Polymer radicals, reaction+metal ions

Commercial poly(butadiene), which is mainly the 1,4 isomer, is also used to improve the impact resistance of polystyrene (Chapter 1). Polydienes also increase the rate of physical disintegration of polyblend containing them. The addition of a styrene-butadiene block copolymer e.g. SBS, page 9 et seq.) to polyethylene also accelerates the peroxidation of the latter. However, this system also requires a polymer-soluble transition metal ion catalyst e.g. an iron or manganese carboxylate) to increase the rate of photooxidation in the environment by the reactions shown in Scheme 5.3. The products formed by breakdown of alkoxyl radicals (PO ) (Scheme 3.4) are then rapidly biodegradable in compost (page 107 et seq.). 

The oxidation of hydrocarbons, including hydrocarbon polymers, takes the form of a free-radical chain reaction. As a result of mechanical shearing, exposure of ultraviolet radiation, attack by metal ions such as those of copper and manganese as well as other possible mechanisms, a hydrocarbon molecule breaks down into two radicals... 

Both typical and exceptional examples of the polymerization of a vinyl monomer containing a transition-metal ion are provided by the radical polymerization of vinylferrocene31. Vinylferrocene and its derivatives are polymerized by a radical or a cationic initiator to form a polymer of high molecular weight. The high polymeriz-ability is based on the property that the ferrocene compounds are extraordinarily stable against chemical reactions. 

The oxidation of polymers can be represented by an autocatalytic mechanism involving the intermediate formation of hydroperoxides (Scheme 1) (B-79MIU501). The termination steps involving reactions of peroxy radicals are thought to predominate in most cases. Metal ion impurities accelerate degradation by promoting decomposition of intermediate hydroperoxides (B-79MI11502). 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serving as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. 

Also, coordination compounds and metal carbonyls are able to undergo a PET, resulting in initiating radicals [63]. Recently investigated examples are iron chloride based ammonium salts [149], vanadium(V) organo-metallic complexes [150], and metal sulfoxide complexes [151]. However, the polymerization efficiency of some systems is only low due to redox reactions between the central metal ion and the growing polymer radical, and the low quantum yields of PET. 

The dormant species in ATRP arises from the polymer chain being capped with a halogen atom (P -X), while in the active state the halogen is chelated to a metal complex, thus allowing monomer to add. This takes advantage of the Kharasch reaction in which halo-genated alkanes add to vinyl monomers by a free-radical reaction that is catalysed by transition-metal ions in their lower-valent state (Fischer, 2001). 

In this Scheme the ligand is often bipyridyl if the metal is Cul and, in early ATRP, ruthenium compounds were used. The nature of the ligand controls the effectiveness of the catalyst. The system is living since the concentration of the transition metal in the higher oxidation state is sufficient to ensure that deactivation to the dormant state is much faster than any other polymer radical termination steps, including bimolecular recombination. Because of the absence of a Trommsdorff effect the reaction may be carried out in the bulk (Matyjaszewski, 1999). The detailed mechanism is complex since the reaction rate depends on the metal, the organic halide, the ligand and the counter-ion (Fischer, 2001). 

They are formed in the following chain of events. First, the initial step, a chain initiation, which happens as a result of thermal or photo-induced breakage of a chemical bond in the polymer. This often happens with the help of a catalyst (water, metal ion) in the vicinity of the reaction site, where the initiation step produces a hydrocarbon free radical, R ... 

Oxygen is omnipresent in our environment and the deterioration of rubbers and plastics by peroxidation is the normal cause of property deterioration in most polymers under ambient conditions. Peroxidation is a free radical chain reaction, shown in summary in reactions 3.1 and 3.2. Under normal conditions it is initiated by hydroperoxides that are formed in each cycle of the peroxidation chain sequence (reaction 3.1). Hydroperoxides are very unstable compounds due to the weakness of the peroxide bond which readily undergoes thermolysis when heated (reaction 3.3). This reaction is powerfully catalysed by transition metal ions. 

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