Two-phase emulsion polymerization kinetics

This free radical distribution function, determined via a Monte Carlo technique, was then incorporated into the two-phase emulsion polymerization kinetics model developed by Nelson and Sundberg [44, 45] to predict the monomer conversion versus time data available in the literature. Thus, the only difference between the kinetic model characterized by the following major governing equations [47] and that of Nelson and Sundberg is the method used for calculation of the average free radical population in each polymer phase. 

Recently, Durant et al. [55] developed a mechanistic model based on the classic Smith-Ewart theory [48] for the two-phase emulsion polymerization kinetics. This model, which takes into consideration complete kinetic events associated with free radicals, provides a delicate procedure to calculate the polymerization rate for latex particles with two distinct polymer phases. It allows the calculation of the average number of free radicals for each polymer phase and collapses to the correct solutions when applied to single-phase latex particles. Several examples were described for latex particles with core-shell, inverted core-shell, and hemispherical structures, in which the polymer glass transition temperature, monomer concentration and free radical entry rate were varied. This work illustrates the important fact that morphology development and polymerization kinetics are coupled processes and need to be treated simultaneously in order to develop a more realistic model for two-phase emulsion polymerization systems. More efforts are required to advance our knowledge in this research field. 

P. Varshney, Two Phase Emulsion Polymerization Kinetics of Styrene and Methyl-Methacrylate, MS Thesis, Department Of Chemical Engineering, University Of New Hampshire, Durham, NH, 1981. 

Rosen [40] is a pioneer in the field of two-phase emulsion polymerization mechanisms and kinetics. He studied the influence of the polymer phase separation on the efficiency of grafting reactions and demonstrated the physical limits of these grafting reactions. This was followed by the work of Chiu [41] with an attempt to develop a mechanistic model that can be used to predict the experimental kinetic data obtained from two-phase emulsion polymerizations that was not successful. 

C. W. Chiu, The Kinetics of Two-Phase Emulsion Polymerization, PhD Thesis, Carn-egie-Mellon University, Pittsburgh, PA, 1978. 

D. Nelson and D. C. Sundberg, Kinetics of Two Phase Emulsion Polymerization, In AIChE Manuscript No. 7739, AIChE Meeting, Houston, TX, March, 1983. 

The kinetics of emulsion polymerization is complex, involving a large number of species and at least two phases. The first quantitative approach to emulsion polymerization kinetics led to extensions by many others.The important events to consider are 1) the free-radical reactions of chain formation initiation, propagation, chain transfer, and termination and 2) the phase transfer events that control particle formation radical entry into particles from the aqueous phase, radical exit into the aqueous phase, radical entry into micelles, and the aqueous phase coil-globule transition. In free-radical emulsion polymerization, the fundamental steps are shown schematically in Fig. 1... 

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

Emulsion systems, while widely used in the polymerization of unsaturated monomers, are used rarely for polycondensation. The emulsion system is one in which two (or more) liquid phases are present, md in which polymerization occurs entirely in the bulk of one of the phases and is almost exclusively kinetically controlled. It thus represents a transition from solution polymerization to interfacial polymerizations. In the case of polycondensation reactions, emulsion polymerization has not been studied in detail. Results thus far indicate that molecular weight and molecular weight distribution are subject to the same statistical considerations as apply to solution and melt polymerizations. 

Mathematical modeling is an essential tool for design and scale-up. As emulsion polymerization involves a minimum of two phases, the kinetics is complex and makes it difficult to ... 

The term zero-one designates that all latex particles contain either zero or one active free radical. The entry of a radical in a particle that already contains a free radical will instantaneously cause termination. Thus, the maximum value of the average number of radicals per particle, n, is 0.5. In a zero-one system, compartmentalization plays a crucial role in the kinetic events of emulsion polymerization processes. In fact, a radical in one particle will have no access to a radical in another particle without the intervention of a phase transfer event. Two radicals in proximity will terminate rapidly however, the rate of termination will be reduced in the process because of compartmentalization, as the radicals are isolated as separate particles. Consequently, the propagation rate is higher and the molecular weight of the polymer formed is larger than in the corresponding bulk systems. Which model is more appropriate depends primarily on the particle size. Small particles tend to satisfy the zero-one model, as termination is likely to be instantaneous. ... 

The kinetics of inverse emulsion polymerization can be classified more or less arbitrarily into two subclasses according to the solubility of the initiator. Note that the solubility in water of oil-soluble initiators was found to be oihanced (up to a factor of 3) by the presence of monomo [23,28,29], making possible initiation by radical pairs formed in the aqueous dispersed phase. On the other hand, homogeneous nucleation mechanisms generating oligoradicals in the continuous phase can also be operative in inverse emulsions due to the maiginal solubility of acrylamide in organic media (1.6 wt% in isoparaffinic solvents and 2 wt% in toluene) [3]. 

When a mixture of water, monomer, surfectants, initiator, and property control agents forms microdroplets of monomer in water with dimensions of 1 pm, an emulsion has been formed (8). If polymerization occurs in the 1 pm droplets by migration of an initiator active site from the aqueous phase to the microdroplet to completely react the droplet, the process is emulsion polymerization. A major problem in emulsion polymerization is developing a mixture which remains a stable emulsion throughout the polymerization. Polymerization kinetics is controlled by the number of microdroplets in the reaction mixture and the polymerization rate of the monomer. Polymer can be recovered as fine particles fiom the completed reaction but is most often used as the emulsion of polymerized product. Emulsion polymerization is commonly used to synthesize two polymer types covered in this book, polyvinyl chloride... 

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