Locus of polymerization

The principal deficiency of source-based nomenclature is that the chemical structure of the monomeric unit in a polymer is not identical with that of the monomer, e.g., -CHX-CH2- versus CHX=CH2. The structure of the constitutional repeating unit (CRU) may also not be clearly identified in this scheme for example, the name polyacrylaldehyde does not indicate whether (i) the vinyl group or (ii) the aldehyde group was the locus of polymerization. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

The preceding discussion has led us to the conclusion that the surface is the only locus of polymerization which needs to be considered in the heterogeneous polymerization of acrylonitrile. Radicals arrive at the surface at a rate determined by the decomposition of the initiator and efficiency of initiation. Propagation occurs on the surface at a rate determined by the activity of monomer at the surface. By analogy with emulsion polymerization, where monomer diffuses into the particles rapidly enough to maintain near equilibrium activity (14), we assume that the activity of the monomer adsorbed on the particle surface is approximately equal to the mole fraction in solution. The propagation rate constant is presumably influenced somewhat by the presence of the solid surface. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

The MC simulation method is particularly suitable for investigating emulsion polymerization that involves various simultaneous kinetic events with a very small locus of polymerization. The MC simulation method will become a standard mathematical tool for the analysis of complex reaction kinetics, both for linear and nonlinear emulsion (co)polymerization. 

Kinetically, each bead acts as a small independent reactor there is little exchange of material between the beads. Since there is no solvent present at the locus of polymerization, the kinetics are those of bulk polymerization, with the molecular weight distribution (MWD) characteristics similar to those of bulk or solution polymerizations. If water-soluble initiator is used in a suspension polymerization, very little polymerization will occur, since few free radicals will reach the locus of polymerization in the monomer beads. 

One of the most unique properties of miniemulsion polymerization is the lack of monomer transport. Recall from Fig. 1 that with macroemulsion polymerization, the monomer must diffuse from the monomer droplets, across the aqueous phase, and into the growing polymer particles. In contrast, in an ideal miniemulsion (nucleation of 100% of the droplets), there is no monomer transport, since the monomer is polymerized within the nucleated droplets. This lack of monomer transport leads to some of the most interesting properties of miniemulsions. For most monomers, macroemulsion polymerization is considered to be reaction, rather than diffusion limited. However, for extremely water insoluble monomers, this might not be the case. In this instance, polymerization in a miniemulsion might be substantially faster than polymerization in an equivalent macroemulsion. For copolymerization in a macroemulsion, where one of the comonomers is highly water insoluble, the comonomer composition at the locus of polymerization might be quite different from the overall comonomer composition, resulting in copolymer compositions other than those predicted by the reactivity ratios. 

Samer [104] carried out similar copolymerizations with similar results. An example of his data is given in Fig. 16. Here 2-ethylhexyl acrylate (EHA) was copolymerized with MMA in batch. The miniemulsion polymerizations (two are shown) follow the copolymer equation, while the macroemulsion polymerization gives EHA incorporation that is lower than predicted by the copolymer equation, presumably due to the low concentration of EHA at the locus of polymerization. The dotted hne in Fig. 16 is for a model derived by Samer that accurately predicts the copolymer composition. Samer derived this model by adapting the work of Schuller [149]. Schuller modified the reactivity ratios for the macroemulsion polymerization of water-soluble monomers to take into accoimt that the comonomer concentration at the locus of polymerization is different from the comonomer composition in the reactor due to the water solubilities of the monomers. Samer used the same approach to account for the fact that the comonomer concentration at the locus of polymerization might be different from that of the reactor due to transport limitations of water insoluble comonomers. 

The polymerization rate in the presence of alkyd was slower than that without alkyd. Doubling the initiator and emulsifier concentration increased the reaction rate, but not to the level achieved with the miniemulsion polymerization without alkyd. This retardation (as reported also by Nabuurs) increased with increasing alkyd level. The latexes obtained from the miniemulsion polymerization of the alkyd-acrylate mixtures were uniform emulsions, and no coagulation occurred during polymerization. Macroemulsion polymerization with alkyd resulted in colloidal instabihty, probably due the inabihty of the alkyd to reach the locus of polymerization. 

Before describing a qualitative picture of emulsion polymerization a note on monomer solubility and type of surface active agents is in order. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. The solubility must be less than about 0.004 mol/L, as otherwise the aqueous phase will become a major locus of polymerization and the system will then not be typical emulsion polymerization. At the same time the monomer must be slightly soluble as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (see later). 

