Suspension polymerization approach

Studies involving the use of organically modified clay particles in heterophase polymerization are rather scarce. Indeed, we are aware of only two reports that combine the emulsion or suspension polymerization approaches and ion-ex-change reaction. In one of these reports, AI BA is immobiHzed in the clay interlayer region to yield exfoliation of MMT in the PMMA matrix through suspension polymerization [135]. In another relevant study, it was demonstrated that exfoliated structures could be obtained by post-addition of an aqueous dispersion of layered silicates (either MMT or laponite) into a polymethyl methacrylate latex suspension produced in the presence of suitable cationic compounds (cationic initiator, monomer or surfactant) [136]. Since the latex particles were cationic and the clay platelets anionic, strong electrostatic forces were developed at the polymer/clay interface. 

Several approaches have been used to obtain molecularly imprinted particles with a controllable size and shape distribution. For example, suspension polymerization has been utilized to obtain monodispersed MIP particles of several microns diameter [18,19]. Additives are put into the suspension to act as pore inducers thereby increasing the specific surface area and efficiency of the material for chromatographic applications. 

The authors considered their description of a series of bulk and suspension polymerizations of vinyl chloride with various initiators as very satisfactory [17] with Q St 15. In spite of that, their approach has been criticized by Ugelstad who considers termination of growing radicals exclusively by mutual collisions to be very improbable [19], A large number of primary particles are formed in a short time interval, and many of these are then easily absorbed by the already solid flakes. Desorption of sorbed radicals has also to be considered. Radical distribution among the two phases should be controlled by these processes, especially at low conversion. 

The problem with applying correlations derived from other systems to emulsion polymerization is twofold. First, normal macroemulsion particles are said to be created with 30 to 40% monomer in them and so the unbiased (zero conversion) termination rate is unknown. Secondly, the diffusional limitations in particles might be quite different from those observed in bulk or suspension polymerizations. It is for these reasons that an empirical approach is suggested. 

FIGURE 9.2 Modeling a suspension polymerization reactor (approach of Vivaldo-Lima et o/ 1998). 

Polymerization of vinyl monomers by free-radical mechanisms is perhaps the most widely encountered and best understood mode of vinyl polymerization. The popularity of free-radical polymerization is due in substantial part to the many advantages that this route to polymers offers to industry. The polymerization process is noteworthy for its ease, convenience, and relative insensitivity to impurities, such as water and oxygen, that plague ionic polymerizations. Indeed, it is common to carry out free-radical polymerizations in water as a suspending medium, as in emulsion and suspension polymerization. Another advantage of free-radical polymerization is that it offers a convenient approach toward the design and synthesis of myriad specialty polymers for use in almost every area. 

For completeness, it should be mentioned here that Stille and co-workers " have presented two routes to immobilized DIOP ligands I04a and I04b by polymerization of suitably derivitized DIOP monomers with styrene and DVB (Scheme 51). In the first approach, the optically active styryl monomer 103, prepared from 102 and 4-styrylbenzaldehyde, was suspension polymerized with styrene and DVB using AIBN as initiator. The resulting... 

Using a quite different approach, polymeric beads of supported ionic liquid palladium catalysts comprised of polymerized ionic liquid monomers and palladium complexes have been synthesized using traditional suspension polymerization methods [86]. Here, polymeric ionic liquid beads were made from polymerization of l-butyl-3-vinylimidazolium bis(trifluoromethyl sulfonyl)imide and poly(vinylalcohol) by heating with AlBN (2,2 -azobis(2-methylpropionitrile)) in the presence of l,l -bis[l,8-octyl)-3-vinylimidazolium bis(trifluoromethyl sulfonyl)imide as cross-linker (Scheme 5.6-6). The ionic liquid support beads proved to be thermally stable up 250 °C which is significantly higher than conventional vinyl resins. 

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