Reactive processing chain modification

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

Classes of Polymer Chain Modification Reactions, Carried out in Reactive Polymer Processing Equipment, 604... 

CLASSES OF POLYMER CHAIN MODIFICATION REACTIONS, CARRIED OUT IN REACTIVE POLYMER PROCESSING EQUIPMENT... 

There are many polymer chain modification reactions of different types that have been carried out on polymer melts processed in single and twin rotor extruders. This activity, (4-6) in the analysis of polymerization reactors, driven by market forces seeking to create value-added polymers from commodity resins, started in the mid-1960s in industrial research laboratories (7). Indeed much of the early work is to be found in the patent literature.1 Although in recent times more publications, both industrial and academic can be found in the open literature, there is still a good deal of industrial secrecy, because the products of reactive polymer processing are of significant commercial value to industry. Below we will deal briefly with two important examples of such reactions. 

Oxidative amino acid side-chain modifications do not result in a stable end product of the oxidation process, but very often highly reactive intermediates are formed. These include chemically reactive groups, like ketones and aldehydes, or the formation of protein hydroperoxides. The presence of such protein hydroperoxides leads to a process called protein peroxidation. Here secondary reactions occur if the protein hydroperoxide decomposes and initiates further oxidative reactions, again forming oxidized protein forms. 

This type of modification process has been used to form sulfhydryl-reactive dextran polymers by coupling amine spacers with crosslinkers containing an amine reactive end and a thiol-reactive end (Brunswick et al., 1988 Noguchi et al., 1992). The result was a multivalent sulfhydryl-reactive dextran derivative that could couple numerous sulfhydryl-containing molecules per polymer chain. 

Over the past 10 years, our understanding of enzymes which effect the difficult chemical process of C6H bond cleavage has increased dramatically (Stubbe, 1989 Klinman, 1996). We know that nature employs both metal ions and reactive organic cofactors, such as radicals and quinones, derived by post-translational modification of aminoacids in the polypeptide chain of the enzyme. The two enzymes to be described in the present review are good examples galactose oxidase employs copper and a tyrosine covalently cross-linked to a cysteine to stabilize a radical whilst amine oxidases employ copper and tyrosine-derived quinones. There is subtle interplay between the roles played by copper in the biogenesis of these novel cofactors and in the catalytic cycle of the oxidases. 

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