Cyanoisopropyl radicals

When heated to 120°C, AIBN decomposes to form nitrogen and two 2-cyanoisopropyl radicals. The ease with which AIBN forms radicals, and the fact that the rate of information does not vary much in various solvents has resulted in wide use of AIBN as a free-radical initiator. AIBN is used commercially as a catalyst for vinyl polymerisation (see Initiators). 

The design and use of new initiators to mediate addition reactions has attracted some attention with new peroxide and azo-derived initiators being described. The additions of 2-cyanoisopropyl radicals (derived from homolysis of the common radical initiator AIBN) to a range of alkynes have been examined. " The reactions were regioselective with alkynes bearing electron-withdrawing substituents but failed with hindered or alkylacetylenes. The same radical addition to Ceo has been studied by EPR. Two different types of adduct radicals were proposed. " " ... 

The byproducts of decomposition of certain dialkyldiazcncs can be a concern. Consider the case of AIBN decomposition (Scheme 3.13). The major byproduct is the ketenimine (lO).61 100"102 This compound is itself thermally labile and reverts to cyanoisopropyl radicals at a rate constant similar lo that for AIBN thermolysis.59,60 102 This complicates any analysis of the kinetics of initiation/2,60... 

Even though AIBN has a low transfer constant, the ketenimine formed by combination of cyanoisopropyl radicals (Scheme 3.13) is anticipated to be more susceptible to induced decomposition (Scheme 3.22).1Cb... 

The reactions of cyanoisopropyl radicals with monomers have been widely studied. Methods used include time resolved EPR spectroscopy,352 radical trappingj53 355 and oligomer00 356 and polymer end group determination. 1 Absolute341 and relative reactivity data obtained using the various methods (Table 3.6) are in broad general agreement. 

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. 

Cyanoisopropyl radicals generally show a high degree of specificity in reactions with unsaturated substrates. They react with most monomers (c.g. S, MMA) exclusively by tail addition (Scheme 3.4). However, Bcvington et al.11 indicated that cyanoisopropyl radicals give ca 10% head addition with VAc at 60 °C and that the proportion of head addition increases with increasing temperature. 

A simple model for the propagating species in MAN polymerization is the cyanoisopropyl radical (15). The reactions of these radicals (from AIBN Scheme 5.8) have been extensively studied. In contrast with the analogous esters 8-10 (Section 5.2.2.1.2), combination is by far the dominant process (Table 5.4). 

Table 5.4 Values of kjk for Reactions involving Cyanoisopropyl Radicals...

Cyanoisopropyl radicals (15) undergo unsymmetrical C-N coupling in preference to C-C coupling.11 1 The preferential formation of the ketenimine is a reflection of the importance of polar and steric influences.1"1 However, the ketenimine is itself thermally unstable and a source of 15, thus the predominant isolated product is often from C-C coupling. 

Preferential C-N coupling is also observed for oligomeric radicals (Scheme 5.9).117 A ketenimine (21) is the major product from the reaction of the "dimeric" MAN radical 18 with cyanoisopropyl radicals (15). Only one of the two possible ketcnimincs was observed a result which is attributed to the thermal lability of ketenimine 19. If this explanation is correct then, although C-N coupling may... 

Barton et al.1X7 have reported that primary radical termination between PBMA and cyanoisopropyl radicals (7) involves largely disproportionation. 

The monomer 359 has been formed in situ by decomposing the initiator (A1BN) in the presence of the corresponding nilroxide in a solution of S or 2-ethoxyethyl acrylate.744 The kinetics dictate that alkoxyamine formation, by coupling of the nitroxide with cyanoisopropyl radicals, will take place before eopolymerization. 

Irotn AIBN decomposition 76-7 from combination of cyanoalky] radicals 37-9 from cyanoisopropyl radicals 76, I 16, 257 from dimeric PMAN 257-8 thermal stability 257... 

Since these isotope effects have been interpreted in terms of a physical transition state, it is instructive to contrast this phenomenon with studies in which the absence of an isotope effect was used to demonstrate a physical rate determining step [97]. The most relevant example is rotational-diffusion control of radical disproportionation in the solid-state photolysis of azobisis-obutyronitrile (AIBN). Since there is normally a primary isotope effect on the disproportionation of cyanoisopropyl radicals to methacrylonitrile and isobutyronitrile, the absence of such an effect in the solid-state photolysis of... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

On the other hand, the transfer constant to the polymer is always low since the cyanoisopropyl radical has a rather weak reactivity. 

Selective thermal isomerization of 5,8-di(l-cyano-l-methylethyl)-l,3,6-cyclooctatriene (10), obtained from 1,4-addition ofa-cyanoisopropyl radicals to 1,3,5,7-cyclooctatetraene, to homogeneous 3,8-di(l-cyano-l-methylelhyl)bicyclo[4.2.0]octa-2,4-diene (12) has been observed in refluxing benzene (75 h) or xylene (20 h)87. The 1,3,5-triene 11 formed by an initial [1,5] hydrogen shift is regarded as an intermediate in this conversion87. 

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