What is the classification and mechanism of polymerization inhibitors?

December 30, 2022 Longchang Chemical

What is the classification and mechanism of polymerization inhibitors?

Free radical polymerization of monomers in the storage or processing and purification process, often due to the role of light, heat and other factors and polymerization, adding a small amount of polymerization inhibitor can avoid this destructive reaction. In the polymerization process, some monomer polymerization to a certain conversion rate after the need to stop or have a tendency to burst polymerization, as long as the timely addition of polymerization inhibitors, it may soon end or stop the reaction. Polymerization inhibitors are primary radicals or chain radicals into stable molecules or the formation of very low activity is not enough to continue the polymerization reaction of a stable radical class of substances. In addition, in the ionic polymerization process sometimes in order to terminate the reaction or make the reaction prepolymer stability, sometimes adding some acidic or alkaline compounds as a blocking agent, usually called stabilizers, due to the type and performance of simple, generally not discussed.

In the polymerization process monomer in storage, transportation is often added to the polymerization induction period (i.e., the polymerization rate of zero for a period of time), the length of the induction period is proportional to the content of the polymerization inhibitor, after the consumption of polymerization inhibitor, the end of the induction period, that is, according to the normal rate of the presence of no polymerization inhibitor. Therefore, the polymerization inhibitor should be removed before the monomer is used. Generally, the polymerization inhibitor is a solid substance with little volatility, so it can be removed during the distillation of the monomer. Commonly used resist to the polymerization of hydroquinone can react with sodium hydroxide to generate a water-soluble sodium salt, so it can be removed by washing with 5% to 10% sodium hydroxide solution. Cuprous chloride and ferric chloride and other inorganic polymerization inhibitors can also be removed by acid washing.

The classification and mechanism of commonly used polymerization inhibitors are as follows.

(1) polyphenols polymerization inhibitor polyphenols and substituted phenols is a class of widely used, the effect of a good polymerization inhibitor, but must be dissolved in the monomer when there is oxygen to show the blocking effect. The mechanism of polymerization is the phenol is oxidized to the corresponding quinone and the chain of free radicals combined to play the role of polymerization. In the presence of phenolic inhibitors, the peroxide radicals are quickly terminated to ensure that there is sufficient oxygen in the monomer to prolong the polymerization period. A large number of experimental results have proved that the inhibitory effect of phenols is actually an antioxidant effect, and their inhibitory activity is related to their molecular structure and properties, so phenols that are easily oxidized to quinone-like structures such as hydroquinone have high reactivity with peroxyl radicals and high inhibitory activity. When benzene ring with electron-absorbing group, the reaction activity with peroxy radical is low, and the blocking activity is also low; on the contrary, with pushing electron group, the reaction activity with peroxy radical is high, and the blocking activity is also strong. Commonly used species are hydroquinone, p-tert-butylcatechol, 2,6-di-tert-butyl-p-methylphenol, 4,4′-di-tert-butylbiphenyl and bisphenol A, etc.

(2) quinone polymerization inhibitors quinone polymerization inhibitors are commonly used molecular polymerization inhibitors, the amount of 0.01% to 0.1% will be able to achieve the expected polymerization blocking effect, but the effect of different monomer blocking different. To benzoquinone is styrene, vinyl acetate effective inhibitor of polymerization, but methyl acrylate and methyl methacrylate only play a role in slow polymerization. The mechanism of quinone blocking is not fully understood, it may be that quinone and radicals undergo addition or disproportionation reactions to produce quinone-type or semi-quinone-type radicals, and then combine with reactive radicals to obtain inactive products, which play a role in blocking polymerization. The ability of quinone to block aggregation is related to both the structure of quinone and the nature of monomer. The quinone nucleus has electrophilic properties, and the substituents on the quinone ring have an effect on the electrophilicity, which, together with the site-blocking effect, results in the difference of the ketone blocking efficiency. The number of radicals that can be terminated per molecule of p-benzoquinone is greater than 1, or even up to 2. Tetrachlorobenzoquinone and 1,4-naphthoquinone can be added to unsaturated polyester resins containing styrene to play a good role in blocking the polymerization and improve storage stability. Tetrachlorobenzoquinone is an effective polymerization inhibitor for vinyl acetate, but has no polymerization inhibiting effect on acrylonitrile.

