Inhibitor of radical polymerization

Quinones (Q) are well known as inhibitors of radical polymerization they terminate chains by the addition of alkyl radicals via the following reactions [7] ... 

Most monomers can be stored unchanged under nitrogen only for short times (hours or days), even in the dark at low temperature. For long-term storage, a suitable stabilizer is therefore indispensable. Effective stabilizers (inhibitors) of radical polymerization are quinones, phenols, amines, nitro compounds, and some metals or metal compounds. The addition of 0.1 to 1 wt% of hydroquinone or 4-ferf-butylpyrocatechol results in sufficient stabilization of many monomers. 

Quinones are the most extensively studied inhibitors of radical polymerization and they represent an important inhibitor class. They have complex behaviour and numerous inhibition mechanisms. 

Another approach is the reaction of diimides with divinyl monomers. Examples of this route, starting from divilylsulfone and pyromellitic, benzophenonetetracarboxylic and cyclopentanetetracarboxylic diimides have been reported (Scheme 22). The polymerizations are carried out in solution in the presence of inhibitors of radical polymerization, and the molecular weights achieved are not very high [129]. By a similar mechanism, polyimides have been prepared from diallylesters and cycloaliphatic [130] and aromatic diimides [131]. 

We [6,34] prepared polymers containing tetraphenylporphyrin (TPP) moieties, which were considered to function as inhibitors of radical polymerizations, and found that TPP moieties in the polymers were forced to interact with each other because they are so close. Accordingly, we introduced paramagnetic metal ions [Cu(II), Ag(II), VO(II), and Co(II)] into the TPP moieties of the polymers and investigated the magnetic behavior. The mag-... 

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 kinetics and mechanism of inhibition by stable radicals has been reviewed by Rozantsev el al,lS3 Ideally, for radicals to be useful inhibitors in radical polymerization they should have the following characteristics ... 

Certain monomers may act as inhibitors in some circumstances. Reactivity ratios for VAc-S copolymerization (r< 0.02, rVu -2.3) and rates of cross propagation are such that small amounts of S are an effective inhibitor of VAc polymerization. The propagating chain with a terminal VAc is very active towards S and adds even when S is present in small amounts. The propagating radical with S adds to VAc only slowly. Other vinyl aromatics also inhibit VAc polymerization.174... 

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

At first, methyl methacrylate (MMA) polymerization was used as a model reaction system. However, phenolic derivatives are well known as antioxidants and inhibitors for radical polymerization (lU), so such a reaction would not be expected to occur. However, by proper choice of vinyl compounds and initiators, it was found that such polymerization is possible, and that a variety of IPNs can be produced. 

They acted on 2-nitratoethanol with chlorides of acrylic (a), methacrylic (b) and croionic (c) acids in the presence of cuprous chloride as a polymerization inhibitor. The radical polymerization was initiated by benzoyl peroxide in nitrogen atmosphere. It lasted 20 min at 100 C. In the absence of a catalyst polymerization occurred at 20 C in the course of a few days. This observation is in agreement with the finding reported in V ol. II, p. 19 that nitrate esters can catalyse polymerization reaction, contrary to C-nitro compounds (Chapter IV). 

It is known that PAC plays the role of inhibitors of radical-chain processes, specifically, polymerization [177]. In all probability, interaction of PAC molecule with being formed radicals in CDA at irradiation takes place. Reaction between radicals and inhibitor with formation of completely inactive products may probably proceed here ... 

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... 

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ccntcrcd radicals yet react with carbon-centered radicals by coupling at or near diffusion controlled rates.This capability was utilized by Rizzardo and Solomon " to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, while an efficient inhibitor, shows little selectivity and its reaction with radicals is complex. ... 

Initiators, accelerators, and inhibitors of cyanoacrylate polymerization are used to modify the cure speed and storage stability of these adhesives. They can also be used to broaden the range of materials which can be bonded with cyanoacrylates. Initiators are those materials which are capable of polymerizing cyanoacrylate esters upon contact. These are, therefore, applied either to the substrate surface ( surface primers ), or mixed with the adhesive just prior to application. Accelerators are materials which do not cause polymerization on contact with monomer, but which increase the cure rate once the adhesive is applied. These chemicals are most often compounded with the monomer in the adhesive formulation. The distinction between these two classes can be blurred, as some additions will not cause immediate polymerization on contact but will shorten shelf life in the long run. Anionic polymerization inhibitors are Lewis or Bronsted acids which retard or completely inhibit anionic polymerization. Radical inhibitors prevent polymerization by adventitious, radical sources and are used to prolong the storage stability of the adhesive they generally do not affect cure speed. 

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 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... 

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