Redox polymerization-initiating

P. L. Nayak and S. Lenka, Redox polymerization initiated by metal ions, J. Macromol. Sci.— Revs. Macromol. Chem. C19, 83 (1980). 

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... 

The synthetic methods of macromolecules having an active pendant group include (1) the transformation reactions of polymer and copolymers, and (2) polymerization and copolymerization of functional monomers having active pendant groups. The macromolecules, either in the shape of film or microbeads, can be used as the substrate. As we have mentioned previously, the rate of polymerization initiated with the Ce(IV) ion redox system is much faster than that initiated by Ce(l V) ion alone, as expressed in / r 1. Therefore, the graft... 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

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]. 

Organic-inorganic redox pairs initiate polymerization, usually but not always by oxidation of the organic component, for example, the oxidation of an alcohol by Ce4+,... 

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. 

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. 

Mino, G., and S. Kaizerman A new method for the preparation of graft copolymers. Polymerization initiated by ceric ion redox systems. J. Polymer Sci. 31, 242 (1958). 

Organic peroxides acyl peroxides (benzoyl, acetyl, lauryl peroxides, etc), alkyl peroxides, hydroperoxides, etc. are used as polymerization initiators. Azocompounds (for example, redox systems, etc. are also widely used as initiators. In... 

Since the 1960 s many researchers have been concerned with the development of feasible and industrially useful methods for the synthesis of cellulose graft copolymers3, 4. Recent investigations have shown that the most efficient approach to this problem involves free radical polymerization initiated by redox systems5. An impressive example is the industrial production of mtilon (cellulose-polyacrylonitrile graft copolymer) and other fibers, particularly those with ion-exchange and acid-resistant properties6"8. ... 

The hydroxytelechelic polymers synthesis involving a free-radical mechanism employs polymerization initiators which are cleaved into free radicals bearing hydroxyl substituents, by heat, light or redox systems. These radicals initiate polymerization of monomers and can give hydroxyl-terminated polymers by recombination. 

Hydroxytelechelic polymer synthesis with redox systems requires hydrogen peroxide as an oxidizing agent and, generally, takes place in aqueous media (to solubilize the salts). This kind of polymerization is possible at lower temperatures compared to polymerizations initiated by thermal decomposition of H202. Therefore, the less frequent transfer reactions improve the polymer functionality and its polydispersity. 

The rate of acrylamide polymerization initiated by the K2S04/2-mercaptoethanol redox system is of first order with respect to monomer concentration and (3/2)-th order with respect to K2SO+ concentration, the overall energy of activation being 134kJ mol-1. 

The phenomena of autoacceleration and post-polymerization also have been observed in polymerization initiated by redox systems 70,71 74,75). 

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. 

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