Cyanoisopropyl radicals from AIBN

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. 

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). 

A simple model for the propagating species in MAN polymerization is the cyanoisopropyl radical (15). The reactions of these radicals (from AIBN Scheme... 

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... 

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] ... 

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 reaction of oligostyrcnc radicals with cyanoisopropyl radicals (7) has been studied by several groups and reported to give exclusively combination (OS C, toluene), " or mainly combination (bO C, ethyl acetate 98 C, toluene "). Moad et a/. " examined S oligomerization in toluene at 98°C using high concentrations of AIBN as initiator. While the major products arose from combination, they also isolated and identified small amounts of disproportionation products thus demonstrating that disproportionation does occur. 

In the decomposition of AIBN, besides the formation of the products (tetramethyl succinonitrile (TMSN), isobutyronitrile (IBN) and MAN) from the normal Carbon-Carbon reaction of the cyanoisopropyl radicals, a ketenimine (5) from a Carbon-Nitrogen reaction also occurs. This arises because one of a pair of cyanoisopropyl radicals can tautomerize to a keteniminyl radical which subsequently reacts with the second cyanoisopropyl radical to form the ketenimine. This reaction is thermally reversible and the ultimate products would be TMSN, IBN and any polymerized MAN. 

Johns and Willing have reported an interesting example of polycyclization involving addition to a cyano group (Scheme 155). The reaction of the diallyl amine 444 with AIBN gives the ketone 448. This would result from addition of the cyanoisopropyl radical to 444 followed by Cy5 cyclization of 445 in a Cy5/Cy6 case to the pyrrolidine radical 446. This radical would cyclize to 447 by Cy6 intramolecular addition to the cyano group in a Cy6/Cy7 case and, finally, the intermediate imine would be hydrolyzed to the ketone 448. 

The reaaions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio-or chemoselective. Reactions, such as head addition, abstraction, or aromatic substitution, often compete with tail addition. In the sections that follow, the complexities of the initiation process will be illustrated by examining the initiation of polymerization of two commercially important monomers, S and methyl methacrylate (MMA), with each of three commonly used initiators, azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO), and di-t-butyl peroxyoxalate (DBPOX). The primary radicals formed from these three initiators are cyanoisopropyl, benzoyloxy, and t-butoxy radicals, respectively (Scheme 7). BPO and DBPOX may also afford phenyl and methyl radicals, respectively, as secondary radicals. 

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