Ordinary Emulsion Polymerization

A so-called ordinary emulsion polymerization, i.e., similar to the case treated by Smith and Ewart, is characterized by large monomer droplets... 

A detailed critique of the validity of Eq. (27) is given in Section III. In brief, two criteria are required for this validity, both of which are well satisfied in ordinary emulsion polymerization systems. These are (i) that the time required for formation of a single polymer chain be much less than that over which significant changes occur in the rate coefficients governing the MWD and (ii) that the average number of free radicals per particle of size steady-state value. 

Addition of small amounts of to solutions of polymers has been shown to facilitate formation of aqueous emulsions of-such solutions. A very recent application involves the addition of Z2 to the monomer phase in an ordinary emulsion polymerization. This results in a decrease in the concentration of monomer in the polymer particles and thereby in a change in the kinetics of polymerization. 

Another interesting appbcation of this method is the preparation of large, monodisperse polymer particles. In this case the starting point is small, monodisperse particles (seed) in tbe size range of 0.3-1 pm, produced by ordinary emulsion polymerization according to procedures known to ipve monodisperse particles (Vanderhoff et al.,1956, 1970 Goodwin et al.. 

If in an ordinary emulsion polymerization, a water-insoluble Z2 is added to the monomer phase, the effect will obviously be to decrease the activity of the monomer in this phase and accordingly to decrease the concentration of the monomer in tbe particles (Azad et al., 1980 Ugelstad cf of.. 1980b,c). The appropriate form of the equilibrium equation for the case in which one has Z equilibrated between Z3 particles and monomer droplets containing Zj will be... 

When neutral salts are added to an emulsion-polymerization system for vinyl acetate, the monomer, in effect, is salted out. The process then takes place at a much higher rate in the monomer-polymer particles than before. This is generally expected for ordinary emulsion polymerizations [136]. 

The most characteristic feature distinguishing miniemulsion polymerization from an ordinary emulsion polymerization is that, in the former case, both particle nucleation and polymerization take place in pre-formed, stable drc Iets of monomer, irrespective of whether water-soluble or oil-soluble initiatois are used. A con nehensive treatmenf of miniemulsion polymerization in general is given in Ch ter 20. The present discussion will, therefore, focus on those aspects most relevant to vinyl chloride polymerization. 

In ordinary emulsion polymerization, the polymerization takes place i homogenous particles consisting of polymer and monomer. In Interval II, whic comprises the region from the end of Interval I (particle formation poiod) t the point where monomer disappears as a separate phase, the volume fraction o... 

It will be apparent that if the particle is larger in volume by several orders of magnitude than the size considered above, it may then accommodate two or more radicals without immediate termination. The typical characteristics of emulsion polymerization vanish, and we have what is often referred to as bead polymerization, which resembles ordinary bulk polymerization in its kinetics. 

Many water-soluble vinyl monomers may be polymerized by the emulsion polymerization technique. This technique, which differs from suspension polymerization in the size of the suspended particles and in mechanism, is widely used for the production of a number of commercial plastics and elastomers. While the particles in the suspension range from 10 to 1000 nm, those in the emulsion process range from 0.05 to 5 nm in diameter. The small beads produced in the suspension process may be separated by filtering, but the latex produced in emulsion polymerization is a stable system in which the charged particles cannot be recovered by ordinary separation procedures. 

In ordinary batch macroemulsion polymerization reactions, monomer macroemulsion droplets (tf = 1 -10 p.m) are in equilibrium with excess surfactant in the form of micelles d 0.01 xm). The emulsion polymerization reaction can be divided into three intervals (Figure 9.14). In Interval I, nucleation of particles takes place by invasion of radicals from the aqueous phase into micelles or by precipitation of oligomer particles in the aqueous phase outside the particles. Macroemulsion droplets play little role in Interval I owing to the fact that they are of a large size... 

The following two approaches to rendering Compn B type expls more insensitive were investigated and reported in Ref 46 the first by coating the RDX crystals with inert materials such as waxes and/or polymeric compds of sufficiently high melting points to prevent their remelting in molten TNT or at ordinary steam temp, and the second by the addition of a wax directly to Compn B by means of a wax/TNT emulsion. 

In conclusion we may say that a conversion near 100% can only be reached within reasonable time if the polymerization temperature is above the glass transition point of the polymer. This conclusion holds for bulk, emulsion, and suspension polymerization but, of course, not for solution polymerization where the solvent-polymer mixture usually has a glass transition point which is well below ordinary polymerization temperatures. 

Although an emulsion technique was found to be satisfactory to synthesize small functional particles for the study of labeled cells in SEM, it was, however, necessary to increase particle size for observations under the ordinary light microscope. For this purpose, a Coy irradiation technique was developed to polymerize 2-hydroxy ethylmethacrylate in absence or in presence of a variety of comonomers.ii... 

Another, more specific method for the preparation of emulsions of Z, involves the addition of Z to a preformed mixture of an ionic emulsifier, a long-chain fatty alcohol, and water. In this way, the rapid formation of a stable emulsion may he obtained at ordinary stirring with relatively modest amounts of emulsifier. The mechanism of Ais process is still not satisfactorily explained. Also, subsequent polymerization (in the case where Zi is a monomer) may lead to polymerization with initiation in monomer droplets. 

Highly purified diallyl maleate and fumarate in an inert atmosphere, are said to polymerize very rapidly [118], However, trace impurities and atmospheric oxygen substantially reduce the polymerization rate under ordinary circumstances. Naturally with the double bond between the two carboxylate groups and the two allylic double bonds, crosslinking takes place at very low conversion. Even so, in copolymer systems such as in poly(vinyl acetate) emulsion copolymers, the cross-linking of a fumarate or maleate within the latex particles appears not to interfere significantly with film formation properties. As a matter of fact these monomers are incorporated in poly(vinyl acetate) latices used in adhesives and in water-based paints. 

In general, the polymerization of vinyl chloride may be carried out in bulk, solution, suspension, and in emulsion. Free-radical initiators are most commonly used although organometallic initiators and radiation initiation have been considered. Since the monomer is a gas at ordinary temperatures and pressures, suitable equipment is required for VCM polymerization. Sealed tubes and capped bottles have been used for this experimental work. In the use of bottles, safety precautions should be considered both from the standpoint of explosion hazards and the problems of exposure of personnel to VCM. 

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