Non-aqueous dispersion polymerization

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

Fig. 8.6. Typical MCLCP non-aqueous dispersion polymerization synthetic...

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). 

The question whether the intramolecularly crosslinked microparticles of non-aqueous polymer dispersions are really microgels is also justified, considering non-aqueous dispersions prepared from acrylic copolymers and melamine/formaldehyde crosslinker with particle sizes of about 300 nm. [45, 343]. In any case, these crosslinked polymeric microparticles are useful constituents of high-solids coatings, imparting a yield stress to those solutions which probably involves attractive forces between the microparticles. 

Examples of dispersion polymerizations using macromonomers are summarized in Table 4. Non-aqueous dispersion (NAD) polymerization of polar... 

The materials we studied are non-aqueous dispersions of polymer particles. Colloidal stability of these particles in hydrocarbon solvents is conferred by a surface covering of a highly swollen polymer (the stabilizer) on a second polymer, insoluble in the medium (the core polymer), which comprises 90 % of the material (11). These particles are prepared by dispersion polymerization polymerization of a monomer soluble in the medium to yield an insoluble polymer, carried out in the presence of a soluble polymer which becomes the stabilizer. In the examples discussed here, the core polymer is formed by free radical polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium... 

The emulsion polymerization technique is a heterophase polymerization technique in which three phases can be distinguished the water phase, the latex particle phase and the monomer droplet phase (the latter is usually present during part of the polymerization reaction). The product of an emulsion polymerization is a latex a submicrometer dispersion of polymer particles in water. Non-aqueous dispersions of latex particles also exist. 

The emulsion can be made in either (a) water or (b) some other nonsolvent. Emulsions based on b) are called organosols or non-aqueous dispersions (NADs). If aqueous emulsions are evaporated to dryness without coalescence of the polymer particles, these may be redispersed in some organic non-solvent to form an organosol However, if the polymer dispersion is produced by direct polymerization of monomers in organic non-solvent, then the product is an NAD. Methods of making emulsions, both aqueous and non-aqueous, will now be considered. 

The resinous thickeners above are thermoplastic and all ultimately soluble in one solvent blend or another. As we have seen, their low shear effects can be destroyed by solvent additions. If, however, the polymers are made as latexes or non-aqueous dispersions by emulsion or dispersion polymerization (Chapter 11) and a small amount of cross-linking is introduced via polyfunc-tional monomer, then insoluble colloidal resin particles are produced. These are called microgels. 

Some interest has been shown in recent years in emulsion polymerizations which occur in systems having non-aqueous dispersion media. A typical example would be the polymerization of methyl methacrylate in a medium such as dodecane, in the presence of a suitable colloid stabilizer. Industrial interest in such reactions and their products has waned somewhat following the increases in world oil prices and increasing sensitivity to the possibility of environmental pollution. However, these reactions and their products remain of considerable academic interest. The subject of non-aqueous polymer dispersions has recently been reviewed by Barrett, following the appearance of a book on dispersion polymerization. ... 

Non-aqueous dispersions (NAD) in which the dispersing medium is not water but an organic solvent may also be made by emulsion polymerization. Although such systems combine the advantages of high solids contents and adjustable drying rates, they find limited commercial use. 

In non-aqueous pigment dispersions, proton transfer from the acid groups of a polymeric dispersant to the surface of basic particles is a process promoting effective stabilization ( ). 

Both polar and non-polar solvents may be used for dispersion polymerization. Whilst the earlier woric on dispersion pofymerization involved the use of petroleum distillates and other alii iatic hydrocarbons, most of the recent studies have used polar solvents such as etiianol and methanol [11,12]. There are few examples of true, radically initiated dispersion polymerizations in which water is tire sole solvent. In many cases using alcohol solvents, water is added in order to fine tune the solvency and thus manipulate the particle size, and size distribution, of the product. Some of the earlier reports on dispersion polymerization in aqueous media were strictly suspension polymnizations. In one of the few true examples of an aqueous system, Margel and l esel [13] polymerized acrolein in water in the presence of a polyglutaraldehyde stabilizer, using eith base... 

The use of water-containing miniemulsions (as described above) is Hmited to particular monomers for anionic polymerization. Since, in many cases, the initiator and active species are sensitive to water, the polymerization caimot be carried out in either water-in-oil or oil-in-water miniemulsions. The anionic polymerization of e-caprolactam in a non-aqueous miniemulsion was first reported in 2005 [103]. As the e-caprolactam monomer is hydrophilic, it was necessary to conduct the polymerization in an inverse miniemulsion. Unfortunately, the molten e-caprolactam-in-oil miniemulsions could not be stabilized efficiently, in contrast to dimethyl sulfoxide (DMSO)-in-oil miniemulsions. Therefore it was necessary to dissolve the e-caprolactam directly in DMSO, to form the dispersed phase. This synthetic strategy, which permitted the production of polyamide-6 nanoparticles, paved the way for a variety of water-sensitive reactions to be performed in miniemulsion nanodroplets, including the creation of hydrophihc polyurethane capsules and particles (see below). Subsequently, poly(e-caprolactone)... 

Fluoropolymers are typically synthesized in aqueous polymerization systems (both emulsion and suspension), non-aqueous systems (Freon-113), or Freon-113/aqueous hybrid systems [8]. Such processes require the use of large quantities of water and CFCs (for non-aqueous polymerizations) or fluorinated surfactants (for emulsion polymerization). Aqueous suspension and dispersion poly-... 

Latices can be prepared by emulsion polymerization (see also Section 20.6.5) or by the subsequent dispersing of polymer solutions or melts in water. These latices have very high solids contents up to 74% in the case of perfect spheres and up to 80% with imperfect spheres. In addition, aqueous dispersions can be economically produced. Water is a nontoxic and non-inflammable solvent. These advantages face the disadvantage that water can only be removed slowly and in a relatively uncontrolled manner and the retained water unfavorably influences the polymer properties. 

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