Cyanoisopropyl radicals styrene

Absolute rate constants for addition reactions of cyanoalkyl radicals are significantly lower than for unsubstituted alkyl radicals falling in the range 103-104 M V1.341 The relative reactivity data demonstrate that they possess some electrophilic character. The more electron-rich VAc is very much less reactive than the electron-deficient AN or MA. The relative reactivity of styrene and acrylonitrile towards cyanoisopropyl radicals would seem to show a remarkable temperature dependence that must, from the data shown (Table 3.6), be attributed to a variation in the reactivity of acrylonitrile with temperature and/or other conditions. 

The secondary benzoate (4) is labile below 300°C whereas the primary benzoate (3) is stable. Both the model compounds were found to be labile at 300°C. Hence it can be concluded that the stability of the polymers is direcdy related to the stability of the respective benzoates. Tliis study clearly shows the effect of one weak link per chain on the polymer properties. An additional route to secondary benzoates is discussed below under termination. In related studies on styrene we have established that both the cyanoisopropyl radicals (from AIBN) and t-butoxy radicals are much more selective in this system and add virtually exclusively to the tail to give polymers tliat are thermally stable. 

These reactions are almost exclusively taking place within the solvent cage when styrene monomer is present. In the absence of monomer, however, with only a solvent present (eg, benzene), close to half of the products are formed outside the solvent cage, indicating that the addition of the cyanoisopropyl radicals to the monomer units is effectively competing with self-reactions of the primary radicals (94). 

Also, the electronics of the monomer greatly affect the rate of radical addition. For example, the relative rate of addition of tert-butoxy radical to various monomers has been found to correlate well with the e value of the monomers [62] (Fig. 12). For example, acrylonitrile has an electron deficient vinyl group (e = 1.2) while the styrene vinyl group is electron rich (e = — 0.8). Methyl radicals (electron rich) add two times faster to acrylonitrile than to styrene while tm-butoxy radicals (electron poor) add 57 times more rapidly to styrene. Therefore, most of the FR used for styrene polymerization tend to be of the electron deficient type (i.e. tert-butoxy, benzoyloxy, and cyanoisopropyl). 

A sample of styrene is polymerized with the help of a little azoisobutyronitrile as initiator. Hydrolysis of the resulting polymer releases 6 g of ammonia from 10 kg of polymer. If all the end groups are derived from the initiating radical (a-cyanoisopropyl), what is the average number of styrene units in the chain ... 

---