McLeish

K. O Connor, T. McLeish. Entangled dynamics of heahng end-grafted chains at sohd/polymer interface. Faraday Discuss Chem Sci 95 67-78, 1994. 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

However, it may be that the acid catalysis mechanism is more complex than that described by Scheme 6-5. Broxton and McLeish (1983) investigated the (ii)-diazo ether of a benzene derivative with a carboxy group in the 2-position. They also observed acid catalysis by added carboxylic acids, but, interestingly enough, the reactivity decreased as the strength of the added acid increased. 

First of all, there are the two products of O-coupling addition of methoxide ion to the diazonium ion, the (Z)- and (jE)-diazo methyl ethers. As discussed in Section 6.2, they are formed in reversible reactions with half-lives of the order of a fraction of a second (Z) to a minute (E). The two diazo ethers are, however, decomposed rapidly to the final dediazoniation products. We show in Scheme 8-47 the products obtained by Broxton and McLeish (1983 b) in the dediazoniation of 4-chloro-3-nitrobenzenedi-azonium ion (8.64) with methoxide ion in CH3OH. The products are 4-chloro-3-nitro-anisole (8.65, 49 9o), 2-chloro-nitrobenzene (8.66, 449o), and 2-nitroanisole (8.67). 

The ionization of alkyl (E)-arylazo ethers is subject to general acid catalysis when the reaction is carried out in the presence of carboxylic acid buffers (see Scheme 6-3), and the ionization is also subject to steric acceleration in the presence of bulky substituents ortho to the azo ether group (Broxton and Stray, 1980 Broxton and McLeish, 1983 a, and earlier work of Broxton s group). 

More recently Broxton and McLeish (1983 b) postulated that the aryldiazene is formed by direct hydride transfer from the methoxide ion to the diazonium ion (Scheme 8-53). In the opinion of Galli (1988, p. 775), and also of the present author, this and the following reactions can hardly compete with the chain reaction discussed above. 

Broxton and Bunnett (1979) determined the products of the reaction of 4-chloro-3-nitrobenzenediazonium ions with ethoxide ion in ethanol, which is exactly analogous to the reaction in methanol discussed earlier in this section. These authors found 12.8% 4-chloro-3-nitrophenetole, 83% 2-chloronitrobenzene, and 0.8% 2-nitrophenetole. When the reaction was carried out in C2H5OD, the first- and second-mentioned products contained 99% D and 69% D respectively. Dediazoniation in basic ethanol therefore results in a higher yield of hydro-de-diazoniation with this diazonium salt compared with the reaction in methanol. This is probably due to the slightly higher basicity of the ethoxide ion and to the more facile formation of the radical CH3-CHOH (Packer and Richardson, 1975). Broxton and McLeish (1983 c) measured the rates of (Z) — (E) interconversion for some substituted 2-chlorophenylazo ethyl ethers in ethanol. 

McLeish, T.C.B. et al. Dynamics of entangled H-polymers Theory, rheology, and neutron scattering. Macromolecules, 32, 6734, 1999. 

Aggeli A, Bell M, Boden N, Keen JN, Knowles PF, McLeish TCB, Pitkeathly M, Radford SE (1997) Nature 386 259... 

Aggeli A, Nyrkova lA, Bell M, Harding R, Carrick L, McLeish TCB, Semenov AN, Boden N (2001) Proc Natl Acad Sci USA 98 11857... 

McLeish, T.C.B., Milner, S. T Entangled Dynamics and Melt Flow of Branched Polymers. VoL 143, pp. 195-256. 

The authors are indebted to E. McCarney, C. A. Royer, D. Segel, M. Kataoka, Y Ka-matari, and G. V. Semisotnov for providing unpublished or previously published data and T. C. B. McLeish, G. Fredrickson, D. Shortle, R. Pappu, and G. Rose for many stimulating conversations regarding the reconciliation problem. ... 

Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ (1998) Phys Rev Lett 81 373... 

Buzza DMA, Fzea AH, Allgaier JB, Young RN, Hawkins RJ, Hamley IW, McLeish TCB, Lodge TP (2000) Macromolecules 33 8399... 

Aggeli, A., Bell, M., Boden, N., Keen, J. N., Knowles, P. F., McLeish, T. C., Pitkeathly, M., and Radford, S. E. (1997). Responsive gels formed by the spontaneous self-assembly of peptides into polymeric beta-sheet tapes. Nature 386, 259-262. 

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