Heterophase polymerization processes

Fig. 13 Different stages of the heterophase polymerization process (a) homogeneous distribution of NPs in a reaction mixture containing monomers, crosslinker and initiator before polymerization (b) early stages of the nucleation process indicating incorporation of NPs into the growing polymer particles and (c) final NP-containing microgel particles...

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. 

Background Knowledge of Heterophase Polymerization Processes 191 Table 4.1 Principal industrial applications of polymer latexes. 

FIGURE 21.12 Schematic representation of heterophase polymerization processes (a) Emulsion polymerization, (b) nano-emulsion polymerization, and (c) microemulsion polymerization. (From Antonietti, M. and Landfester, K., Prog. Polym. ScL, 2002, 27, 689-757. With permission.)... 

Heterophase polymerization in general is a current trend in polymer science as it allows solvent-free polymer synthesis polymer powders are obtained when the dispersion agent is removed. DeSimone improved this procedure significantly by performing the heterophase polymerization in supercritical C02, which simplifies the synthesis of powders with excellent handling of the polymerization and evaporation process.7 9... 

As compared to classical inverse heterophase polymerization techniques such as polymerization in inverse micro emulsions [47] or dispersion polymerization [75,76], polymerization of inverse miniemulsions is favored by the very efficient use of surfactant and the copying process from the droplets to the par-... 

Characteristic kinetic and morphological features of vinyl chloride radical polymerization processes were reviewed in89. While developing a mathematical model for the polymerization of this monomer, Canadian90 and Soviet91 investigators concentrated their attention on the heterophase nature of the process. In both cases the dependence of kinetic constants on the viscosity of the medium was disregarded. 

Investigations of water-in-oil polymerizations employing new monomers or emulsifiers for which kinetic or colloidal characterization is incomplete, require careful nomenclature designation. Under such circumstances a general description such as Water-in-Oil Polymerization or Heterophase Polymerization is recommended until the physical and chemical nature of the polymerizations can be identified. The designations inverse-suspension, inverse-emulsion and inverse-microemulsion should be reserved for processes for which a relatively advanced level of understanding exists. 

Novel applications in science and technology require highly efficient and, if possible, environmentally friendly methods and techniques for the generation of functional nanocomposite materials. Serving the environmental aspect, water-based formulation techniques that avoid the use of organic solvents are the focus of attention. Besides the well-known water-based emulsion and mlcroemul-sion polymerization processes, the miniemulsion polymerization technique is a highly versatile heterophase system that is suitable for the preparation of complex nanoparticles. 

ICP = blends of isotactic propylene homopolymer with ethylene-propylene rubber. These materials are commonly called "impact copolymers," "heterophasic copolymers," or, incorrectly, "block copolymers." These are typically prepared during the polymerization process using a series of reactors. L = low rubber (less than about 15% rubber by weight typically witli an ethylene content of less than about 10%). H = high rubber content blends (greater than about 15% rubber by weight typically with an ethylene content of at least 7%). 

Another possibility for altering a is a complete change of the dispersion medium. Besides water other classes of suitable liquids are alkanes or perfluorocar-bon fluids. By using such materials a drastic decrease in / can be achieved (cf. Table 8.4) when compared to water (/w = 72.8 mN m ). Indeed, it is the state of the art (also in larger-scale technical processes) to use certain petroleum fractions as continuous phases for the polymerization of hydrophilic monomers (so-called inverse heterophase polymerizations) to prepare, for instance flocculants for waste-water treatment or as aids for paper production (14). The large-scale application of perfluorocarbon fluids, however, is restricted... 

Suspension polymerization processes (Fig. 8.1) are characterized as being conducted in a heterophase medium, in which the reactive phase (monomer) is insoluble in a continuous phase, typically water. Its proper dispersion in the aqueous medium is guaranteed through a twofold mechanism. The first being agitation, which is generated by a mechanical impeller. In addition to mechanical agitation. 

Classical emulsion polymerization processes are conducted in a heterogeneous medium, but they have some characteristics that differentiate them from other processes. As a result, their unique characteristics may be exploited for the production of specific materials. There are specific requirements that must be met for this process to occur in a heterophase reaction system. First, the reactive organic phase (monomer) must be almost completely insoluble in the continuous phase (water). Subsequently,... 

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