Homogeneous dispersion polymerization particle size

To achieve dependable performance, it is important to consider the morphology and structure of the catalyst granule, in the addition to the nature of the active centers and the polymerization kinetics. Proper combination of these factors will allow for a porous structure consisting of crystal and properly-sized, homogeneously dispersed, primary particles within the growing polymer granule, allowing the monomer equal access to the active centers (Fig. 1). 

In the previous chapter, preparation process and microstructure investigation of a novel C/C-ZrB -ZrC-SiC composite were reported, in which polymeric precursors of ZrB2-ZrC were used to fabricate an ultra-high temperature ceramics (UHTCs) matrix for the first time but with the traditional polymeric precursor infiltration and pyrolysis (PIP) technique. The formed ZrB -ZrC-SiC complex matrix exhibits homogeneous dispersion of nano-sized ZrB and ZrC particles in continuous SiC ceramic all with particle dimensions less than 200 nm. This kind of composite... 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

The method of preparing superparamagnetic particles developed by Charmot [109] uses hydrophobic non-porous polystyrene seed particles of narrow size distribution. A seeded polymerization is carried out to increase the particle size (1.35 pm) and a terpolymer is formed around the seed particles by a dispersion polymerization of styrene, DVB and 4-vinylpyridine in toluene. The toluene containing cobalt precursor swells the latex particles, which results in a homogeneous distribution of the metal precursor. A thermolysis reaction is conducted in the presence of 4-vinylpyridine, and the release of carbon monoxide indicates the decomposition of the metal salt into cobalt. The main problem of this method is the particle surface deformation during the evolution of carbon monoxide. The amount of crosslinker, however, cannot be reduced below a certain level without significantly modifying the properties of the particles. 

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