Polymerization heterogeneous phase

Polymerization of Spiropentane by Metal-Halide Complexes. Heterogeneous-phase polymerization with trialkylaluminum metal-halide... 

Heterogeneous reaction, 25 728-729 Heterogeneous reactors, 27 568 Heterogeneous stereospecific polymerization, 20 410-411 Heterogeneous vapor-phase fluorination, 22 863... 

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 metallorganic compounds (I, II) employed in presence of a heterogeneous phase containing an amorphous compound of a low-valency, strongly electropositive transition metal, generally polymerize a-olefins to amorphous polymers. In a similar fashion, the soluble reaction products of such metallorganic compounds with compounds of transition metals, chemisorbed on amorphous substrates, polymerize a-olefins to amorphous polymers 6, 9). 

Finally, in the fourth section the fundamentals of the modelling concerning two basic olefin polymerization processes are examined heterogeneous slurry polymerization and gas-phase polymerization. The SPERIPOL process for making High Impact PolyPropylene (HIPP) is then described as an illustrative example for combining fundamentals and elements of product and technology development. 

The rate of phenol degradation catalyzed by decatungstate in homogeneous phase and in heterogeneous phase (PVDF-W10 membrane) was similar however, when the catalyst is immobilized in the polymeric membranes a higher mineralization degree of the phenol was observed [42],... 

In addition to the studies in which supported catalysts are exclusively used for gas-phase polymerizations one study is available in which the supported catalyst is optimized in a solution process prior to its application in the gas phase. Tris-allyl-neodymium [Nd(/ 3- C3H5)-dioxane] which is a known catalyst in solution BD polymerization is heterogenized on various silica supports differing in specific surface area and pore volume. The catalyst is activated by MAO. In solution polymerization the best of the supported catalysts is 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. In addition to the polymerization in solution, the supported allyl Nd catalyst is applied for the gas-phase polymerization of BD [578,579] the performance of which is characterized by macroscopic consumption of gaseous BD and in-situ-analysis of BD insertion [580]. 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

Polymerization from the gaseous phase (disregarding dimer to tetramer formation) is an example of a heterogeneous reaction where the active centres are present in the condensed phase and the monomer in the gaseous phase. Polymerization does not, of course, proceed in the gaseous state but on the surface of the component carrying the active centres, i. e. also in the condensed phase. These polymerizations are of industrial importance. 

Heterogeneous systems comprising (a) heterogeneous bulk polymerizations, (b) heterogeneous solution polymerizations, (c) suspension systems, (d) emulsion systems, (e) dispersion polymerization, (0 gas phase polymerization, and (g) interfacial polymerizations. 

Any chemical reaction that yields polymeric material can be considered polymerization. However, polymerization in the conventional sense, i.e., yielding high enough molecular weight materials, does not occur in the low-pressure gas phase (without a heterogeneous catalyst). With a heterogeneous catalyst, polymerization is not a gas phase reaction. Therefore, the process of material deposition from luminous gas phase in the low-pressure domain might be better represented by the term luminous chemical vapor deposition (LCVD). Plasma polymerization and LCVD (terms explained in Chapter 2) are used synonymously in this book, and the former... 

Polymerization in gas phase must cope with larger entropy change than polymerization in liquid phase. Therefore, polymerization of gas phase monomers such as olefins is carried out in superatmospheric pressure and/or in the presence of heterogeneous catalyst. Polymerization in gas phase in low pressure (in vacuum) does not occur easily due to the limitation of the ceiling temperature of polymerization, and there are only few cases in which the deposition of polymeric material from gas phase starting material occurs in vacuum. Those main exceptional cases are plasma polymerization and parylene polymerization. 

Polymerization can be catalytic or noncatalytic, and can be homogeneously or heterogeneously catalyzed. Polymers that form from the liquid phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are formed and remain in suspension sometimes emulsions form. In some processes solid polymers precipitate from a fluidized gas phase. Polymerization processes are also characterized by extremes in temperature, viscosity, and reaction times. For instance, many industrial polymers are mixtures with molecular weights of 104 to 107. In polymerization of styrene the viscosity increased by a factor of 106 as conversion increased from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1800 K (3240°R). Initiators of the chain reactions have concentration as low as 10-8 g-moFL, so they are highly sensitive to small concentrations of poisons and impurities. 

Radical polymerization can be carried out under homogenous as well as heterogenous conditions. This difference is classified based on whether the initial mixture and/or final product are homogenous or heterogenous. Some homogenous mixtures become heterogenous as polymerization proceeds as a result of insolubility of the resulting polymer in the reaction media. There are many other specialized processes that are used to synthesize materials via free-radical polymerization. These include interfacial polymerization, gas phase reactions ( popcorn polymerization ), as well as the use of specialized media like supercritical fluids. Current research efforts include the study of such... 

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