Polymerization, emulsion mechanism and kinetics

Chem CS (2006) Emulsion polymerization mechanisms and kinetics. Prog Polym Sci 31 443... 

The inverse emulsion polymerization mechanisms and kinetics can be found in the literature [10,66-68]. The area of inverse emulsion polymerization has not been studied extensively, except perhaps for the inverse microemulsion polymerization of acrylamide. The most important applications for these acrylamide-based products are as polymeric flocculants in water treatment. The two major advantages of this polymerization process are the very high polymer molecular weight and a colloidal system that results in rapid dissolution of the polymer in water. 

There is no doubt that the discipline of interfacial phenomena is an indispensable part of emulsion polymerization. Thus, the goal of this chapter is to offer the reader an introductory discussion on the interfacial phenomena related to the emulsion polymerization process, industrial emulsion polymerization processes (primarily the semibatch and continuous reaction systems), some important end-use properties of latex products, and some industrial apphcations. In this manner, the reader may effectively grasp the key features of emulsion polymerization mechanisms and kinetics. Some general readings in this vital interdisciphnary research area [1-6] are recommended for those who need to familiarize themselves with an introduction to the basic concepts of colloid and interface science. 

The basic framework of emulsion polymerization mechanisms and kinetics is primarily built on the aforementioned pioneering studies, and many other excellent contributions appeared thereafter. Several very useful empirical or approximate equations for calculating n were also developed for the emulsion polymerization system in the absence of the bimolecular termination of free radicals in the continuous aqueous phase (i.e., Y = 0). For example ... 

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. 

In general, the number of latex particles per unit volume of water determined at the end of polymerization and the slope obtained from the least-squares best-fitted linear portion of the monomer conversion versus time curve are taken as the Np and Rp data in the study of polymerization mechanisms and kinetics. In traditional emulsion polymerization, monomer droplet nucleation, can be neglected and only naiceflar nucleation, homogeneous nucleation and flocculation of latex particles need to be taken into consider-... 

In addition to batch emulsion polymerization (which is commonly used in the laboratory to study reaction mechanisms) for preliminary development/screen-ing of new latex products and to obtain approximate kinetic data for process development and reactor scale-up, the versatile semibatch and continuous emulsion polymerization processes are widely used for the production of commercial latex products. A major reason that batch reactors are not used for commercial production is due to the very exothermic nature of free radical polymerization and rather limited heat transfer capacity in large-scale reactors. Furthermore, continuous and especially semibatch reaction systems offer the operational flexibility to produce latex products with controlled polymer composition, particle morphology, and particle size distributions. These parameters will have an important influence on the performance properties of latex products. In this chapter, we will focus on the aspects of polymerization mechanisms and kinetics involved in semibatch and continuous emulsion polymerization systems. Those who are interested in the previous studies of semibatch and continuous emulsion polymerization processes should refer to the review articles cited in references 1. ... 

The rational design of a reaction system to produce a desired polymer is more feasible today by virtue of mathematical tools which permit one to predict product distribution as affected by reactor type and conditions. New analytical tools such as gel permeation chromatography are beginning to be used to check technical predictions and to aid in defining molecular parameters as they affect product properties. The vast majority of work concerns bulk or solution polymerization in isothermal batch or continuous stirred tank reactors. There is a clear need to develop techniques to permit fuller application of reaction engineering to realistic nonisothermal systems, emulsion systems, and systems at high conversion found industrially. A mathematical framework is also needed which will start with carefully planned experimental data and efficiently indicate a polymerization mechanism and statistical estimates of kinetic constants rather than vice-versa. 

In contrast to emulsion polymerization, the reaction kinetics of microemulsion polymerization is characterized by two polymerization rate intervals the interval of constant rate characteristic of emulsion polymerization is missing [42,49,53], as shown in Fig. 2. Polymer particles are generated continuously during the reaction by both micellar and homogeneous mechanisms. As the solubility of the monomer in the continuous domain increases, homogeneous... 

The mechanism and kinetics of vinyl chloride polymerization in emulsion have been studied extensively, mainly involving the use of chemical initiators, and has been summarized by Talamini and Peg-gion and by Ugelstad et alia (4). The mechanism shows a gradual change as the number of particles (N) is increased. 

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. 

