor concentration on the incident light intensity, as schown by Eq. 5. 

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