Initiation in heterogeneous polymerization

Many polymerizations are carried out in heterogeneous media, usually water-monomer mixtures, where suspending agents or surfactants ensure proper dispersion of the monomer and control the particle size of the product. 

Suspension polymerizations are often regarded as mini-bulk polymerizations since ideally all reaction occurs within individual monomer droplets. Initiators with high monomer and low water solubility are generally used in this application. The general chemistiy, initiator efficiencies, and importance of side reactions are similar to that seen in homogeneous media. 

Emulsion polymerizations most often involve ihe use of water-soluble initiators e.g. persulfate see 3.3.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert and Lovell and El-Aasser for a more comprehensive treatment. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efTiciencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entiy. 

Microemiilsion and miniemiilsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively V5 50-300 nm) and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

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Electron-transfer initiation also occurs in heterogeneous polymerizations involving dispersions of an alkali metal in monomer. Initiation involves electron transfer from the metal to monomer followed by dimerization of the monomer radical-anion to form the propagating... 

ATRP is a very potent method for preparing block copolymers by sequential monomer addition as well as star polymers using multifunctional initiators. Furthermore, it can be applied also in water and in heterogeneous polymerization systems, for example, emulsion or dispersion polymerization. 

The plasticization of polymers by CO2, which causes a decrease in the glass-transition temperature (Tg) of the polymer, is another important feature that must be taken into accoimt. This plasticization facilitates the occurrence of important effects that are essential to polymer synthesis. Plasticization of the polymer allows enhanced diffusion of monomer and initiator into the polymer phase which often results in increased polymerization rates in heterogeneous polymerizations, removal of residual monomer, solvent, or catalyst from the polymer, and the formation of blends by polymerization within a CO2-swollen host polymer. An important emerging issue is the role that CO2 plays in manipulating the loci of reactants such as monomer/comonomer/initiator partition coefficients. Each of these topics is discussed in this chapter. 

The hyperbranched PILs represent effective stabilizers in heterogeneous polymerization reactions. When employing water-soluble PEHO-ClImOTs as the emulsifier in the 2,2 -azobis(2-methylpropionitrile) (AIBN) initiated mini-emulsion polymerization of styrene, stable polystyrene (PS) latexes were obtained. Characterization by means of transmission electron microscopy (TEM) indicated the presence of spherical PS nanoparticles (see Figure 7.5). Further studies revealed that, by modifying parameters such as the PIL/monomer ratio and the duration of the ultra-sonication treatment prior to the polymerization, the mean diameter of the obtained PS particles can be varied from 40 nm to 110 nm. In contrast to PEHOClImOTs, the use of the low-molecular-weight ILs 1,3-dimethylimidazolium tosylate or 1-butyl-3-methylimidazolium tosylate did not result in the formation of stable latexes. 

Rissoan, G., Randriamahefa, S., Cheradame, H. Heterogeneous Cationic Polymerization Initiators, In Cationic Polymerization Faust R-> Shaffer, T. D., Eds., American Chemical Society Washington, D.C., 1997 pp. 135-150. 

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

Reactive radical ions, cations and anions are frequent intermediates in organic electrode reactions and they can serve as polymerization initiators, e.g. for vinylic polymerization. The idea of electrochemically induced polymerization of monomers has been occasionally pursued and the principle has in fact been demonstrated for a number of polymers But it appears that apart from special cases with anionic initiation the heterogeneous initiation is unfavorable and thus not competitive for the production of bulk polymers A further adverse effect is the coating of electrodes... 

Cyclopropanone Polymerization. Triethylamine is an efficient initiator for the polymerization of cyclopropanone. This initiator caused polymerization to start almost immediately as evidenced by the rapid increase in temperature and the formation of a precipitate within 2-3 minutes. From the data in Table 1 there does not appear to be any correlation between the amount of initiator added and the molecular weight of the resultant polymer. One possible explanation for this is that the polymer was synthesized under heterogeneous conditions thus limiting the access of monomer to growing polymer chains. 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

Certain experimental considerations can complicate the kinetics. Cu+ is difficult to obtain and maintain in pure form. Only a few percent of Cu2+ present initially or formed as a result of insufficent deoxygenation in the polymerization can alter the process. Reaction systems may be heterogeneous or become heterogeneous with conversion as the medium changes. 

Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media in ionic polymerizations is often not clear since heterogeneous inorganic initiators are often involved. Further, it is extremely difficult in most instances to obtain reproducible kinetic data because ionic polymerizations proceed at very rapid rates and are extremely sensitive to the presence of small concentrations of impurities and other adventitious materials. The rates of ionic polymerizations are usually greater than those of radical polymerizations. These comments generally apply more to cationic than anionic polymerizations. Anionic systems are more reproducible because the reaction components are better defined and more easily purified. 

Another consideration in the application of the various kinetic expressions is the uncertainty in some reaction systems as to whether the initiator-coinitiator complex is soluble. Failure of the usual kinetic expressions to describe a cationic polymerization may indicate that the reaction system is actually heterogeneous. The method of handling the kinetics of heterogeneous polymerizations is described in Sec. 8-4c. 

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