Emulsion polymerization mechanism

Figure 3. Presentation of the emulsion-polymerization mechanism in an aqueous phase.

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. 

This picture of emulsion polymerization mechanisms, mainly due to Harkins (29, 30), is generally accepted as is borne out by its description in several recent textbooks on polymerization (4, 10, 13, 24, 68). An extensive review has been given by Gerrens (26). 

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

These factors alone may contribute significantly to variations in experimental results obtained in the polymerization of vinyl chloride. For example, at superatmospheric pressures, variation in the liquid volume of the monomer to the total free space of a closed reactor means a variation in the amount of gaseous monomer that can condense and/or diffuse into the polymer at some stage of the polymerization process. This vapor-phase monomer level may represent a monomer reservoir somewhat analogous to the monomer droplets postulated for the emulsion polymerization mechanism by Smith and Ewart. 

Copolymerization is a very important tool for investigating emulsion polymerization mechanism. In other words, not only the rate of polymerization and the particle size, but also copolymer and particle composition can be used to derive the mechanism. The pair of monomers with different solubilities is considered as a model system for such investigation. The fraction of monomers in the aqueous phase and in polymer particles may be controlled by the nature of monomers and the monomer feed composition. The composition of the copolymer is a function of the monomer feed composition in particles and is only slightly affected by the monomer composition in the aqueous phase. This follows from the much higher rate of polymerization in articles compared to that in the homogeneous (aqueous) phase. 

In summary, there has been a tremendous effort devoted to the fundamental aspects of emulsion polymerization mechanisms, kinetics and processes since the early twentieth century. Representative review or journal articles concerning the conventional emulsion polymerization can be found in literature [20-25, 48-60]. The research areas related to both miniemulsion [42,61-64] and microemulsion [44-47, 65] polymerizations have received increasing interest recently. 

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 ... 

Poehlein [81] identified major problems encountered with the development of continuous emulsion polymerization processes. It was shown that the development of commercial continuous emulsion polymerization processes involves the consideration of many factors associated with process design and product quality. These factors include the effects of inhibitor, polymerization rate, particle size distribution, copolymer composition, addition strategy of feed streams, unsteady-state operation, and reactor design on continuous emulsion polymerization processes. The author then used a two-continuous stirred tank reactor series to elucidate key continuous emulsion polymerization mechanisms and generate the knowledge necessary for the development of commercial continuous processes. 

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. 

Kinetics and Mechanisms. Early researchers misunderstood the fast reaction rates and high molecular weights of emulsion polymerization (11). In 1945 the first recognized quaHtative theory of emulsion polymerization was presented (12). This mechanism for classic emulsion preparation was quantified (13) and the polymerization separated into three stages. 

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

The quahty of the water used in emulsion polymerization has long been known to affect the manufacture of ESBR. Water hardness and other ionic content can direcdy affect the chemical and mechanical stabiUty of the polymer emulsion (latex). Poor latex stabiUty results in the formation of coagulum in the polymerization stage as well as other parts of the latex handling system. 

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). 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Emulsion Polymerization. Poly(vinyl acetate) and poly(vinyl acetate) copolymer latexes prepared in the presence of PVA find wide appHcations in adhesives, paints, textile finishes, and coatings. The emulsions show exceUent stabiHty to mechanical shear as weU as to the addition of electrolytes, and possess exceUent machining characteristics. 

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. 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

Water-soluble initiators that can generate active free radicals are used in emulsion polymerization. The generation of active free radical can occur by two different mechanisms (1) thermal decomposition, and (2) chemical interaction. 

Therefore, the polymerization progresses within the micelle structure by following the traditional mechanism of emulsion polymerization. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

The early history of redox initiation has been described by Bacon.23 The subject has also been reviewed by Misra and Bajpai,207 Bamford298 and Sarac.2,0 The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when initiation is required at or below ambient temperature and drey are frequently used for initiation of emulsion polymerization. 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small dial entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of n will usually be less than 0.4. In these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates or particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical termination, when it occurs in the particle phase, will usually be between a short species (one that lias just entered) and a long species. 

Copolymerization of macromonomers formed by backbiting and fragmentation is a second mechanism for long chain branch formation during acrylate polymerization (Section 4.4.3.3). The extents of long and short chain branching in acrylate polymers in emulsion polymerization as a function of conditions have been quantified.20 ... 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

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