Emulsion radical polymerization kinetics

Radical Polymerization Kinetics in Emulsion Polymerization 769 Smoluchomkifast coagulation rate ... 

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

These kinetic results have led to a variety of interpretations. Magat and associates have proposed kinetic schemes based on the idea that a steady state does not exist and that only small (primary) radicals can terminate polymer chains (52, 93). Bamford and Jenkins have criticized this concept of emulsion-type polymerization (77). They point out that if the source of initiation were removed no more small radicals would be formed and polymerization should continue indefinitely. They cite a photosensitized reaction at 60° in which the light was shut off at about 15% conversion, whereupon the rate fell to one-half of its original value in 60 seconds. Bamford and Jenkins point also to evidence from the fast reaction that argues against emphasis on termination between polymer radicals and small radicals. 

In this review we summarize and discuss the amphiphilic properties of polyoxyethylene (PEO) macromonomers and PEO graft copolymer molecules, the aggregation of amphiphilic PEO macromonomers into micelles, the effect of organized aggregation of macromonomers on the polymerization process, and the kinetics of radical polymerization and copolymerization of PEO macromonomer in disperse (dispersion, emulsion, miniemulsion, microemulsion, etc.) systems [1-5]. 

Although there have been several references on the theory of emulsion polymerization kinetics, it is surprising that its commercial importance and multiphase kinetics have not generated more interest. Smith and Ewart (46) predicted a constant rate of polymerization per particle, based on initiation in the water phase and three ranges of radical concentration per particle. Van der Hoff (55) confirmed this for concentrations... 

The first interval is the interval of particle nucleation (interval I) and describes the process to reach an equilibrium radical concentration within every droplet formed during emulsification. The initiation process becomes more transparent when the rate of polymerization is transferred into the number of active radicals per particle n, which slowly increases to n 0.5. Therefore the start of the polymerization in each miniemulsion droplet is not simultaneous, so that the evolution of conversion in each droplet is different. Every miniemulsion droplet can be perceived as a separate nanoreactor, which does not interact with others. After having reached this averaged radical number, the polymerization kinetics is slowing down again and follows nicely an exponential kinetics as known for interval III in emulsion polymerization or for suspension polymer-... 

In the previous sections, methods of qualitatively controlling the course of propagation were described. Indirect control as well as the quantitative effects caused by intentional control of the other partial processes in polymerization have still to be mentioned. The separation of initiation from propagation alters the kinetic character of the whole reaction. With ionic polymerizations, initiation can be separated from propagation by the selection of conditions suitable for rapid initiation. With radical polymerizations, this is not possible. Therefore both partial processes must be separated in space. Fortunately, radical active centres operate both in polar and in non polar media. Thus it is not difficult to confine initiation and propagation to mutually immiscible components of the medium. Emulsion polymerization remains the most important representative of quantitative control of propagation. 

An approach similar to that taken by Nomura and Harada was used by Samer to quantify the effects of droplet nucleation on emulsion polymerization kinetics in a CSTR. In their simplified analysis, it was assumed that radical capture by particles and droplets is proportional to the ratio of particle and droplet diameters. This assumption is reliable at low to moderate residence times, when polymer particles still closely resemble monomer droplets with respect to composition and surface characteristics. For predominant droplet nucleation, the maximum particle generation is limited by the concentration of monomer droplets in the feed. In Fig. 11 the steady state particle generation is given as a function of the residence time and temperature. Nucleation efficiency is defined as the number of particles divided by the number of droplets in the... 

As far back as 1948, Smith and Ewart [ 13] included the effects of radical desorption in emulsion polymerization kinetics, and in 1965 Romatowski et al. [282-284] showed that radicals resulting from chain transfer to monomer indeed escape from the particles. 

The kinetic schemes described in this chapter apply to free-radical polymerizations in bulk monomer, solution, or in suspension. Suspension polymerizations ([Section 10.4.2.(iii)]) involve the reactions of monomers which are dispersed in droplets in water. These monomer droplets contain the initiator, and polymerization is a water-cooled bulk reaction in effect. Emulsion systems also contain water, monomer and initiator, but the kinetics of emulsion polymerizations are different from those of the processes listed above. Chapter 8 describes emulsion polymerizations. 

Macroradicals are isolated I rom each other in emulsion polymerizations because they grow in particles, which can accommodate either one or zero radicals at any instant. The distinguishing feature of the kinetics of such reactions is that the polymerization rate and polymer molecular weight are proportional to the number of particles, as distinct from free-radical polymerizations in bulk, solution or suspension. An interesting consequence is that the rate of polymerization will be inversely proportional to the particle size. This holds affixed final polymer content, which is the way such reactions are usually performed. Polymer molecular weight may also be affected by particle size under the same conditions. 

The kinetics of emulsion polymerization is complex, involving a large number of species and at least two phases. The first quantitative approach to emulsion polymerization kinetics led to extensions by many others.The important events to consider are 1) the free-radical reactions of chain formation initiation, propagation, chain transfer, and termination and 2) the phase transfer events that control particle formation radical entry into particles from the aqueous phase, radical exit into the aqueous phase, radical entry into micelles, and the aqueous phase coil-globule transition. In free-radical emulsion polymerization, the fundamental steps are shown schematically in Fig. 1... 

Due to the kinetics of the polymerization in small partides, emulsion polymerization allows for high polymerization rates and also high polymer molecular weights at the same time. This differs from bulk free-radical polymerization, where an increase in rate occurs at the expense of molecular weight and vice versa. However, this is obviously a feature of classical free-radical polymerization and must therefore not be considered further in the context of most transition metal-catalyzed polymerizations. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

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