Redox process polymerization reactions

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

Certain polymerizations (e.g.. S, see 3.3.6.1) can be initiated simply by applying heal the initiating radicals are derived from reactions involving only the monomer. More commonly, the initiators are azo-compounds or peroxides that are decomposed to radicals through the application of heal, light, or a redox process. 

Thus, because the mass action law applies, the behavior of the microcomponent is expected to be the same at normal concentration and at the tracer scale. In ultra diluted systems, the thermodynamical behavior of an element does not depend on concentrations. However, some discrepancies can occur in particular cases, for example as a result of the different degrees of consumption of ligands leading to unusual complexation reactions or in unexpected redox processes [4], Tracer scale chemistry is also characterized by a kinetics hindrance for reactions between two microcomponents in a given system this trend excludes polymerization reactions or... 

A kinetic model is presented by Pohlman and McColl (1989) to describe the initial and rapid redox processes between polyhydroxyphenolic acid and soil or Mn oxide suspensions. The oxidation process of polyhydroxybenzoic acid by soil and Mn oxides follows second-order kinetics. The rate constants derived from the model are similar in magnitude in both suspensions for the organic reductants studied (Table 8-16). Polyhydroxyphenolic acids with para- and ortho-OH groups are rapidly oxidized by Mn oxides with spectral evidence suggesting that the reaction leads to polymeric humic products probably via semiquinone or benzoquinone derivatives. By contrast, polyhydroxyphenolic acids with meta-oriented phenolic-OH groups are not oxidized by soil or Mn oxide suspensions within a 120-min reaction period. Presumably these compounds are not capable of being oxidized to semiquinone or benzoquinone intermediates. The rapid disappearance of polyhydroxyphenolic acids is accompanied by formation of Mn and colored humic products in both soil and Mn oxide suspensions. These results provide further evidence that abiotic oxidation of certain organics can lead to the formation of humic substances and the mobilization of Mn in nature. 

U ecent work has led to the synthesis of a variety of compounds in which metal atoms or ions are held in close proximity by chemical linkages. These polymetallic compounds represent a new class of materials that have distinctive chemical and physical properties, and in some systems the properties can be varied systematically by chemical synthesis. The compounds are of interest because of possible cooperative chemical and electronic interactions between the chemically linked metal centers. In the future it may prove possible (a) to create solid state materials that have controllable, and perhaps unusual, electrical conductivity properties (b) to prepare polymeric complexes which in solution have properties that are intermediate between those of solid state materials and those of simple monomeric complexes and (c) to devise chemical systems in which cooperative chemical interactions lead to net, multiple-electron redox processes, or to simultaneous, two- or more site reactions. 

An alternative method is to use a semi-coitinuous process in which inverse emulsion and initiator solution are progressively added in the course of the reaction. When redox initiators operating at room temperature are used, a common practice is first to inject the oxidant part in the water-in-oil emulsion and to add the reducing portion progressively [11]. This initiation mode is an additional way to control the polymerization reaction since the half-life of a redox initiator is shorter than that of an azo initiator 10 h at 64 °C for AIBN (2,2 -azobis(isobutyrtHiitrile)) compared to 8 min at 45 °C for /-butyl hydroperoxide/cobalt stearate pair [16]. 

The preceding sections have dealt with polymerization by either insertion or GTP mechanisms. Of course, vinyl monomers are also polymerizable by radical, anionic, or cationic mechanisms. In this short section, we summarize the processes which are reasonably well understood from a mechanistic viewpoint, and which involve the intervention of transition metal alkyls (or hydrides), either during initiation, propagation, or chain transfer/termination. A much larger class of polymerization reactions where redox-active transition metal complexes are used to mediate radical polymerizations by reversible atom transfer (ATRP) or other means has been extensively and recently reviewed from a mechanistic perspective and will only be briefly mentioned here. 

Atom transfer radical polymerization (ATRP) reactions mediated by transition metals have also garnered interest.Essentially, ATRP reactions are similar to traditional free-radical polymerization reactions in that they can be described by initiation, propagation, and chain-transfer steps involving carbon radicals. Transition metals mediate this process via redox processes (M => and promoting chain transfer by donation of a... 

The batch process involves the addition of the monomer emulsion to the reaction vessel, which is cooled below 15°C. This is followed by the addition of the redox initiator, usually as an aqueous solution. The metering of the initiator can sometimes control the polymerization rate. The polymerization reaction results in an initial temperature rise during the first half an hour to one hour and then falls because of the cooling and removal of the heat of reaction. 

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