(3) aromatic amine polymerization inhibitors aromatic amine polymerization inhibitors are both alkene monomer inhibitors, but also polymer materials, antioxidant aging agent. Aromatic amine compounds are not as effective as phenols in blocking polymerization, only for vinyl acetate, isoprene, butadiene, styrene, but no blocking effect on acrylates and methacrylates. Nitrobenzene acts as a polymerization inhibitor by generating stable nitroxide radicals with free radicals. Aromatic amines and phenols are similar in their mechanism of polymerization, and for some monomers, the use of the two in a certain ratio will have a better effect on polymerization than a single use. For example, hydroquinone and diphenylamine mixed, or tert-butyl catechol and phenothiazine mixed, the effect of polymerization than either alone when the effect is increased by 300 times. The blocking activity of aromatic amine polymerization inhibitors is related to the nature of their molecular substituents, and the blocking activity of aniline will be enhanced when it has an electron-pushing group at the para position. When the hydrogen in the amino group is replaced by methyl, the blocking activity is significantly reduced. For aniline, the activity of the amino group is higher at the 1-position than at the 2-position, and the activity increases with more amino groups, and decreases significantly when the naphthalene ring carries an electron-absorbing group. The hydrogen on the amino group of p-phenylenediamine is substituted by alkyl, aryl derivatives, the blocking activity are higher. Commonly used aryl amine polymerization inhibitors include p-toluidine, diphenylamine, benzidine, p-phenylenediamine, N-nitrosodiphenylamine, etc.

(4) Free radical polymerization inhibitor 1,1-diphenyl-2-trinitrophenylhydrazine is a typical free radical type polymerization inhibitor. Due to the strong conjugate stabilization and huge spatial site resistance, this compound can exist in the form of free radicals, which cannot dimerize by itself and cannot initiate monomers, but can trap active radicals, which is an ideal polymerization inhibitor. Although the free radical-type polymerization inhibitor blocking effect is excellent, but the preparation is difficult, expensive, monomer refining, storage and transportation, termination of polymerization are less used this inhibitor, limited to the determination of the initiation rate.

(5) inorganic compounds polymerization inhibitor inorganic salts is through charge transfer and the role of polymerization, ferric chloride polymerization blocking efficiency, and can be eliminated by chemical dose 1,1 free radicals. Sodium sulfate, sodium sulfide, ammonium thiocyanate can be used as an aqueous phase polymerization inhibitor. Sodium sulfide, sodium dithiocarbamate and methylene blue and other nitrogenous, sulfur compounds in some monomers also have effective polymerization blocking effect. Transition metal salts with variable valence have a polymerization blocking effect on some monomers because these substances can terminate the polymerization reaction by bursting the active chain by electron transfer. Other compounds such as cuprous oxide, cobalt methacrylate, etc. have good polymerization blocking effect.

The choice of polymerization inhibitor is mainly required to have a high polymerization blocking efficiency, it should also consider its solubility in the monomer, and the adaptability of the monomer, can be easily removed from the monomer by distillation or chemical method of polymerization inhibitor. It is best to choose a polymerization inhibitor that can act as a blocker at room temperature and decompose rapidly at reaction temperature, so that it can be removed from the monomer to reduce the trouble and to ensure the smooth polymerization reaction.

①Miscibility with monomer and resin is good, only miscible can play a role in polymerization.

②Can effectively prevent the occurrence of polymerization reaction, so that monomer, resin, emulsion or adhesive has sufficient storage period.

③Polymerization inhibitor in the monomer is easy to remove or does not affect the polymerization activity. It is best to choose the effective polymerization inhibitor at room temperature, and at a suitably high temperature to lose the polymerization inhibitor, so that the inhibitor does not have to be removed before use. For example, tert-butyl catechol, p-phenol monobutyl ether is this type of polymerization inhibitor.

④ does not affect the physical and mechanical properties of adhesives and sealants curing. Polymer inhibitor in the preparation of adhesives in the process of oxidation due to high temperature discoloration and affect the appearance of the product.

⑤ several inhibitors used in conjunction, can significantly improve the effect of polymerization. For example, unsaturated polyester resin with hydroquinone, tert-butyl catechol and copper naphthenate 3 kinds of inhibitors, hydroquinone activity is the strongest, in the miscible with styrene and polyester can withstand high temperatures of about 130 ℃, in 1 min without copolymerization, can be safely mixed dilution. Tert-butyl catechol is poor at high temperatures, but at a slightly lower temperature (for example, 60 ℃), its blocking effect is 25 times higher than that of hydroquinone, and can have a longer storage period. Copper naphthenate acts as a blocker at room temperature and a promoter at high temperature: also, for example, in the presence of oxygen. For tert-butyl catechol and phenothiazine, hydroquinone and diphenylamine mixed use, the blocking effect is about 300 times higher than either alone.

For example, iodine at 10-4 mol/L is an effective polymerization inhibitor, but more than this amount can trigger polymerization reactions. Iodine is generally not used alone, but a small amount of potassium iodide should be added to increase the solubility and improve the efficiency of polymerization blocking.

(7) Non-toxic, harmless, no environmental pollution.

⑧Stable performance, cheap and easy to obtain.

 

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