Cyanoisopropyl radicals reaction with monomers

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). 

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. 

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. 

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

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. 

Primary radical termination may involve combination or disproportionation with the propagating radical. It is often assumed that small radicals give mainly combination even though direct evidence for this is lacking. Both pathways are observed for reaction of cyanoisopropyl radicals with PS (Scheme 19). The end group formed by combination is similar to that formed by head addition to monomer differing only in the orientation of the penultimate monomer unit. 

The reaction of oxygen with carbon-centered radicals (e.g., cyanoisopropyl. Scheme 15) affords an alkylperoxy radical (17) 62,63 spedes may initiate polymerization, thus forming a relativdy unstable peroxidic end group (18). With respect to most carbon-centered radicals, the alkylperoxy radicals (17) show an enhanced tendency to abstract hydrogen. The alkylperoxy radicals may abstract hydrogen from polymer, monomer, or other components in the system, " forming a potentially reactive... 

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