Seeded emulsion polymerization molecular weight additives

Azad and Fitch (5) investigated the effect of low molecular weight hydrocarbon additives on the formation of colloidafr particles in suspension polymerization of methyl methacrylate and vinyl acetate. It was found that the additives n-octane, n-dodecane, n-octadecane, n-tetracosane and mineral oil exerted a thermodynamic affect depending upon water-solubility and molecular weight. Since these effects on emulsion polymerization have not been considered by the earlier investigators, we have chosen n-pentane and ethyl benzene as additives with limited water-solubility and n-octadecane, and n-tetracosane as water-insoluble ones. Seeded emulsion polymerization was chosen so that the number of particles could be kept constant throughout the experiments and only the effect of the other parameters on the rate could be determined. 

Figure 1 gives the conversion-time curves for the seeded emulsion polymerization of styrene in the absence and presence of various low molecular weight additives. Table I summarizes the results given in Figure 1. The rates of polymerization were determined from the straight line portion of the conversion-time curves (below 40% conversion) by least squares analysis of the experimental points. Table I also gives the calculated rates assuming a mere dilution of the monomer in the seed by the additive. It is clear that in every case the rate of polymerization is retarded much more than that due to dilution alone.

Figure 1. Effect of addition of low molecular weight hydrocarbons as additives upon the progress of the seeded emulsion polymerization of styrene at 50°C.

This chapter serves as an introduction to the origin of nonuniform latex particles. First, a brief discussion of the seeded emulsion polymerization technique that has been widely used to prepare composite polymer particles with a variety of morphological structures is given. This is followed by the illustration of the effects of important factors such as initiators, monomer addition methods, polymer molecular weight, volume ratio of the second-stage monomer to the seed polymer, and polymerization temperature that affect the morphological structures of latex particles. The development of morphological structures of nonuniform latex particles will then be covered at the end of this chapter. 

The combination of high molecular weight with high polymerization rate is one reason for the popularity of the emulsion technique. Seeded polymerizations can be useful for making large-particle-size latexes by the emulsion technique. Thus a completed seed latex may be diluted to give the desirable value of Np particles per liter of emulsion. No additional surfactant is added, so no new polymer particles are formed. When monomer is fed and initiator is added, polymerization occurs in the previously formed particles, so that each one grows as monomer diffuses into it and is converted. 

A compatibilization technique specifically suitable for emulsion polymerization involves the in-situ polymerization of a polymer in the presence of a previously polymerized polymer. As applied to emulsion polymerization, this is typically referred to as core-shell polymerization. The procedure involves the initial polymerization of seed particles. The addition of other monomers can result in two distinct results. One result involves swelling of the monomer in the particles followed by phase separation once a critical molecular weight is achieved (if the two polymers are thermodynamically immiscible). The other result would involve the... 

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