Inhibition radical polymerization

Aromatic nitro compounds inhibit radical polymerization. However, they do not seem to inhibit ionic polymerization [137]. On the contrary Rumanian authors reported [190] that nitrobenzene as welt as nitromethane and nitro ethane increase the rate of cationic polymerization of W-vinylcarbinol. 

However, one should take into consideration that fact, that atomic radicals, being formed, especially of tertiary atom, may take part in the chain transmission onto monomer [178]. Kinetic dependences of the rate of inhibited radical polymerization of styrene in the presence of XXXV, XXXVI, XXXVII (Figure 2.17 - 2.19) show that the rate of polymerization reaction decreases with the increase of azomethines concentration and depends on structure, structural features and efficiency of the chain of PAC conjugation. 

At this same time there are data that arylsulfanamides may inhibit radical-chain process of destruction. Kinetics of inhibited radical polymerization of methyl methacrylate, initiated by dinitrile of azo - bis - isobutyric acid, was studied to evaluate inhibiting activity of carbazolsulphonamides. 

One unusual and surprising characteristic of tertiary aromatic amines is that in addition to acting as accelerators, the same compounds in low concentration in the presence of oxygen and an initiator may act as inhibitors of polymerization (41, 42). This behavior has also been attributed to the ability of the amine to engage in charge-transfer complex formation. Oxygen also inhibits radical polymerization and results in uncured films at the surface of dental sealants (42). 

There are also a t riety of other parameters that may lead to alterations of the initiation, propagation, and termination reactions, adding even more complexity to the reartion behavior. For instance, oxygen is well known to inhibit radical polymerizations due to its high reactivity toward radicals and can influence network behavior with sample depth. Although a standard radical mechanism with an initiator is discussed in this section, other techniques (e.g., thiol-ene polymerizations) will be discussed in subsequent sections. 

Benzoquinone can be used to inhibit radical polymerizations. This compound reacts with a radical intermediate, R-, to form a less reactive radical that does not participate in chain propagation steps and thus breaks the chain. 

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 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... 

Oganova et a/. observed that certain cobalt (II) porphyrin complexes reversibly inhibit BA polymerization presumably with formation of a cobalt (111) intermediate as shown in Scheme 9.27. Thus, it seemed reasonable to propose these species may function as initiators in living radical polymerization.250 259... 

PA exerts an inhibiting action on free radical polymerizations. Bartlett et al (Ref 28) ascribe this to the following reactions ... 

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-... 

One serious fault of these materials is that the presence of an electron-rich phenolic hydroxyl group inhibits free-radical polymerization. Thus, composite resins placed over them do not polymerize completely. 

In addition, Stansbury Brauer (1985), taking advantage of the fact that vanillates do not inhibit free-radical polymerization, incorporated... 

It is commonly known that vinyltin compounds undergo no free-radical polymerization and do not readily copolymerize with various vinyl monomers which can be attributed to their inhibiting effect towards radical reactions48,79. ... 

The net result is a decrease in initiator efficiency and an attendant increase in polymer molecular weight. In fact, all of our work on radical polymerization of phenol-containing vinyl monomers suggests that inhibition and transfer problems are at most minor, if AIBN is used as initiator and oxygen is carefully excluded from the reaction mixtures (9). 

Owing to their liquid or semisolid nature, monomers are easy to process into polymers. For radical polymerization the use of solid AIBN for liquid monomers at room temperature and liquid MEKP for semisolid monomers or a mixture of liquid and semisolid monomers with some heating is convenient. During the course of curing at 85- 100°C for 22 h the problem of surface inhibition of free radicals by oxygen from the air can be avoided by inert-gas blanketing. 

Radical polymerization of maleic anhydride and fullerene was used to obtain a new material, the photodynamic properties of which have been studied in vitro and in vivo. HeLa and bone tumor cell growth were inhibited by treatment with fullerene and light, so the polymer was tested on mice affected by bone tumor. After injection and irradiation, tumor size and weight were reduced and the mouse survival time was extended (Jiang and Li, 2007). The photodynamic properties of a supramolecular cucurbit[8]uril-fullerene complex have been studied by the same authors (Jiang and Li, 2006) who attributed HeLa cell death mainly to the damage of membrane phosphohpids and proteins. 

Normal polymerization commences only after complete consumption of the oxygen this is then accelerated by the formation of additional initiating radicals through the thermal decomposition of the polymeric peroxide. Thus, molecular oxygen at first inhibits the polymerization, but after its consumption there is an accelerating action. 

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