Cyanoalkyl radicals

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

While it is desirable and important to have some knowledge of radical stabilities, the following sections will show that this is only one, and often not the major, factor in determining the outcome of radical reactions. 

In radical polymerization and in most radical reactions the radical species arc present only in low concentrations (total concentration 10 8-1 O 7 M). Radicals are 

Largely for these reasons, radicals are most often characterized indirectly by examining the products of their reaction. Many of the methods used to study radical reactions have been applied to study initiation of polymerization. Some of these techniques are detailed in Section 3.5. 

The reactions of cyanoisopropyl radicals with monomers have been widely studied. Methods used include time resolved EPR spectroscopyradical trapping 

Absolute and relative reactivity data obtained using the various metliods (Table 3.6) are in broad general agreement. 

The Chemistry of Radical Polymerization Table 3.6 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 10 -lO -is i 341 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. 

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, 

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Thermal or photochemical decomposition of azonitriles (e.g. AIBN) affords o-cyanoalkyl radicals (Scheme 3.71 ).29... 

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

However, there are also examples of addition across a strained carbon-carbon single bond, as occurs with bicyclobutane1 and derivatives (Scheme 4.21, Scheme 4.22).180,181 Interestingly, l-cyano-2,2,4,4-letramethylbieylobulane (31) is reported to provide a polykctcniminc (Scheme 4.22).183 This is the only known examples of a a-cyanoalkyl radical adding monomer via nitrogen. 

While a polar mechanism has been suggested for the formation of ketenimines by attack of triphenylphosphine or triethyl phosphite on the nitrogen of chlorodiphenylacetonitrile (36), ketenimines form from 1-cyanoalkyl radicals as well (32, 50). 

For combination processes involving cyanoalkyl radicals, reversible C,N-coupling occurs in competition with C,C-coupling. Steric factors appear to be important in determining the relative amounts of C,C- and C,N-coupling and... 

It is particularly typical of grafting amine-containing monomers peroxides cause oxidation of amino groups, which leads to a loss of commercial value of a grafted product. It should be remembered that most of the azo compounds are unsuitable as initiators not only because of half-life (tq.s) but also because cyanoalkyl radicals formed are inactive in abstraction reactions of hydrogen from chains. An exception is phenylazo-compounds (33) being a source of phenyl radicals that are most active in hydrogen abstraction reactions (Table 10.3). 

Figure 25 General mechanism of the reaction of cyanoalkyl radicals with metal complexes and olefin monomers.

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