According to the core-shell model, the growing particle is actually heterogeneous rather than homogeneous, and it consists of an expanding polymer-rich (monomer-starved) core surrounded by a monomer-rich (polymer-starved) outer spherical shell. It is the outer shell that serves as the major locus of polymerization and Smith-Ewart (on-off) mechanism prevails while virtually no polymerization occurs in the core because of its monomer-starved condition. Reaction within an outer shell or at the particle surface would be most likely to be operative for those polymerizations in which the polymer is insoluble in its own monomer or under conditions where the polymerization is diffusion-controlled such that a propagating radical cannot diffuse into the center of the particle. 

In conventional emulsion polymerization, the disappearance of the VAc/BA droplets at ca. 25% conversion results from the transfer of monomer from monomer droplets to the locus of polymerization (monomer-swollen polymer particles). The presence of HD in the minidroplets reduces the free energy of mixing of the constituent monomers in the droplets. Therefore, the difference in the free energy of mixing between the monomer and polymer (particles) in mini-emulsion copolymerization is less than that in conventional emulsion polymerization. As a consequence, a smaller flux of monomer from the monomer droplets to polymer particles is achieved during mini-emulsion polymerization. In addition, HD cannot be transported from the droplets to particles because of its extremely low water solubility. Thus, the HD concentration in the droplets is greater than that in the particles, and monomer is retained in the droplets to minimize the HD concentration gradient. 

One of the key features of this reaction mechanism is the particle nucleation beyond the first maximal Rp. Furthermore, the disappearance of monomer droplets in the conventional emulsion polymerization at ca. 30-50% conversion results from the transfer of monomer from the monomer droplets to the locus of polymerization (polymer particles). In mini-emulsion polymerization, the concentration of monomer decreases and monomer droplets may exist throughout the polymerization. In the context of the micellar nucleation model,both Np and the concentration of monomer in the particles in the constant reaction rate region contribute to Rp. Therefore, the first maximal Rp observed in the course of mini-emulsion polymerization does not necessarily correspond to the end of particle nucleation. This is because Np may increase in the course of polymerization, but the contribution of the increased Np to Rp can be outweighed by the decreased monomer concentration in the reaction loci. 

M] in Equation 12.44 has been replaced by [Mp] in Equation 12.45. This makes sense, since the monomer concentration that feeds the radicals in emulsion polymerization (at the appropriate reaction site) is indeed the monomer concentration in the polymer (latex) particles. Since [R] in Equation 12.44 represents total radical concentration, it has been replaced by the product (N n), which represents the total number of radicals present at the reaction site (the main locus of polymerization, which is inside the monomer-swollen polymer particles). is the total number of particles (usually per lit of water) and n represents the average number of radicals per particle. N, Avogadro s number, appears in the equation simply for unit conversion. Needless to say. Equation 12.45 is completely analogous to Equation 12.44. 

Emulsions, suspensions, and dispersions are examples of colloidal systems. It is important to mention that these terms are not always used consistently in the literature and that this situation may be confusing for students and nonpolymer scientists [24]. From the point of view of polymer science and engineering, these terms refer to heterogeneous polymerizations, particularly polymerizations in aqueous/alcoholic dispersed media. Thus, the aforementioned terms have connotations that have to do with the initiator, monomer, and polymer solubility in each phase as well as with particle size and the main locus of polymerization. These aspects are treated in detail later for the moment, let us assume that there are no chemical reactions and that such terms are used in the context of colloid science. 

Given the three phases present in an emulsion polymerization system, the locus of polymerization can conceivably be in the monomer droplets, in the aqueous phase within the micelles, or possibly at an interface. Some polymerization obviously takes place in the aqueous phase but with a limited contribution to the overall polymerization because of the low solubility of the monomer in water. The monomer droplets also do not provide the loci for polymerization because the negatively charged sulfate anions find the soap-stabilized monomer droplets virtually impossible to penetrate. Also, the primary sulfate radical anions are oil insoluble. The absence of polymerization in the monomer droplets has been verified... 

A pseudo-bulk system is one in which the compartmentalized nature of the locus of polymerization has no effect on any kinetic property (rate, molar mass or particle size distributions). A system in which n is appreciably greater than 0.5 will always be pseudo-bulk there are so many radicals in a particle that the polymerization will be indistinguishable from the equivalent bulk one. However, a system with a low value of n can also be pseudo-bulk, if (for example) radical desorption results in the desorbed radical suffering no other fate except to re-enter another particle [1,3]. It is then apparent that the polymerization process will not see the walls between particles. Because pseudo-bulk kinetics can occur in systems where n 0.5, a pseudo-bulk system is different from the Smith-Ewart Case 3. 

Act to stabilize the monomer droplets in an emulsion form (i.e., the droplets act as a reservoir to supply the growing polymer particles with additional monomer, or can act as the locus of polymerization if the monomer emulsions are comparable in size to monomer-swollen micelles, as in the case of miniemulsion polymerization (see Chapter 20)). 

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