Polymerization redox

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... 

Takeishi, et. al, have described the redox polymerization of methyl methacrylate in the absence of solvent (6). With 18-crown-6 as the phase transfer catalyst and potassium persulfate/sodiurn bisulfite as the redox couple, polymerization was observed at temperatures <50 C whereas little or no polymerization occurred under these conditions in the absence of bisulfite. Above 55 C, however, polymerization occurred even in the absence of bisulfite. From the limited kinetic data reported (6), one can estimate (13) that the rate of polymerization (Rp) is approximately proportional to the square root of crown concentration (Equation 1) ... 

Other exceptions to the first-order dependence of the polymerization rate on the monomer concentration occur when termination is not by bimolecular reaction of propagating radicals. Second-order dependence of Rp on [M] occurs for primary termination (Eq. 3-33a) and certain redox-initiated polymerizations (Sec. 3-4H-2). Less than first-order dependence of Rp on [M] has been observed for polymerizations (Sec. 9-8a-2) taking place inside a solid under conditions where monomer diffusion into the solid is slower than the normal propagation rate [Odian et al., 1980] and also in some redox polymerizations (Sec. 3-4b-2) [Mapunda-Vlckova and Barton, 1978]. 

Some redox polymerizations involve a change in the termination step from the usual bimolecular reaction to monomolecular termination involving the reaction between the propagating radicals and a component of the redox system. This leads to kinetics that are appreciably different from those previously encountered. Thus, in the alcohol-Ce4+ system (Eq. 3-39), termination occurs according to... 

In many redox polymerizations, monomer may actually be involved in the initiation process. Although not indicated above, this is the case for initiations described under item 3b and, of course, for item 4. Rp will show a higher dependence on [M] in these cases than indicated by Eqs. 3-41 and 3-45. First-order dependence of Rt on [M] results in j- and 2-order dependencies of Rp on [M] for bimolecular and monomolecular terminations, respectively. 

Ed, the activation energy for thermal initiator decomposition, is in the range 120-150 kJ mol-1 for most of the commonly used initiators (Table 3-13). The Ep and Et values for most monomers are in the ranges 20-40 and 8-20 kJ mol-1, respectively (Tables 3-11 and 3-12). The overall activation energy Er for most polymerizations initiated by thermal initiator decomposition is about 80-90 kJ mol-1. This corresponds to a two- or threefold rate increase for a 10°C temperature increase. The situation is different for other modes of initiation. Thus redox initiation (e.g., Fe2+ with thiosulfate or cumene hydroperoxide) has been discussed as taking place at lower temperatures compared to the thermal polymerizations. One indication of the difference between the two different initiation modes is the differences in activation energies. Redox initiation will have an Ed value of only about 40-60 kJ mol-1, which is about 80 kJ mol-1 less than for the thermal initiator decomposition [Barb et al., 1951], This leads to an Er for redox polymerization of about 40 kJ mol-1, which is about one half the value for nonredox initiators. 

Ej has a value of about —60 kJ mol-1 for thermal initiator decomposition, and Xn decreases rapidly with increasing temperature. Ej is about the same for a purely thermal, self-initiated polymerization (Fig. 3-16). For a pure photochemical polymerization Ej is positive by approximately 20 kJ mol-1, since Ed is zero and X increases moderately with temperature. For a redox polymerization, Ej is close to zero, since Ed is 40-60 kJ mol-1, and there is almost no effect of temperature on polymer molecular weight. For all other cases, Xn decreases with temperature. 

Redox polymerizations are usually carried out in aqueous solution, suspension, or emulsion rarely in organic solvents. Their special importance lies in the fact that they proceed at relatively low temperatures with high rates and with the formation of high molecular weight polymers. Furthermore, transfer and branching reactions are relatively unimportant. The first large-scale commercial application of redox polymerization was the production of synthetic rubber from butadiene and styrene (SBR1500) at temperatures below 5 °C (see Example 3-44). 

Most emulsion polymerizations are performed with water-soluble initiators however, the following experiment describes a redox polymerization where one component (dibenzoyl peroxide) is water-insoluble, while the other is water-soluble. 

According to Eq. (25), a cyclic phosphite monomer (MN) 38 is oxidized to a phosphate unit yielding copolymer 40 whereas the a-keto acid monomer (ME) 39 is reduced to the corresponding a-hydroxy acid ester. Thus, the term redox copolymerization has been proposed to designate this type of copolymerization in which one monomer is reduced and the other monomer oxidized. The redox copolymerization clearly differs from the so-called redox polymerization in classical polymer chemistry where the redox reaction between the two catalyst components (oxidant and reductant) is responsible for the production of free radicals. 

Many monomers can be grafted by redox polymerization to the magnetic core [magnetic material embedded in poly(vinyl alcohol) matrix nominally 100% cross-linked with glutaraldehyde] to get whisker-type resins " . Acrylic acid and acryl-... 

Note that the initiation step dominates the overall temperature dependence of the rate of polymerization. When the method of initiation varies, Er will also change. For redox initiation, for example, d is of the order of 40-60 kJ/mol and R for redox polymerizations is about 40 kJ/mol. For photochemical or radiation-induced polymerizations, d is practically zero and the rate of polymerization in such cases does not change much with the reaction temperature. 

Cerium(lV) compounds with suitable reducing agents, readily initiate the redox polymerization of, for example, vinyl monomers [22]. This property is used to initiate graft polymerization of vinyl monomers onto cellulose, wool, starch, cotton, etc. in order to, e.g. improve mechanical strength, resist moisture penetration and reduce micro-organism attack. 

During redox polymerization of some monomers. J. Polymer. Sci. C16, 2763 (1967). 

A ferrocene modified siloxane redox polymeric electron transfer system in carbon paste electrodes for aldose biosensors using PQQ-dependent aldose dehydrogenase was reported by Smolander et al. [86]. Polymethyl(ll-ferrocenyl-4,7,10-trioxa-undecanyl)methyl(12-amino-4,7,10-trioxa-dodecyl)-siloxane (1 1 random co-polymer) (Fig. 3.6) was found to be an efficient electron transfer system yieldii better electrode operational stabiUty than those constructed with dimethylferrocene fi ee mediator. The hydrophilic nature of the pendant chain and side chain on dimethyl siloxane units favorably interact with enzjnne causing efficient electron transfer from coenzyme PQQ of aldose dehydrogenase to the electrode surface. 

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