Initiation in the aqueous phase

Most monomers polymerizing by the radical mechanism are almost insoluble in water. Intensive stirring of a mixture of such a monomer with water produces an emulsion which remains stable, however, only in the presence of a surface active compound (tenside), e. g. soap. By the addition of a water-soluble initiator to this emulsion, the monomer polymerizes at a rate several times higher than would be observed by any other radical method with an initiator of equal efficiency. At the same time, a higher polymer with a narrower molecular mass distribution is formed. At the initial stages of the reaction, the monomer is present as three types of particle in tenside-stabilized monomer droplets of diameter 10-3 to 10 4cm (about 1012 such droplets are present in 1 cm3 of emulsion of average concentration) in solubilized micelles about 10 nm in size and concentration 1018 cm 3 and in the growing, emulsifier-stabilized monomer—polymer particles 50-100 nm in size. This situation is illustrated schematically in Fig. 14(a). 

The proceeding monomer—polymer transformation is accompanied by an increasing need for stabilization of the monomer—polymer particles. The necessary emulsifier is drawn from the micelles so that, at a certain moment, the micelles disppear [at a conversion x = 0.1-0.2 see Fig. 14(b). 

Finally, the monomer supply from the droplets is exhausted so that, in the final stage of emulsion polymerization, only the monomer—polymer particles are present in the system in an amount of about 1016cm-3 [see Fig. 14(c)]. 

The first comprehensive qualitative concept of emulsion polymerization was presented by Harkins [127], 

The simple Harkins model is the basis of most quantitative theories of emulsion polymerization. 

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Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) 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 Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

In normal emulsion polymerization the diffusion of monomers from droplets allows particles to grow. The polymerization is usually initiated in the aqueous phase and the oligomeric radicals either enter micelles or merge with other growing species. In the crosslinking ECP of EUP the ratio EUP/comonomer and the solubility or insolubility of both components and the initiator in the aqueous and non-aqueous phases respectively are parameters which decide whether diffusion of the reactants in the aqueous phase plays a role and where the initiation takes place. 

The section on suspension polymerization indicated the differentiation between suspension and emulsion (or latex) polymerizations. Emulsion polymers usually are formed with the initiator in the aqueous phase, in the presence of surfactants, and with polymer particles of colloidal dimensions, i.e., on the order of 0.1 gm in diameter [17]. Generally, the molecular weights of the polymers produced by an emulsion process are substantially greater than those produced by bulk or suspension polymerizations. The rate of polymer production is also higher. As a large quantity of water is usually present, temperature control is often simple. 

The general question of polymerization on or in the particle vs. polymerization in the true aqueous phase has been discussed for a number of years. Evidence other than that cited above suggests that both sites may be important depending on conditions. Polymer chains might be initiated in the aqueous phase and then grow further after transport to the particles. Unreported work in this Laboratory suggests that polymerization in the two sites concurrently, may account for some results on molecular weight distribution and on copolymerization. Mino (100) has shown from his own experiments and those of others that the parti-... 

The reduction in rate per unit quantity of aqueous phase which occurs at low ratios of aqueous phase to monomer phase may be due to serious depletion of Initiator in the aqueous phase. The Initiator Is considerably more soluble in the butadiene phase than in the aqueous phase, and therefore may have been present largely In the monomer phase in those systems which contained large volumes of butadiene. 

Since emulsion polymerization is initiated in the aqueous phase, the undesirable formation of latex polymer can be minimized in suspension systems by using water-soluble inhibitors, like sodium nitrite. 

The fraction 1 — will therefore represent the probalnlity for termination of radicals stemming from initiator in the aqueous phase before they can be absorbed. 

Free radicals, initiated in the aqueous phase, can form polymer particles by a number of mechanisms. Smith and Ewart (1948) considered two quantitative models for predicting the number of particles formed. One of these models is shown as Eq. (1) below. 

In the absence of micelles, some polymerization still occurs. In this case the polymer particle nuclei are initiated in the aqueous phase where... 

Model / Where all the radicals generated by the initiator in the aqueous phase nucleate the micelles. Therefore, the rate of formation of particles equals the initiation rate. Consequently,... 

ITie dependence of the pertechnetate extraction with cyclohcxanol on the concentration of acid, initially in the aqueous phase, demonstrates the rapid extraetion increase upon the addition of small amounts of aeid. After a maximum extraction coefficient is reached, an exponential decrease sets in (Fig. 7.4.A). Curves similar to those in Fig. 7.4.A were also observed with eyelohexanone, tri-/t-butyl phosphate (TBP), and with solutions of TBP in a liquid hydrocarbon. 

Precipitation of polymer chains initiated in the aqueous phase. 

According to the initiation-in-the-aqueous-phase mechanism, radicals generated in the aqueous phase add monomer units until the oligomeric radicals exceed their solubility and precipitate. The precipitated oligomeric radicals form spherical particles which adsorb emulsifier and absorb monomer to become primary particles. 

Therefore, the two mechanisms to be discussed further and compared in this paper are those of initiation in micelles and initiation in the aqueous phase. For initiation in micelles, the disappearance of the micelles marks the end of the particle nucleation stage the particle growth stage begins with the formation of the... 

For initiation in the aqueous phase, the principal function of the emulsifier is to stabilize the oligomeric radicals as they precipitate from the aqueous phase. Therefore, the initiation and propagation of polymerization in the aqueous phase follows the general kinetic scheme for mass, solution, and suspension pol3nnerization the expression for the number-average degree of polymerization is 5 ... 

For initiation in the aqueous phase to produce a monodisperse latex, the primary particles generated early in the reaction must act as nuclei to capture all primary radicals formed thereafter and these nuclei must grow without flocculation until the end of the polymerization. Therefore, the emulsifier must adsorb rapidly enough to stabilize these initial nuclei but not so rapidly as to stabilize the primary particles formed later in the reaction. This condition may be met if the emulsifier concentration is relatively low or if the emulsifier is omitted the sulfate endgroups introduced by the persulfate initiator are often sufficient to stabilize latex particles at relatively low monomer-water ratios (30). Table III gives the increase in particle size and surface charge resulting from the flocculation of the primary particles described in... 

The presence of these very small particles in samples polymerized to high conversions using different emulsifiers suggests that the predominant mechanism of particle nucleation is initiation in the aqueous phase, followed by flocculation with the particles of the main distribution. Such a mechanism would be reasonable for the emulsion polymerization of the more water-soluble monomers such as vinyl acetate or vinyl chloride indeed, such particles were found in vinyl acetate emulsion polymerizations (48). Nevertheless, this finding was not expected for the 50 50 styrene-butyl acrylate mixture, as both monomers are only sparingly soluble in water, and it suggests that initiation in the aqueous phase is far more prevalent, than has been thought. 

The rate of generation of radicals is based on the presence of initiator in the aqueous phase alone ... 

The Smith-Ewart theory was developed for monomers such as styrene, with very low water solubility. Monomers such as acrylonitrile, with appreciable water solubility (on the order of 10%), may undergo significant homogeneous initiation in the aqueous phase. In some emulsion systems, the particles flocculate (coalesce) during polymerization, not only making a kinetic description difficult, but also sometimes badly fouling reactors. Good reviews of this subject are available [8-11], as well as complete books [7,12-15]. 

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g., persulfate) and polymer chains are initiated in the aqueous phase. A number of mechanisms... 

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