Nitrosobenzene polymerization

Fig. 21.—A comparison of the effects of 0.1 percent of benzo-quinone (curve II), 0.5 percent of nitrobenzene (curve III), and 0.2 percent of nitrosobenzene (curve IV) on the thermal polymerization of styrene at 100°C. Curve I represents the polymerization of pure styrene. (Results of Schulz. )...

Fig. 3-9 Inhibition and retardation in the thermal, self-initiated polymerization of styrene at 100°C. Plot 1, no inhibitor plot 2, 0.1% benzoquinone plot 3, 0.5% nitrobenzene plot 4, 0.2% nitrosobenzene. After Schulz [1947] (by permission of Verlag Chemie GmbH and Wiley-VCH, Weinheim).

Rather interestingly, radical cations have also been identified in styrene polymerizations catalysed by more conventional Lewis acid/co-catalyst systems [BF3/0(C2Hs)2, AlCla/PhOH, BFs/PhOH] by employing 2,4,6-tri-t-butyl-nitrosobenzene as a trap. One wonders once again therefore if the mechanisms which are generally accepted to operate do indeed represent a total picture. 

FIGURE 3.4 Polymerization of styrene at 373 K in the presence of curve 1, no inhibitor curve 2, 0.1% benzoquinone curve 3, 0.5% nitrobenzene curve 4, 0.2% nitrosobenzene. The time t and percentage conversion p are plotted. 

Curve 1 represents the polymerization of styrene in the absence of any agents. When benzoquinone is added, the polymerization is completely inhibited until all the benzoquinone has been consumed and then the reaction proceeds normally (eurve 2). The time interval a is the induction period and reflects the time taken for the benzoquinone to react with all the radicals formed until no more benzoquinone is left unreacted. In the presence of nitrobenzene, curve 3, the polymerization continues but at a much reduced rate. The action of nitrosobenzene is much more complex (curve 4). It acts first as an inhibitor but probably produces a substance during this period, which then acts as a retarder, and both effects are observed. 

Initially, it was decided that the donor substituent should contain the aromatic diamine functionality necessary for polymerization (27). However, the target monomer M-5 proved impossible to prepare by classical electrophilic substitution reactions. All attempts to couple triaiylamines substituted with peripheral electron donor groups with diazonium salts failed and resulted instead in deaiylation 18). The desired monomer M-5 was finally prepared by the Mills reaction of tris-p-aminophenylamine with p-nitronitrosobenzene 12 as shown in Scheme 2. This is an exceptionally versatile synthetic procedure limited only by the ability to prepare substituted nitrosobenzene partners. 

Figure 20 S. Influence of the addition of benzoquinone (BQ), nitrosobenzene (NSB), and nitrobenzene (NB) on the thermal polymerization of styrene S at 100°C. (After G. V. Schulz.)...

Since from 2-n i trobi phenyl, carbazole [see eqA36)] was obtained in 37.5% yield, an arylnitrene intermediate was suggested. Such labile organic intermediate is presumably bound to iron.This species could produce azobenzene by coupling, azoxybenzene by reaction with nitrosobenzene formed by reduction of nitrobenzene (3.2.1.), amines by hydrogen abstraction and polymeric tars by decomposition. 

Cationic polymerizations are propagated by addition of monomer on to the macrocation. Examples are the polymerization of isobutylene [see reaction (18-38)] or of nitrosobenzene. 

For example, in the case of thermal polymerization of styrene [1], benzoquinone acts as an inhibitor. When the inhibitor has been consumed, polymerization regains its momentum and proceeds at the same rate as in the absence of the inhibitor. Nitrobenzene [1] acts as a retarder and lowers the polymerization rate, whereas nitrosobenzene [1] behaves differently. Initially, nitrosobenzene acts as an inhibitor but is apparently converted to a product which acts as a retarder after the inhibition period. Impurities present in the monomer may act as inhibitors or retarders. The inhibitors in the commercial monomers (to prevent premature thermal polymerization during storage and shipment) are usually removed prior to polymerization or, alternatively, an appropriate excess of initiator may be used to compensate for their presence. 

---