For emulsion polymerization

Soap. A critical ingredient for emulsion polymerization is the soap (qv), which performs a number of key roles, including production of oil (monomer) in water emulsion, provision of the loci for polymerization (micelle), stabilization of the latex particle, and impartation of characteristics to the finished polymer. 

Table 14. Recipe for Emulsion Polymerization by Continuous Addition ...

Partially hydrolyzed poly(vinyl alcohol) grades are preferred because they have a hydrophobic /hydrophilic balance that make them uniquely suited for emulsion polymerization. The compatibUity of the residual acetate units with the poly(vinyl acetate) latex particles partly explains the observed stabilization effect. The amount of PVA employed is normally 4—10% on the weight of vinyl acetate monomer. The viscosity of the resulting latex increases with increasing molecular weight and decreasing hydrolysis of the PVA (318). 

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

Water-soluble polymers eomprise a major elass of polymerie materials and are used in a wide variety of applieations. Synthetie water-soluble polymers inelude poly(vinyl aleohol), poly(aerylamide), poly(aerylie aeid), poly(ethylene oxide), poly(vinyl pyrrolidone), eellulosies, and many eopolymers of these types. Their end uses are quite varied and their applieations depend mainly on their viseosify-ing, rheologieal, and surfaee-aetive properties (1). For example, poly (vinyl aleohol) is used in adhesives, fibers, textile and paper sizing, paekaging, as a stabilizer for emulsion polymerization, and as a preeursor for the manufaeture of poly(vinyl butyral), whieh is used in automotive windshields. Poly(vinyl aleohol) is also the world s largest volume, eommodity, water-soluble polymer. 

Figure 3 The schematical representation of a typical laboratory scale stirred reactor system for emulsion polymerization.

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

The rates for emulsion polymerization are expressed per cc. of water phase, and the amount of styrene usually is less than the quantity of water. Hence the rates per unit weight of monomer will be somewhat greater than the figures given above.)... 

Models for emulsion polymerization reactors vary greatly in their complexity. The level of sophistication needed depends upon the intended use of the model. One could distinguish between two levels of complexity. The first type of model simply involves reactor material and energy balances, and is used to predict the temperature, pressure and monomer concentrations in the reactor. Second level models cannot only predict the above quantities but also polymer properties such as particle size, molecular weight distribution (MWD) and branching frequency. In latex reactor systems, the level one balances are strongly coupled with the particle population balances, thereby making approximate level one models of limited value (1). 

TABLE 4-1 Composition of a GR-S Recipe for Emulsion Polymerization of Styrene-Butadiene"... 

Fig. 4-3 Plot of percent conversion versus time for emulsion polymerizations of styrene with different concentrations of potassium laurate at 60° C. The moles of emulsifier per polymerization charge (containing 180 g H2O, 100 g styrene, 0.5 g K2S2O8) are 0.0035 (plot 1), 0.007 (plot 2), and 0.014 (plot 3). After Williams and Bobalek [1966] (by permission of Wiley-Interscience, New York).

As mentioned earlier, the necessary components for emulsion polymerization are monomer, emulsifier, initiator, and water. The monomer is insoluble in water in principle, although a small amount of water-soluble monomer can be used as a comonomer in some practical cases. On the other hand, the initiator is usually water soluble. These relations mean that the loci for radical formation and radical polymerization are different, that is, in aqueous and organic phases, respectively. This might look irrational, but the unique advantages of emulsion polymerization lie in it. When radicals are formed from a water-soluble initiator in an aqueous medium in Figure 11.1.2, top, left, each radical undergoes one of the following reactions ... 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

In some compositions suitable for emulsion polymerized types,... 

A common chain transfer agent is dodecylmercaptan (14). Further, dimeric a-methylstyrene and terpinolene have been claimed to be useful transfer agents (16). Initiators for emulsion polymerization are shown in Table 8.6. 

Finally, addition polymerization of suitably substituted furans allows incorporation of the furan nucleus into heterocyclic polymers (77MH1102). 2-Vinylfuran apparently exhibits free radical polymerizability comparable with that of styrene, although rates, yields and degrees of polymerization are low under all conditions except for emulsion polymerization. Cationic polymerization is quite facile and leads not only to the poly(vinylfuran) structure (59), as found in free radically produced polymers, but also to structures such as (60) and (61) in which the furan nucleus has become involved. Furfuryl acrylate and methacrylate undergo free radical polymerization in the manner characteristic of other acrylic esters. 

Advantages of emulsion polymerization are rapidity and production of high-molecular-weight polymers in a system of relatively low viscosity. Difficulties in agitation, heat transfer, and transfer of materials are minimized. The handling of hazardous solvents is eliminated. The two principal variations in technique used for emulsion polymerization are the redox and the reflux methods. 

Formulas for emulsion polymerization also include buffers, free radical initiators, such as potassium persulfate (KiSiOs), chain transfer agents, such as dodecyl mercaptan (G sSTT). The system is agitated continuously at temperatures below 100°C until polymerization is essentially complete or is terminated by the addition of compounds such as dimethyl dithiocarbamate to prevent the formation of undesirable products such as cross-linked polymers. Stabilizers such as phenyl Beta-naphthylamine are added to latices of elastomers. 

Hydroxyalkyl acrylates and polyols are acetoacetylated with diketene to give comonomers used in adhesives, polymers, and coatings, especially the new low solvent coatings, and for emulsion polymerization. The most widely used compound is 2-acetoacetoxyethyl methacrylate (AAEMA) (152). 

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