Other parts of this book contain detailed discussions of emulsion polymerization mechanisms, kinetics, and reaction en neering. Thus, the intent of this chapter is to introduce the reader to some of the basic concepts of continuous reactor systons. A rather simple steady-state reactor model will be presented for a sin e CSTR system. This model will be compared with a batch reactor model for the same reaction mechanism and the model differences highlighted. 

It is at present clearly impossible to understand all the aspects of these systems. Nevertheless the mechanism and kinetics of some emulsion systems are reasonably well understood—those in which the monomer is water- insoluble and in which the polymer is soluble in the monomer. An outline is given of this mechanism and the kinetics of polymerization are developed on the basis of this mechanism. This theoretical kinetic behavior is then compared with experimental data, both from the literature and from unpublished results. Whenever possible, the influence of monomer water solubility and monomer solubility of the polymer is commented on. These comments are mostly of a qualitative nature and sometimes even speculative. The present state of our knowledge does not permit going beyond such comments, although recently the literature has given a few attempts at quantitative interpretation of emulsion polymerization of water-soluble monomers. 

In an inverse emulsion polymerization, a hydrophilic monomer, frequently in aqueous solution, is emulsified in a continuous oil phase using a water-in-oil emulsifier and polymerized using either an oil-soluble or water-soluble initiator the products are viscous latices comprised of submicroscopic, water-swollen, hydrophilic polymer particles colloidally suspended in the continuous oil phase. The average particle sizes of these latices are as small as 0.05 microns. The technique is applicable to a wide variety of hydrophilic monomers and oil media. The inverse emulsion polymerization of sodium p-vinylbenzene sulfonate initiated by both benzoyl peroxide and potassium persulfate was compared to the persulfate-initiated polymerization in aqueous solution. Hypotheses for the mechanism and kinetics of polymerization were developed and used to calculate the various kinetic parameters of this monomer. 

The heterogenous nature of emulsion polymerization processes, and the stepwise mechanisms involved add complications to the understanding of the kinetics. Despite the investigations conducted in emulsion polymerization over the last few decades, the scientific representation of these complex processes remains incomplete. With recent advances in fundamental chemistry and colloid science, model uncertainties have diminished. 

A considerable amount of work has been published during the past 20 years on a wide variety of emulsion polymerization and latex problems. A list of 11, mostly recent, general reference books is included at the end of this chapter. Areas in which significant advances have been reported include reaction mechanisms and kinetics, latex characterization and analysis, copolymerization and particle morphology control, reactor mathematical modeling, control of adsorbed and bound surface groups, particle size control reactor parameters. Readers who are interested in a more in-depth study of emulsion polymerization will find extensive literature sources. 

The main issues in dealing with the mechanisms and kinetics of emulsion poly merization involve the understanding of the processes by which latex particle form and grow, which includes the evolution of particle size (or particle number and size distribution, the development of molar mass and molar mass distribution the polymerization rate profile during the course of the polymerization, and hov these are influenced by the basic polymerization parameters such as monomeifs) surfactant(s) type and concentration, initiator type and concentration, tempera ture, and mode and rate of monomer addition. Research over the past 50 yean has dealt with all of these issues resulting in tremendous advances in our bask knowledge of emulsion polymerization, as is reflected in this book and othei references listed in the references of this chapter. 

Moreover the miniemulsion, microemulsion and conventional emulsion polymerizations techniques show quite different particle nucleation and growth mechanisms and kinetics [271]... 

The mechanism and kinetics of the inverse emulsion polymerization of p-sodium styrene sulfonate was investigated using both oil-soluble and water-soluble initiators (53). Table XI gives the recipe used in these polymerizations. The p-sodium styrene sulfonate was dissolved in the water, and the solution was emulsified in the o-xylene using the Span 60 (sorbitan monostearate ICI America) emulsifier in some experiments, the benzoyl peroxide was dissolved in the o-xylene in others, the potassium persulfate was dissolved in the aqueous phase. The polymerizations were carried out at 40-70°. The rates of polymerization were measured dilatometrically, the molecular weights by solution viscosity, and the latex particle sizes by electron microscopy. 

The mechanism and kinetics of these types of heterophase acrylamide polymerization reactions have been studied [13-15]. Depending on the initiator type and oil quality the polymerization can physically and kinetically resemble either an emulsion or a suspension polymerization. With paraffinic oil phases or water-soluble initiators its mechanism resembles the one for suspension polymerization. 

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