Emulsion polymerization Harkins-Smith Ewart mechanism

In the polymerization of styrene, using potassium persulfate as initiator. Roe ( ) observed that the total number of particles in latices depended on the composition of the mixed surfactants and not on the total number of micelles. Therefore, he devaluated the micellar nucleation mechanism for emulsion polymerization as proposed by Harkins( ) -Smith-Ewart(j). 

Emulsion polymerization first gained industrial importance during World War II when a crash research program in the United States resulted in the production of styrene-co-butadiene [SBR] synthetic rubber. The Harkins-Smith-Ewart model [5-6] summarized the results of early research, which focussed on this and similar systems. Current thinking is not entirely in accord with this mechanism. It is still worthwhile to review it very brielly here, however, because it is still widely referenced in the technical literature and because some aspects of the model provide valuable insights into operating procedures. 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

The foundations of emulsion polymerization were described originally by Harkins [39]. The first theoretical treatment was proposed by Smith and Ewart [40]. The theory was later modified to some extent by O Toole [41] and more fundamentally by Garden [42], who proposed an unsteady-state mechanism for the concentration of free radicals in the emulsion particles. Tauer [43], Gilbert [44] and Lovell and El-Aasser [45] have produced recent reviews. 

Polystyrene can be easily prepared by emulsion or suspension techniques. Harkins (1 ), Smith and Ewart(2) and Garden ( ) have described the mechanisms of emulsTon polymerization in batch reactors, and the results have been extended to a series of continuous stirred tank reactors (CSTR)( o Much information on continuous emulsion reactors Ts documented in the patent literature, with such innovations as use of a seed latex (5), use of pulsatile flow to reduce plugging of the tube ( ), and turbulent flow to reduce plugging (7 ). Feldon (8) discusses the tubular polymerization of SBR rubber wTth laminar flow (at Reynolds numbers of 660). There have been recent studies on continuous stirred tank reactors utilizing Smith-Ewart kinetics in a single CSTR ( ) as well as predictions of particle size distribution (10). Continuous tubular reactors have been examined for non-polymeric reactions (1 1 ) and polymeric reactions (12.1 31 The objective of this study was to develop a model for the continuous emulsion polymerization of styrene in a tubular reactor, and to verify the model with experimental data. 

The anionic and cationic emulsion polymerization of dimethylcyclosiloxanes in aqueous phase, has received little study to date. The acid-catalysed emulsion polymerization of a series of cyclic dimethylsiloxane monomers, with varying emulsifiers, has recently been studied. Whilst qualitatively the mechanism appears consistent with the Harkins model, quantitatively the system, as might be expected, shows wide deviations from Smith-Ewart predicted behaviour. This emulsion polymerization system is interesting in that initiation, propagation, and termination proceed by ionic pathways. 

The ocxiventional model for describing the mechanism of emulsion polymerization was used as a starting point Qrigincilly proposed by Harkins / it was made more quantitative by Stnith and Ewart u) and has recently been exta )ded by Garden While there is much controversy over the applicability of the Smith-Ewart model/ and many systems have been found in vhich it does not apply/ the properties of the present system/ particularly the low solubility of the major monomer in water/ seemed favorable for its use ... 

The preceding discussion of free-radical addition polymerization has considered only homogeneous reactions. Considerable polymer is produced commercially by a complex heterogeneous free-radical addition process known as emulsion polymerization. This process was developed in the United States during World War II to manufacture synthetic rubber. A rational explanation of the mechanism of emulsion polymerization was proposed by Harkins [4] and quantified by Smith and Ewart [5] after the war, when information gathered at various locations could be freely exchanged. Perhaps the best way to introduce the subject is to list the feed sent to a typical reactor. 

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