Heterogeneous media polymerization

The authors [1] studied kinetics of poly (amic acid) (PAA) solid phase imidization in the presence of nanofiller (Na+-montmorillonite) and in its absence. It was found out, that the kinetic curves conversion (imidization) degree Q versus reaction duration t were have typical for polymerization reactions shape with autodeceleration showing imidization rate reduction as time is passing. As it is known [2], such curves Q(t) are specific for reaction passing in heterogeneous medium and are described by the simple relationship ... 

Supercritical carbon dioxide has been used as a dispersing medium for the manufacture and processing of polymeric materials. The process allows for the synthesis of high molar mass acrylic polymers in the form of micrometer-sized particles with a narrow size distribution. This procedure represents an environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations (Fox, 1994). 

Explanation may be related to the heterogeneity of the reaction medium polymerization location is mostly limited to within the polymer particles, so the copolymer composition is dependent on the composition of the monomer mixture inside the particles, which may be different from that of the whole reactor, monitored by the apparatus. So, compositions of various phases, monomer droplets, polymer particles, aqueous phase, were measured using gas chromatographic (GC) analysis, after they were separated by ultracentrifugation (33,000 rpm). 

Monomers, such as styrene which are good solvents for their polymers do not retard the bulk polymerization rate. However, this rate does not increase in a viscous good solvent medium that is present toward the end of the polymerization. Heterogeneous solution polymerization in nonvlscous poor solvents (1 ) and in viscous poor solvents is faster than rates observed in good solvents. 

Polyoxymethylene is prepared by the polymerization of anhydrous formaldehyde (see Example 3.22) or of 1,3,5-trioxane (see Example 3.24). Paraformaldehyde, obtained by polycondensation of formaldehyde hydrate, cannot be acetylated in heterogeneous medium. Acetic anhydride and fV,fV-dimethylcyclohexylamine are carefully fractionated and sodium acetate is dehydrated by heating. 

Further in this chapter we will discuss polymerization processes, specifically processes performed in a heterogeneous medium, including suspension, emulsion, and mini emulsion. We shall discuss their preeminent properties, such as the type of nucleating particle, a greater efficiency in the heat exchange, inorganic fillers and their features, and the addition of special characteristics of each process and the analytical methods used in the characterization of polymer/inorganic hybrid. 

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,... 

Mini emulsion polymerization processes have multiple advantages apparent in reactions conducted in a heterogeneous medium, but they also maintain distinct characteristics observed in classic polymerization processes, such as perceived in emulsion and suspension. The main difference is evident during the nucleation particle process, which does not require the presence of micelles in the medium. This is a result of the nucleation process, which starts directly in the reactive species (monomer droplets) and is dispersed throughout the continuous phase. Hence, after the polymerization reaction, this process yields a final product comprising very stable polymer particles with sizes that range from 50 to 500 nm [25-27],... 

Polymerization processes are carried out in bulk or in a solvent with or without added stabilizers, and may proceed under homogeneous or heterogeneous conditions. Polymerization in the presence of a solvent medium offers processing advantages in terms of lowered viscosity and elimination of localized heating that may lead to adverse polymerization rates. But the presence of traditional solvents introduces environmental... 

An effective medium approximation (EMA) has been applied to determine the optical constant of amorphous carbon where the entire film is considered as a composite heterogeneous medium of amorphous diamond-like carbon, amorphous graphite polymeric carbon, and voids. The EMA expression for the dielectric function e, of the composite medium can be expressed as... 

Yanovskii, Yu. G., Bashorov, M. T., Kozlov, G. V., Kamet, Yu. N. (2012). Polymeric Mediums as Natural Nanocomposites Intercomponont Interactions Geometry. Proceedings of All-Russian Conf. Mechanics and Nanomechanics of Structurally-Complex and Heterogeneous Mediums Achievements, Problems, Perspectives . Moscow, IPROM, 110-117. 

However, and without any doubt, the in situ monomer polymerization in the presence of pre-prepared magnetic nanoparticles is the most common strategy to prepare nanocomposites based on conducting polymers. In this sense the polymerization can be carried out in either homogeneous medium or heterogeneous medium, i.e., in emulsion using surfactants, providing different types of nanocomposites, from films to core-shell particles. 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70— 100°C and pressures of 0.1—2 MPa (15—300 psi). The polymerization reactions are carried out ia an iaert Hquid medium (eg, hexane, isobutane) or ia the gas phase. Molecular weights of LLDPE resias are coatroUed by usiag hydrogea as a chain-transfer ageat. 

In practice, the taffy process is generally employed for only medium molecular-weight resins (1) (n = 1-4). The polymerization reaction results in a highly viscous product (emulsion of water and resin) and the condensation reaction becomes dependent on agitation. At the completion of the reaction, the heterogeneous mixture consists of an alkaline brine solution and a water—resin emulsion and recovery of the product is accompHshed by separation of phases, washing of the taffy resin with water, and removal of water under vacuum. 

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

The heterogeneity of the reaction medium is also important in determining the molecular weight and in solution polymerization of maeromonomers. The magnitude of the effect varies according to the solvent quality. PS macromonomer chains in good solvents (e.g. toluene) have au extended conformation whereas in poor solvents (e.g. melhylcyclohexane) chains are tightly coiled.89 As a consequence, the radical center may see ail environment that is medium dependent (see also Sections 7.6.5 and 8.3.7). 

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