Polymerization heterogeneous solution

When a monomer such as acrylonitrile is polymerized in a poor solvent, macroradicals precipitate as they are formed. Since these are living polymers, polymerization continues as more acrylonitrile diffuses into the precipitated particles. This heterogeneous solution polymerization has been called precipitation polymerization. 

Relatively stable macroradicals have also been obtained by the polymerization of vinyl chloride (15) or by the copolymerization of this monomer with vinyl acetate (32) in poor solvents—i.e., by heterogeneous solution polymerization. Appropriate solvents for this type polymeriza-... 

No product was obtained when attempts were made to copolymerize styrene and maleic anhydride in benzene at 50° C in the absence of bis-azoisobutyronitrile. Likewise, no free radicals were detectable when these solutions were examined using EPR techniques. Negative results were also noted in solutions of the alternating copolymer prepared in acetone. However, the presence of free radicals was noted when the alternating copolymer produced by heterogeneous solution polymerization in benzene was examined. This peak was observed with freshly prepared and aged copolymer samples that had been stored in an inert environment. However, no peak was observed in product that had been washed with methanol. 

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. 

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. 

Ultra-high-molecular-weight PVK is obtained by the heterogeneous solution polymerization in methanol/tert-butyl alcohol with a low-temperature free radical initiator, such as 2,2 -azobis-(2,4-dimethylvaleronitrile) (ADMVN). In this solvent system, the polymerization rate of NVK, is in a nearly proportional concentration of ADMVN, thus suggesting a heterogeneous nature for the polymerization. ... 

W. S. Lyoo. Synthesis of ultrahigh molecular weight poly(V-vinylcarbaz-ole) with a high yield using low-temperature heterogeneous-solution polymerization. J. Polym. ScL, Part A Polym. Chem., 39(4) 539-545, February 2001. 

The above examples are all heterogeneous solution polymerizations. Heterogeneous solution polymerization occurs in most cases when the Hildebrand solubility parameter values of the solvent and polymer differ by at least 1.8h(122). The presence of macroradicals in these precipitated polymers has been demonstrated by electron spin resonance (esr). 

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 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). 

Heterogeneous catalysts have a tendency to cause gel formation in the solution polymerization of dienes. This adversely affects the polymerization and the quality of the end products. 

If an inert good solvent is used in solution polymerization, the gel thus obtained will have a supercoiled (expanded) structure (Gel B). Gel B swells in good solvents much more than Gel A which is synthesized in bulk. If the amount of the crosslinking divinyl monomer in the reaction mixture is increased while the amount of solvent remains constant, highly crosslinked networks are formed that cannot absorb all solvent molecules present in the reaction mixture and a heterogeneous structure results (Gel C). A part of the solvent separates from the gel phase during polymerization and the formed Gel C consists of two continuous phases, a gel and a solvent phase. If the amount of solvent is further increased, a... 

Monomers may also be polymerized in solution using good or poor solvents for homogeneous and heterogeneous systems, respectively. In solution polymerizations, solvents with low chain transfer constants are used to minimize reduction in chain length. 

Tagata and Homma47 analyzed in the aforementioned manner the compositional heterogeneity of two typical commercial SBR samples, E-SBR and S-SBR, which had 23.5 and 20.0 average styrene wt.%, and were produced by an emulsion polymerization and by a solution polymerization with an organometallic catalyst, respectively. The result was that the former gave a distinctly narrower compositional distribution than the latter. GPC experiments on these samples were also carried out, which indicated that the above situation was just the opposite for the molecular-weight distributions. 

Polymerization activity of proteinoid or polynucleotide-phosphorylase has been compared in ADP solution at pH 8.5 48). The results show that the activity of the neutral proteinoid is approximately 20 times lower than that of the enzyme, and the lower molecular-weight fraction of the proteinoid has negligible activity. The polymerization by polynucleotide-phosphorylase is increased approximately three times when Leuchs polylysine is supplied, polylysine and enzyme yield a heterogeneous solution 48). 

Polymer in solution Any (e.g., a condensation product or another polymer phase) Heterogeneous bulk or solution polymerization Salt precipitating from a condensation reaction. Prepolymerized rubber precipitating from a solution of polystyrene in styrene monomer... 

Tris-allyl-neodymium Nd(//3-C3I Ishdioxane which performs as a single site catalyst in solution polymerization was heterogenized on various silica supports which differed in specific surface area and pore volume. The catalyst was activated by MAO. In the solution polymerization the best of the supported catalysts was 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. 

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]. 

These results may be compared with viscosities obtained in a similar way from conifer bark extracts which, while heterogeneous, contain polymeric pro-cyanidins or mixed polymeric procyanidins and prodelphinidins as their predominant components (2). For example, Weissman (25) reported a viscosity of 65 mPa-s for a 30% solution of the water extract from Pinus oocarpa bark, and Dix and Marutsky (26) obtained a value of 31 mPa-s for a similar solution from Picea abies bark. These viscosities are similar to those observed for the 30% procyanidin polymer solutions. They indicate that the viscosities of these bark extract solutions are dominated by the proanthocyanidins and that there is little influence from any accompanying polysaccharides-as already suggested by Weissmann (25)-in contrast to wattle extracts where gums play an important role in determining solution viscosities (7). 

Solution polymerization. Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are soluble. Monomers are polymerized in a solution that can be homogeneous or heterogeneous. Many free radical polymerizations are conducted in solution. Ionic polymerizations are almost exclusively solution processes along with many Ziegler-Natta polymerizations. Important water-soluble polymers that can be prepared in aqueous solution include poly(acrylic acid), polyacrylamide, poly(vinyl alcohol), and poly(iV-vinylpyrrolidinone). Poly(methyl methacrylate), polystyrene, polybutadiene, poly(vinyl chloride), and poly(vinylidene fluoride) can be polymerized in organic solvents. 

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