urbaf2

AROMATIC NITRO COMPOUNDS 215

Stopping the polymerization consisted here in the formation of a nitro compound

(I) and an ether (IV).

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).

TABLE 27

INHIBITION CONSTANTS IN THE POLYMERIZATION OF VINYL ACETATE AT 45°C

Inhibitor

Nitrobenzene

p- Nitrotoluene

Dinitrodurene

o- Dinitrobenzene

m- Dinitrobenzene

p- Dinitrobenzene

sym-Trinitrobenzene

Sulphur

Z 1

(initial inhibition constant)

38

40

2.5

96

105

267

890

410

More recent investigations by Ihrig and Wong [83] in which the authors used

optically active nitrodiphenyl derivatives for studying the mechanism of the reaction,

confirmed the second scheme, i.e. the attack on a growing radical by a nitro

group.
Recently Inamoto and Simamura [74] investigated the interaction of l-cyano-
1-methylethyl radicals and various nitro compounds (nitrobenzene, m- dinitroben-

zene, nitromethane, tetranitromethane) and Bevington and Ghanem [84] have studied

the effects of picric acid and m- dinitrobenzene on the sensitized radical polymeri-

zation of styrene. Picric acid proved to be a rather inefficient inhibitor. m- di-

nitrobenzene was found to be a polymerization retardant. By using^14 C-labelled

specimens of the nitro compounds the authors determined the amounts of nitro

compounds incorporated in the polymer. The average number of retardant mole-

cules per polymer molecule was found to be 0.5-0.7.

On the basis of these experiments and of those of Inamoto and Sinamura,

Bevington and Ghanem suggest the interaction of polymer radical with m- dinitro-

benzene is likely to be:

(36a)

where P represents a polystyrene chain with a terminal initiator fragment.