Polymerization in gas phase

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. 

Later the growth model developed for a neodymium catalyst system was applied for the butadiene polymerization in gas phase. The ideas concerning the initial polymerization stages are in agreement with a core—shell model .The subsequent polymerization stages correspond to the polymeric flow model . 

Kinetics of Ethylene Polymerization in Gas-Phase and Slurry Reactors 45... 

AH technologies employed for catalytic polymerization processes in general are widely used for the manufacture of HDPE. The two most often used technologies are slurry polymerization and gas-phase polymerization. Catalysts are usuaHy fine-tuned for a particular process. 

The Phillips-type catalyst can be used in solution polymerization, slurry polymerization, and gas-phase polymerization to produce both high density polyethylene homopolymers and copolymers with olefins such as 1-butene and 1-hexene. The less crystalline copolymers satisfy needs for materials with more suitable properties for certain uses that require greater toughness and flexibiUty, especially at low temperatures. 

Propylene Polymerization Kinetics in Gas Phase Reactors Usii Titanium Trichloride Catalyst... 

ATOL [Atochem polymerization] A gas-phase process for making polyethylene. Developed by Atochem and first commercialized in 1991. It uses a Ziegler-Natta catalyst containing titanium and magnesium halides. First commercialized at Gonfreville, France, in 1991. 

Exxpol [Exxon polymerization] A gas-phase process for making polyethylene from ethylene. The process uses single-site catalysis (SSC), based on a zirconium metallocene catalyst. Developed by Exxon Chemical Company in 1990 with plans to be commercialized in 1994. 

Charge Exchange on the Surface of Discharge Electrode. In general, the polymerization process in plasma may be divided into three processes, i. e. the ionization of monomer, the transportation of active particles and polymerization. In a certain discharge condition, the polymerization was supposed to occur in gas phase and powder like polymers were obtained. In our experimental condition, no powder was obtained. Therefore, the polymerization must be initiated on the substrate. 

Formula HfCR MW 320.30 tetrahedral and mononuclear structure in gas phase, halide bridging polymeric structure in solid phase. 

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 potential use of the reaction was examined, but the episulfides decompose partially during the process, and the yield of cyclohexene episulfide under optimum conditions was 20%. Since episulfides may be obtained in much higher yields from the photodecomposition of COS to give triplet sulfur atoms in gas-phase reactions,26 the method hardly has synthetic importance. It was assumed that the low yield is due to deactivation of the sulfur atoms by collision with solvent molecules, causing them to polymerize to molecular sulfur instead of reacting with cyclohexene.22... 

Experimental trends in Si shielding observed experimentally arise from variations in the coordination number (i.e. the number of atoms in the 1st coordination sphere), the extent of polymerization of the silicate tetrahedra, the degree of replacement of one net-work forming cation by another (e.g. coupled Na+, Al+3 for Si+4 substitution), the size of the rings of tetrahedra present and the Si-O-Si angles (1,2). Similar trends are seen in gas-phase molecules, species in aqueous solution and in both crystalline and amorphous solids. Polarized double-zeta basis set Hartree-Fock level calculations using small molecular cluster models reproduce these trends semiquantitatively, as we will show. 

The success of the modified patterns treatment shows that radical reactivities in the gas phase are governed to a major extent by polar forces as given quantitative expression in the Hammett equation. These correlations support the conclusions reached in earlier sections that both the polarity of the alkene and the polar character of the radical are important. They also help to establish a common pattern of behaviour for radicals in gas-phase addition reactions and in liquid-phase polymerization processes. 

Reports on steady increases of polymerization rates with increasing polymerization temperatures usually refer to an upper limit of polymerization temperature of around 60 °C. At temperatures > 60 °C catalyst deactivation becomes more prominent and overall catalyst activities decrease. There are two reports which point in this direction. A decrease of catalyst activities at elevated temperatures was observed for NdV/DIBAH/fBuCl [455] and for NdN/TIBA/EASC [388]. Pires et al. studied the solution polymerization of BD whereas Ni et al. studied the polymerization of BD in the gas phase. The rate maximum observed by Pires et al. was at 80 °C whereas the reaction maximum in the gas-phase polymerization was at 50 °C. The reduction of polymerization rates at elevated temperatures can be explained by the decay of the number of active species. In gas-phase polymerization deactivation becomes evident at lower temperatures (50 °C) compared to the solution pro-... 

As in Nd-catalyzed solution processes in gas-phase polymerization of BD regulation of molar mass is a serious problem as there are no agents for the control of molar mass readily available. Vinyl chloride and toluene are no viable options. Vinyl chloride is ruled out due to ecological reasons and toluene is not applicable due to low transfer efficiencies and the required low concentrations if applied in a gas-phase process. For the control of molar mass and MMD in the polymerization of dienes a combination of different methods is recommended [457,458] (1) temperature of polymerization, (2) partial pressure of BD, (3) concentration of cocatalyst (or molar ratio of Al/MNd)> (4) type of cocatalyst, (5) residence time of the rare earth catalyst in the polymerization reactor. 

In a scientific paper on the control of molar mass in gas-phase polymerization, the importance of Ai/ Nd is also emphasized, hi addition, a combination of the cocatalysts TIBA and DIBAH is recommended. By the use of two aluminum alkyl compounds the concentration ratio of two different active Nd-species is adjusted. As these two species produce different molar masses and MMDs the combination of TIBA and DIBAH allows for the control of these two parameters [229,230]. 

Polymerization frequently is performed in gas-phase reactors at intermediate pressures. The role of heterogeneous catalysts and the interaction between reaction kinetics and mass transfer can only be understood if sorption effects, solubilities of gases in solids, volume changes, and diffusivities at reactor conditions are known. 

The way in which a plasma polymer is formed has been explained by the rapid step growth polymerization mechanism, which is depicted in Figure 5.3. The essential elementary reactions are stepwise recombination of reactive species (free radicals) and stepwise addition of or intrusion via hydrogen abstraction by impinging free radicals. It is important to recognize that these elementary reactions are essentially oligomerization reactions, which do not form polymers by themselves on each cycle. In order to form a polymeric deposition, a certain number of steps (cycle) must be repeated in gas phase and more importantly at the surface. The number of steps is collectively termed the kinetic pathlength. 

The ceiling temperature T can be considered the upper temperature at which a pyrolytic process will reach equilibrium. It may be seen, therefore, as a recommended temperature for pyrolysis. However, in practice, the application for macromolecules of the above relations is not straightforward. The theory was developed for ideal systems (sometimes in gas phase), and although in principle this theory should hold true for any system, its application to condensed phases or polymeric materials may be accompanied by effects difficult to account for (phase change, melting, cage effect [2], etc.). The reaction rate could also be low at calculated Tq values. For this reason, temperatures 50° C or 100° C higher than Tq must frequently be used as practical values of the temperature used in pyrolysis. 

Formation of polymers is governed by thermodynamic and kinetic factors (see also Section 2.3). The free enthalpy of polymerization is an important parameter, which is known for various monomers. Tables with values for AH° and AS are given in literature [4]. Several values for AH for the formation of some polymers having in the backbone chain only carbon atoms are given in Table 2.2.1 (1 cal = 4.1868 J international, 1 cal = 4.184 J thermochemical). Some of the values are given for ideal gas phase, although few monomers and no polymers are in gas phase. Since in a reaction the reactant and the product can be in different aggregation states, the state of both participants must be indicated. 

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