Emulsion polymerization general characteristics

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Abstract Emulsion homopolymers and copolymers (latexes) are widely used in architectural interior and exterior paints, adhesives, and textile industries. Colloidal stabihzators in the emulsion polymerization strongly affect not only the colloidal properties of latexes but also the fdm and mechanical properties, in general. Additionally, the properties of polymer/copolymer latexes depend on the copolymer composition, polymer morphology, initiator, polymerization medium and colloidal characteristics of copolymer particles. 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with... 

In general, for most investigations, y radiation has been used because of its high degree of penetration and the ccanparative ense of estimating dose-depth characteristics and because radical fluxes comparable to those used with chemical initiation can easily be achieved- There have also been a few, comparatively brief, studies using electron accelerators to initiate emulsion polymerization in emulsion. These have mainly been conducted in Japanese laboratories. 

Apart from the advantages of radiation emulsion polymerization indicated above and those characteristic of radiation-induced polymerization in general, the following advantages should also be mentioned. 

The procedure by which an emulsion polymerization is carried out has a profound effect upon the resulting latex and polymer properties. Indeed, latexes and polymers with quite different performance characteristics can be produced from the same reaction formulation by appropriate control of the type of polymerization process and conditions used. In this chapter, the two types of batchwise process are described with particular emphasis upon the control that can be exercised over polymerization conditions and product properties. An enormous volume of material on this subject exists in the scientific and patent literature, and cannot be fully documented here. Instead, general principles are presented and illustrated using specific examples which serve as good starting points for researching relevant literature on the topics covered. 

The most characteristic feature distinguishing miniemulsion polymerization from an ordinary emulsion polymerization is that, in the former case, both particle nucleation and polymerization take place in pre-formed, stable drc Iets of monomer, irrespective of whether water-soluble or oil-soluble initiatois are used. A con nehensive treatmenf of miniemulsion polymerization in general is given in Ch ter 20. The present discussion will, therefore, focus on those aspects most relevant to vinyl chloride polymerization. 

In principle, the characterization of polymers prepared in emulsion could be achieved by using approaches and techniques. similar to those available for polymers in general. Special attention for products of emulsion polymerization, however, seems justified since emulsion polymers usually show a number of features, characteristic of their origin. Therefore, in this chapter, latex polymer characterization will be considered in relation to the pertaining emulsion polymerization conditions. 

Nomura et al. [74,75] proposed an experimental method to study the competitive particle nucleation mechanisms (micellar nucleation versus homogeneous nucleation) in a given emulsion polymerization system. This approach involves the emulsion copolymerization of relatively hydrophobic styrene with relatively hydrophilic monomers such as methyl methacrylate or methyl acrylate. The composition of copolymer produced during the very early stage of polymerization (far lower than 1% monomer conversion), which reflects the characteristic of copolymer at the locus of particle nucleation, is then determined. Emulsion copolymerization of styrene with methyl methacrylate (or methyl acrylate) was carried out, where sodium dodecyl sulfate was used to stabilize the emulsion polymerization system and where the weight ratio of styrene to methyl methacrylate (or methyl acrylate) was kept constant at 1 1. The experimental results show that the compositions of copolymers obtained from emulsion polymerizations in the presence and absence of monomer-swollen micelles are quite different. This provides supporting evidence of the generally accepted Smith-Ewart theory that micellar nucleation controls the particle nucleation process in the emulsion copolymerization of styrene with... 

It is not straightforward to successfully manufacture a particular latex product, which is generally developed in a laboratory batch or semibatch reactor, in a commercial continuous emulsion polymerization system (e.g., a continuous stirred tank reactor). This is simply because the characteristics of continuous stirred tank reactors are dramatically different from those of batch and semibatch reactors. As a consequence, the particle nucleation process and kinetics experienced in batch or semibatch emulsion polymerization systems cannot be directly applied to continuous systems consisting of stirred tank reactors. 

ATRP-functionalized polymer particles are generally synthesized by emulsion polymerization, where the functionalizing monomer (e.g., benzyl halides and a-halo carboxylates carrying the terminal acrylic or methacrylic gronps) is polymerized onto polymer-seed polymer latex particles [157-166]. The process is formally achieved in two different steps, where the polymer seed is formed first with the desired characteristics and the functionalizing monomer is polymerized afterward on the surface. On the other hand, ATRP initiator can be introduced onto particle surface by a chemical reaction between the ATRP initiator and a reactive functional group attached previously at the latex particle surface [167-172]. 

Microemulsion polymerisation has shown a great advantage over conventional polymerisation strategies such as emulsion polymerisation with respect to the end particle size, polydispersity and reproducibility of the product characteristics. Although we still face severe problems regarding the polymerisation of microemulsions (see Section 11.2 in Chapter 11), it has been employed for the synthesis of polymeric nanoparticles of pharmaceutical interest. Microemulsion polymerisation involves free-radical polymerisation in a large number of monomer-swollen microemulsion droplets and represents a thermodynamically stable, transparent one-phase reaction system. Generally, the microemulsion droplet is considered as initiation locus for the polymerisation. The type of microemulsion used for the polymerisation depends on the monomer properties [148]. 

For the successful preparation of emulsions, the wetting conditions on the membrane surface are crucial. It is necessary that the membrane surface is only wetted by the liquid that forms the continuous phase. The droplet size correlates with the membrane pore size by a simple relation, Dd = /Dm where / is a value typically between 2 and 8 (35). Droplets can be produced with diameters in the pm-, as well as in the sub-micrometre range. This technique has been successfully applied to produce monodisperse emulsions and multiple emulsions, as well as to carry out polymerizations leading to polymer particle in the pm size range with narrow size distributions (36, 37). Further advantages (38) are as follows the droplet size is controllable and generally a quite narrow DSD can be achieved, the method is reproducible and the scale-up is easy just by increasing the number of membrane modules, the characteristic features are independent of scale-up, batch as well as continuous operations modes are possible, the continuous phase is exposed to a lower stress. 

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