Kinetic Data for Reactions of Carbon-Centered Radicals

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 a-cyanoalkyl radicals can, in principle, react with substrates either at 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M 1 s 1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or 

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05 

Acyl radicals undergo dccarbonylation. For aliphatic acyl radicals the rate constant for decarbonylation appears to be correlated with the stability of the alkyl radical formed. Values of the dccarbonylation rate constant range from 4 s for CH3C 0) to 1.5x10s s l [lor (CH.,)2C(Ph)C( )0] at 298 °C.3M The loss of carbon monoxide from phenacyl radicals is endothermic and the rate constant is extremely low (ca 10 8 s 1 at 298 nC).388 Consequently, the reaction is not observed during polymerization experiments. 

The Chemistry of Radical Polymerization Table 3.6 Kinetic Data for Reactions of Carbon-Centered Radicals 

A number of reportsindicate that primary radical termination can be important during polymerizations initiated by azonitriles. However, for the case of S polymerization initiated by AIBN, NMR end group determination shows that primary radical termination is of little importance except when very high rates of initiation are employed (e.g. with high initiator concentrations at high temperatures). Cyanoalkyl radicals give a mixture of combination and disproportionation in their reactions with other radicals (see also Sections 2.5, 

Absolute rate constants for the attack of aryl radicals on a variety of substrates 

The reactions are extremely facile in comparison with additions of 

---

The kinetic data for reactions of carbon-centered radicals with various silanes and the silanthrane derivatives 1-6 are numerous as shown in... 

---