Allyl acetate emulsion polymerization

Anomolous results have been observed in some emulsion polymerizations—inverse dependencies of N, Rp, and Xn on surfactant concentration. Some surfactants act as inhibitors or retarders of polymerization, especially of the more highly reactive radicals from vinyl acetate and vinyl chloride [Okamura and Motoyama, 1962 Stryker et al., 1967]. This is most apparent with surfactants possessing unsaturation (e.g., certain fatty acid soaps). Degradative chain transfer through allyl hydrogens is probably quite extensive. 

The preceding indicates that in bulk and in benzene solution, the polymerization of allyl acetate leads to polymers of relatively low molecular weight. It had been expected that in emulsion processes, a higher-molecular-weight polymer would be formed. However, with a persulfate initiator, the emulsion polymerization of this monomer led to a product with the same molecular weight as was found upon bulk polymerization [18]. 

The polymerization of allyl acetate has been studied reasonably extensively in the effort to elucidate the mechanisms of the polymerization process. The utility of poly(allyl acetate) as a resin is believed to be negligible. Nevertheless, this compound has been polymerized in bulk, in solution, and in emulsion. There is mention of a cationic polymerization process and of radiation-induced charge-transfer processes. 

In Experiment 1, 9.28 moles of allyl acetate per liter of latex was emulsified with 0.087 mole of sodium lauryl sulfate, buffered with 0.45 mole of sodium pyrophosphate, and initiated with 0.0920 mole/liter of potassium persulfate. In experiment 2, again 9.28 moles of allyl acetate per liter of latex was polymerized in the presence of 0.087 mole of sodium lauryl sulfate, 0.45 mole of sodium pyrophosphate, and 0.366 mole/liter of potassium persulfate. Since the MW of allyl acetate is 100, the above information implies that the basic monomer to water ratio is an unlikely 928 gm of monomer to approximately 70 gm of water. If indeed these are the experimental facts, then the fact that the polymers produced resembled those produced in bulk or in solution is not surprising. A reaction mixture consisting of nearly 93% pure monomer, naturally would be expected to produce a polymer similar to one produced from a pure (i.e., bulk) monomer and not one similar to an emulsion polymer. Compositions of less than 60% monomer in water would ordinarily be expected to produce latices. Perhaps the data in question refer to a ratio of 9.28 moles of monomer to one liter of water. 

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. 

Besides vinyl acetate monomer, three other components are neeessary to earry out an emulsion polymerization water, an emulsifier and/or a proteetive eolloid, and a water-soluble initiator. Most commonly, anionic long-chain alkyl sulfonates are used as surfactants in amounts up to 6%. Studies have shown that the rate of polymerization is dependent on the amoimt of emulsifier present, with the rates inereasing as the amoimt of emulsifier is increased up to a certain point and then falling olF as free-radieal ehain transfer to the surfaetant beeomes a serious competing side reaetion [240]. In general, surfactants are used in eombination with a protective colloid. Especially useful as protective colloids are poly(vinyl alcohol), hydroxyethyl cellulose, alkyl vinyl ether-maleic anhydride and styrene-allyl alcohol copolymers, and gum arable. Water-soluble initiators, particularly potassium persulfate, alkali peroxydisulfates, hydrogen peroxide, and various redox systems, are most commonly used. 

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