Cross disproportionation

As in the case of PS (Section 8.2.1) polymers formed by living radical polymerization (NMP, ATRP, RAFT) have thermally unstable labile chain ends. Although PMMA can be prepared by NMP, it is made difficult by the incidence of cross disproportionation.42 Thermal elimination, possibly by a homolysis-cross disproportionation mechanism, provides a route to narrow polydispersity macromonomers.43 Chemistries for end group replacement have been devised in the case of polymers formed by NMP (Section 9.3.6), ATRP (Section 9.4) and RAFT (Section 9.5.3). 

Rate Constants of the Cross-Disproportionation of Two Different Peroxyl Radicals (Experimental Data)... 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

One more mechanism responsible for the synergistic action of two phenols has recently been discovered during the study of disproportionation of phenoxyl radicals [42,43], This reaction is possible only for phenoxyls possessing C—H groups in the ortho- or para-position [43], For instance, 2,4,6-tris(l,l-dimethylethyl)phenoxyl is unable to disproportionate, whereas ionol radicals disproportionate. It was found that the cross-disproportionation of ionol and a-tocopherol radicals occurs much more rapidly than homodisproportionation (323 K [42]) ... 

In this reaction, a-tocopherol is regenerated at the expense of the ionoxyl radical. Rapid cross-disproportionation diminishes the total concentration of phenoxyl radicals (thereby preventing their participation in chain propagation) and reduces the reactive phenol (a-tocopherol is more reactive toward ROOH than ionol) into methylenequinone, which terminates the chains in the reaction with peroxyl radicals. 

Like acridinium salts, isoquinolinium salts disproportionate very easily in alkaline solution. In an extensive study. Bunting and Kabir (78JOC3662) investigated the crossed disproportionation of 166 with 167 to give 168 and 169 (Scheme 27). 

This interpretation of the mechanism of disproportionation is supported by two observations on crossed disproportionation reactions. In a kinetic study285 of the reduction of the 2-methylisoquinolinium cation by a series of2-benzyl-5-nitroisoquinolinium cations, the pH dependence of the reaction was observed to be consistent with hydride transfer from the pseudobase alkoxide ion 133 to the 2-methylisoquinolinium cation. Clark and Parvizi111... 

The irreversible decay of the dormant alkoxamine chains stops the monomer conversion rather abruptly at the time t = H( nk. For methyl methacrylate polymerizations this stop has been observed, and it has been demonstrated that it is caused by a considerable fraction of cross-disproportionation between the nitroxide and the propagating radicals.51,97112 Unfortunately, the factors governing disproportion-ation-to-combination ratios in radical—radical reactions are not well understood up to now, but stereo-electronic effects are certainly very important.112 Hence, one cannot yet predict a nitroxide structure that will allow living methacrylate polymerizations up to large conversions. 

In order to avoid side reactions, particularly isomerization, alkaline ions can be added during the impregnation stage of catalyst preparation. By this method, 1-butene production can be greatly reduced during metathesis of propylene and thus avoid the synthesis of other olefins by cross disproportionation between the normal metathesis products and the isomerized products. 

The study of cross-disproportionation of phenol radicals of ionols and a-toco-pherol allowed one to establish that this reaction is much faster than homodisproportionation... 

As a result, fast cross-disproportionation decreases the total concentration of the phenoxyl radicals and participation of phenoxyls in chain propagation and reduces the most active phenol (tocopherol more rapidly reacts with RO 2 than ionol) the methylenequinone that formed, in turn, terminates chains in the reaction with RO-2. 

Arden, E.A., L. Phillips, and R. Shaw (1964), Reactions in the diethyl peroxide-nitric oxide system. Part II. The kinetics of cross disproportionation and combination reactions between the ethoxyl radical and nitric oxide, J. Chem. Soc., 1964, 5126-5129. 

---