Free-radical oligomerizations

Peroxidases are ubiquitous, and many are b-type heme proteins. Several good reviews summarize years of peroxidase research and describe peroxidase applications [57 -61]. Some of the reactions catalyzed by peroxidases are listed in Tab. 10.2 and include oxidation of aromatic and heteroatom compounds, epoxidation, enantioselective reduction of racemic hydroperoxides, free radical oligomerizations and polymerizations of electron-rich aromatics, and the oxidative degradation of lignin [58, 60],... 

Exploration of the template controlled free-radical oligomerization of other activated olefins began with standard monomers utilized in bulk polymer synthesis and the template 63. Vinyl acetate and acrylonitrile led only to uncontrolled polymerization, while vinylene carbonate did not react under the standard experimental conditions. More exotic monomers, such as vinyl trifluoroacetate and rert-butyl acrylate, were also unsuccessful. Only methyl acrylate polymerization was arrested by template 64 to provide the macrocyclized product 96 in modest yield as a mixture of five diastereomers (Scheme 8-25). Subsequent studies with the more effective thiophenyl-bearing template 63 at lower temperatures improved this yield to 35%. The diastereomer distribution was reminiscent of the methyl methacrylate-derived product, although no stereochemical assignments were made in this case either. 

Attempts to extend the template mediated free-radical oligomerization of MMA to the longer (NBD)4-derived template 67 did not afford unequivocal results (Scheme 8-26). 

Richards, D. H., Controlled molecular weight in the free radical oligomerization, J. Polym. Sci. Polym. Lett. Ed., 6, 417, 1968. 

Oligomeric Vinylphosphonate. A water-soluble oligomer, Fyrol 76 [41222-33-7] is produced by reaction of bis(2-chloroethyl) vinylphosphonate and dimethyl methylphosphonate with elimination of all the chlorine as methyl chloride (127,128). This Hquid, containing 22.5% P, is curable by free-radical initiation, on cotton or other fabrics. Nitrogen components, such as A/-methylolacrylamide or methylolmelamines, are usually included in the finish, which can be durable to multiple launderings (129,130). 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Antioxidant free radicals (A H) produced in this way are relatively stable, but they may react with another lipid free radical to form a quinone [15.3] and [15.4]. Quinones can react with amine or thiol groups of proteins, forming polymerisable yellow or red coloured compounds (Pokorny, 1987). Antioxidant free radicals may also react with either another antioxidant free radical or a lipid free radical, forming dimers [15.5] or copolymers [15.6] and [15.7], respectively. Oligomeric or condensated products of antioxidant free radicals usually possess moderate antioxidant activities too (Pokorny et al, 1974) ... 

However, an important problem arises during the peroxidative removal of phenols from aqueous solutions PX is inactivated by free radicals, as well as by oligomeric and polymeric products formed in the reaction, which attach themselves to the enzyme (Nazari and others 2007). This suicide peroxide inactivation has been shown to reduce the sensitivity and efficiency of PX. Several techniques have been introduced to reduce the extent of suicide inactivation and to improve the lifetime of the active enzyme, such as immobilization. Moreover, Nazari and others (2007) reported a mechanism to prevent and control the suicide peroxide inactivation of horseradish PX by means of the activation and stabilization effects of Ni2+ ion, which was found to be useful in processes such as phenol removal and peroxidative conversion of reducing substrates, in which a high concentration of hydrogen peroxide may lead to irreversible enzyme inactivation. 

Research Focus Synthesis of oligomeric porphyrins for use as free radical traps in humans. Originality The preparation of oligomers and polymerizable porphorins for use in... 

---