Precipitation from solution heterogeneous

Manufacture. PVBs are manufactured by a variety of two-stage heterogeneous processes. In one of these an alcohol solution of poly(vinyl acetate) and an acid catalyst are heated to 60—80°C with strong agitation. As the poly(vinyl alcohol) forms, it precipitates from solution (77). Ethyl acetate, the principle by-product, is stripped off and sold. The precipitated poly(vinyl alcohol) is washed to remove by-products and excess acid. The poly(vinyl alcohol) is then suspended in a mixture of ethyl alcohol, butyraldehyde, and mineral acid at temperatures above 70°C. As the reaction approaches completion the reactants go into solution. When the reaction is complete, the catalyst is neutralized and the PVB is precipitated from solution with water, washed, centrifuged, and dried. Resin from this process has very low residual vinyl acetate and very low levels of gel from intermolecular acetalization. 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

Using Mossbauer spectroscopy to monitor the formation of p-hematin under in vitro reaction conditions, Adams et al. have demonstrated that the reaction is a psuedo-zero-order process [109]. Such a process is consistent with a mechanism whereby a small concentration of heme is kept soluble via acetate, functioning as a phase-transfer catalyst, in a heme-saturated solution. In the rate limiting step, the soluble heme aggregates to P-hematin, which in turn grows until it precipitates from solution. There are clearly complicated heterogeneous reaction equilibria involved in the aqueous chemical formation of p-hematin. Consequently, it should be emphasized that the detailed mechanistic analysis of the complex solubilization of the species involved in the chemical synthesis... 

Condensation Methods Used for preparing colloidal dispersions in which either precipitation from solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. 

Precipitation from solution = precipitation polymerization Monomer Initiator Polymer So in M D or So in M D in M HOPE, PP, PMMA, PS, PVC, SAN Powder In all of these polymerizafirm Important example Low-pressure methods, the polymer is polymraization of HDPEP or produced in a finely in hydrocarbon (HC) solvent distributed, heterogeneous PMA, PMMA also in HC, PS form. Viscosity remains low, in methanol, PAN in water efficient heat dissipation, and therefore rapid... 

Talamini and Peggion [145] visualize the process as a modified heterogeneous solution polymerization. The monomer has an appreciable solubility in the aqueous phase. These authors estimate the solubility to be on the order of 0.5 moles per liter. (Presumably this is under the pressure conditions of a typical reactor. Our Table I gives the solubility as 0.1 or approximately 0.02 moles per liter at standard temperature and pressure.) Polymerization starts in the aqueous solution. The polymer that forms separates. The emulsifier in the solution protects the particle from coagulation. By imbibing monomer on the surface of the polymer particle, growth takes place until latex-sized particles form. When the surfactant is consumed by adsorption on these particles, radicals precipitate from solution onto existing particles. Then the number of particles remains constant, very much as in a conventional emulsion polymerization. The total surface area of the polymer particle appears to be involved in the polymer process. 

The values of these ratios change appreciably by passing from the heterogeneous (suspension) to the homogeneous (DMF) system. In the case of copolymerization in suspension in the presence of the K2S208—AgN03 oxidation-reduction system at 30—40 °C, the ratios were found to be ry = 0,77 0,2 and r2 = 1,09 0,04, whereas in the case of the copolymerization in solution they are = 0,52 and r2 = 1,7. The difference in these values seems to be the result of the different solubility of the monomers in water and of the different rate of diffusion of the monomers to the surface of the precipitated copolymer20. From this it follows that 4 is the more reactive monomer in this binary system. 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

In cases where one of two resultants can be separated from the reactants and the other resultant, by precipitation as a solid, by condensation as a liquid, or by volatilization as a gas or vapor, the yield of the desired substance from a given amount uf reactants can sometimes he materially increased. In the ease of heterogeneous systems, whenever a solid participant is present, the t tmt eniwlitm of said solid is considered constant. The precipitation and solution or solids are in this category, as well as the reactions between a gas and a solid, e.g.. the system ferrule nic oxide plus hydrogen gas plus iron plus water vapor. 

Thus, the thermal stabilization of PVC which resulted from the heterogeneous grafting of as little as 3-5% cis-1,4-polybutadiene was more than a simple additive effect and indicates a synergistic interaction. This was demonstrated further by dissolving up to 10% cis-1,4-polybuta-diene in a chlorobenzene suspension or solution of PVC and isolating the polymer blend by precipitation with methanol. Films pressed from the polymer blend were generally deeply colored and contained incompatible, probably gelled or crosslinked, areas. 

---