Classical emulsion polymerization

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

Classical emulsion polymerization is divided into three kinetic stages. At the start of the process, the unsaturated monomers are dispersed into small droplets, stabilized with surfactants. Additional surfactant aggregates into micelles. These micelles are very small ( 10nm) relative to monomer droplets ( 1-10 pm). During stage 1 the initial formation of polymer... 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

Classical emulsion polymerization processes are conducted in a heterogeneous medium, but they have some characteristics that differentiate them from other processes. As a result, their unique characteristics may be exploited for the production of specific materials. There are specific requirements that must be met for this process to occur in a heterophase reaction system. First, the reactive organic phase (monomer) must be almost completely insoluble in the continuous phase (water). Subsequently,... 

Figure 8.2 A representation of a classical emulsion polymerization process and depiction of the radical initiators and their entry mechanism. The diffusion of monomers into the micelles and the micellar structure are shown as a process formed via self-organization, containing monomer droplets (head as the hydrophilic part, and the tail as the hydrophobic part) with a surfactant concentration above the critical micelle concentration are also illustrated.

A/-ISOpropyl Acrylamid6. Homopolymers from A(-isopropyl acrylamide (NlPAAm) are well known and have been of great interest because of their low critical solution temperature (LOST) behavior in water-solution. The polymer is soluble at low temperatures in water and precipitates above the LOST upon heating. Those kinds of polymers are potentially useful in the pharmaceutical or medical area. The homo- and copolymerization with water-soluble comonomers such as 2-hydroxyethyl methacrylate and characterization of the corresponding polymers of NlPAAm are described in some papers. In the case of copolymerization with hydrophobic comonomers, the use of an organic solvent is necessary, otherwise the polymerization has to be carried out in emulsion. But the different solubilities of the monomers in water often make the copol3unerization of water-soluble and water-insoluble monomers by classical emulsion polymerization difficult. Because of the above discussed property of cyclodextrin derivatives, these problems can be avoided if the copolymerization reactions are performed in the presence of cyclodextrin derivatives. 

The classic emulsion polymerization, in which the particle size and particle size distribution are controlled by a water-soluble initiator and a surfactant (17-21). They are monodisperse with a particle size typically less than 0.1 /u.m. 

There is a common misconception that these products are made by an emulsion polymerization. They are not, even though the product is often called a "latex", emulsion, dispersion, etc. For a classic emulsion polymerization to occur one must have liquid monaqueous phase. During the polymerization of TFE, as it is done in many ccmimercial processes, the temperature is well above the critical temperature of TFE and the pressure is well below the critical pressure of the monomer. There are many different ideas as to the exact mechanism of the polymerization, but this writer at least, is not aware of any results that show convincingly exactly what happens. The dispersion is really a suspensoid, with particles varying in shape, but with dimensions of the order of 0.2-0.4 micr[Pg.271]

The free-radical polymerization of vinylpyridine in emulsion has also been described [567-571]. In contrast to classical emulsion polymerizations in which the micelles are the sole propagating sites, for monomers with high hydrophilicity such as vinylpyridines, the aqueous phase is apparently important as the locus of polymerization. 

Classically, emulsion polymerizations involve monomers that are solvents for the homopolymer. Strictly speaking only these systems follow the theory of Harkins as quantified by Smith and Ewart. According to this theory, the propagation rate is a function of the number of polymer particles that are... 

Under saturation conditions, the rate of polymerization of VC is found to be independent of or slightly dependent on the emulsifier concentration. The number of particles as well as the molecular weights of PVC increase with increasing emulsifier concentration. These results indicate that kinetic events typical for the classical emulsion polymerization in particles are not dominant. The polymerization in small particles was more sensitive to emulsifier than that in large particles. This results from the dominant role of the emulsifier layer zone which is more active in small partides. Here the particle size distribution increases with increasing [E], This indicates that the secondary nucleation of particles takes place and/or the nucleation period is proportional to [emulsifier]. 

More information about the effect of VC monomer on the polymerization process can be obtained from the copolymerization of VC. Conversion curves for the systems without or with a low VC concentration showed a sigmoidal shape typical of the classical emulsion polymerization. The addition of higher amounts of VC leads, however, to deviation from the emulsion copolymerization kinetics, i.e., the conversion curves take a shape similar to that for the solution or precipitation polymerization. These results show that growth events in the VC emulsion systems are somewhat restricted and termination becomes more efficient. 

The low activation energy typical for the classical emulsion polymerization was also estimated in the emulsion polymerization of VC. 

Free-radicals for classical emulsion polymerizations are generated in the aqueous phase by the decomposition of water-soluble initiators, usually potassium or ammonium persulfate ... 

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