Nitro compounds, polymerization inhibition

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 ... 

I. Kende and M. Azori Kinetics of inhibition of styrene polymerization by nitro compounds. IUPAC Symposium on Macromolecular Chemistry, Moscow 1960, Section II 31. 

Another example of the homolytic reaction of nitro compounds with free radicals is their inhibiting effect on addition polymerization of vinyl compounds, especially by the higher nitrated compounds which have the most pronounced inhibiting properties. 

Bartlett and Kwart [81] gave the figures referring to the so-called inhibition constants in the polymerization of vinyl acetate under influence of various substances, and mainly nitro compounds (Table 27). 

As Kite [85] observed, aromatic nitro compounds inhibited the polymerization of vinyl acetate more strongly than that of methyl methacrylate. 

Although it was possible to obtain the monomer 2,4,6-trinitrostyrene (Wiley and Behr [44]) every attempt to polymerize it has proved useless. This is understandable in the light of the fact that aromatic nitro compounds inhibit polymerization processes (p. 214). Similarly 2,4-dinitrostyrene has not been polymerized. 

Inhibition of polymerization by aromatic nitro compounds 115 (1/214-216) Initiating explosives 462-505 (111/120-240)... 

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. 

A number of papers have been dedicated to homolytic degradation of TN under the action of ultraviolet or 7-rays. The reaction of TNM with bases (e. benzidine) beings with the formation of CT complexes [183] yielding radical anions which in turn are split into radicals (NO ) and anions (e.g. nitroformanion Irradiation with 7-rays at 77K yielded radicals N02 and C(N02)3 (184). As most nifro compounds TNT inhibits polymerization induced by radiati-[185, 186] and free radical polymerization [I8O- 191]. This is rationalized f the fact that TNM is a radicals acceptor. The higher the number of nitro groui in nitro alkanes the stronger the inhibition of polymerization (189). 

A further important class of retarders, generally less effective than quinones, are the aromatic nitro compounds. They show very different effectiveness toward the retardation of different monomer types the polymerization of vinyl acetate is inhibited, the styrene polymerization is retarded, but there is nearly no influence on the poljunerization rate of acrylates and methacrylates. The effectiveness, however, increases with the number of nitro groups per molecule. 

Aromatic nitro compounds act as inhibitors and show greater tendency toward more reactive and electron-rich radicals. Nitro compounds have very little effect on methyl acrylate and methyl methacrylate [5,10,11] but inhibit vinyl acetate and retard styrene polymerization. The effectiveness increases with the number of nitro groups in the ring [1 13]. The mechanism of radical termination involves attack on both the aromatic ring and the nitro group. The reactions are represented as follows ... 

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