Precipitation and Dispersion Polymerizations

In precipitation polymerization, the reaction mixture is initially homogeneous, as in solution polymerization, but it is a precipitant for the polymer. Thus, the initially formed macromolecules collapse and coagulate to create particle nuclei, which gradually flocculate into irregularly shaped and polydisperse particles. Such a process concerns for instance the synthesis of polytetrafluoroethylene in water or polyacrylonitrile in bulk. 

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Table 11.2.1 Comparison Between Precipitation and Dispersion Polymerizations Defined by Arshady...

Investigations of the polymerization kinetics of aniline and its relatives (as opposed to the oxidation kinetics observed in the initial electrooxidation step) are few. Most studies reported so far are devoted to morphological aspects, nucleation [285-288], nucleation dimensionality, and related features. Preferably, electrochemical (i.e., traditional) methods of investigation were employed. The first application of proton resonance spectroscopy to the investigation of polymerization kinetics of PANI has been reported [289]. Only chemical oxidation (precipitation and dispersion polymerization) was employed the spectroscopy was used just to monitor the concentration of the monomer in the solution phase. Various oxidizing compounds of different effectiveness were studied. An investigation of the chemical oxidation with... 

Dispersion polymerization may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerization. In dispersion emd precipitation polymerization, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerization the initiator is chosen to be soluble in the disperse phase of the monomer. The rate of dispersion polymerization is much faster than precipitation or solution polymerization. The enhancement of the rate in precipitation polymerization over solution polymerization has been attributed to the hindered termination of the growing polymer radicals. 

Spherical beads possess better hydrodynamic and diffusion properties than irregularly shaped particles. It is, hence, desirable to apply MIPs in a spherical bead format, especially for flow-through applications. Methods to synthesize spherical polymer beads are often classified according to the initial state of the polymerization mixture (i) homogeneous (i.e. precipitation polymerization and dispersion polymerization) or (ii) heterogeneous (i.e. emulsion polymerization and suspension polymerization). In addition, several other techniques have been applied for the preparation of spherical MIP beads. The techniques of two-step swelling polymerization, core-shell polymerization, and synthesis of composite beads will be detailed here. 

In-situ processes such as emulsion, suspension, precipitation or dispersion polymerization and interfacial polycondensations are the most important chemical techniques used for microencapsulation [85-90]. An image of microcapsules with an aqueous core and silicone shell prepared using in-situ polymerization is shown in Figure 1.10. 

The radicals from polymer and monomer are generated by two different mechanisms. The monomer molecules are dissociated by the high temperatures inside the hot-spot, whereas the polymer chains are fractured by the high strain rates outside the bubble [141]. The majority of the radicals in an ultrasound-induced polymerization reaction originate from the polymer chains [142] see Figure 21.12. It has to be noted that the radicals are only formed in the immediate vicinity of the ultrasound source where cavitation occurs. Subsequently, these radicals are dispersed throughout the reactor. In the literature several types of ultrasound-induced polymerizations have been reported, namely bulk, precipitation, and emulsion polymerization. 

Monosized polystyrene particles in the size range of 2-10 /am have been obtained by dispersion polymerization of styrene in polar solvents such as ethyl alcohol or mixtures of alcohol with water in the presence of a suitable steric stabilizer (59-62). Dispersion polymerization may be looked upon as a special type of precipitation polymerization and was originally meant to be an alternative to emulsion polymerization. The components of a dispersion polymerization include monomers, initiator, steric stabilizer, and the dispersion medium... 

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

In 1994, we reported the dispersion polymerization of MM A in supercritical C02 [103]. This work represents the first successful dispersion polymerization of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, we took advantage of the amphiphilic nature of the homopolymer PFOA to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymerization (> 3000) in supercritical C02. These polymerizations were conducted in C02 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN were employed as the initiators. The results from the AIBN initiated polymerizations are shown in Table 3. The spherical polymer particles which resulted from these dispersion polymerizations were isolated by simply venting the C02 from the reaction mixture. Scanning electron microscopy showed that the product consisted of spheres in the pm size range with a narrow particle size distribution (see Fig. 7). In contrast, reactions which were performed in the absence of PFOA resulted in relatively low conversion and molar masses. Moreover, the polymer which resulted from these precipitation... 

Vinyl ethers constitute a third class of monomers which have been cationically polymerized in C02. While fluorinated vinyl ether monomers such as those described in Sect. 2.1.2 can be polymerized homogeneously in C02 because of the high solubility of the resulting amorphous fluoropolymers, the polymerization of hydrocarbon vinyl ethers in C02 results in the formation of C02-insoluble polymers which precipitate from the reaction medium. The work in this area reported to date in the literature includes precipitation polymerizations and does not yet include the use of stabilizing moieties such as those described in the earlier sections on dispersion and emulsion polymerizations (Sect. 3). 

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