Aryl radicals reaction with monomers

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

The reaction with aryl radicals of this kind has found an interesting application in the direct polymerization of vinylic monomers like styrene, methyl methycrylate (MMA), or hydroxyethyl methacrylate (HEMA) on the diamond. Furthermore, the covalent attachment of an initiator molecule allows for an atom transfer radical polymerization (ATRP) to take place immediately on the diamond surface and results in a covalently bound composite material (Figure 6.41). 

The second method involves one-electron oxidation of arenediazoate, hyponitrite, or cyanate anions by reaction with arenediazoazodiazonium ions. Aryl radicals then initiate polymerization [32] of the monomer/methyl methacrylate, and the corresponding stabilized oxyl radical is associated with the growing end of the poljrmer ... 

The process of organomagnesium synthesis yields a mixture of al-kyl(aryl)chlorosilanes with various numbers of radicals attached to the silicon atom. However, by regulating the ratio of the parent components and changing the conditions of the process, one can shift the reaction towards the preferential formation of a certain monomer. 

The types of compounds that can be polymerized readily by the radical-chain mechanism are the same types that easily undergo free-radical addition reactions. Alkenes with aryl, ester, nitrile, or halide substituent groups that can stabilize the intermediate radical are most susceptible to radical polymerization. Terminal alkenes are generally more reactive toward radical-chain polymerization than more highly substituted isomers. The dominant mode of addition in radical-chain polymerization is head-to-tail. The reason for this orientation is that each successive addition of monomer takes place in such a way that the most stable possible radical intermediate is formed. For example, the addition to styrene occurs to give the phenyl-substituted radical to acrylonitrile, to give the cyano-substituted radical ... 

All the Schiff s base complexes discussed so far are low-molecular weight complexes. An erbium(lll)-containing methacrylate metallopolymer was prepared by free-radical polymerization of a Schiff s base monomer LllH, followed by complex formation with hydrated erbium nitrate (Haase et al., 1996). By XRD, the presence of a smectic mesophase was shown. A special feature of the polymer is that the complexing group is an Ai-aryl substituted Schiff s base, rather than an Ai-alkyl substituted one. No further structural data were reported. A detailed study of the mesophase behavior of lanthanide complexes of Ai-aryl substituted Schiff s base ligands LllH was reported by Rao et al. (2002, 2010). The complexes were synthesized by reaction between the... 

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