Acrylonitrile polymerization solvent effects

Solvents affect free-radical polymerization reactions in a number of different ways. Solvent can influence any of the elementary steps in the chain reaction process either chemically or physically. Some of these solvent effects are substantial, for instance, the influence of solvents on the gel effect and on the polymerization of acidic or basic monomers. In the specific case of copolymerization then solvents can influence transfer and propagation reactions via a number of different mechanisms. For some systems, such as styrene-acrylonitrile or styrene-maleic anhydride, the selection of an appropriate copolymerization model is still a matter of contention and it is likely that complicated copolymerization models, incorporating a number of different phenomena, are required to explain all experimental data. In any case, it does not appear that a single solvent effects model is capable of explaining the effect of solvents in all copolymerization systems, and model discrimination should thus be performed on a case-by-case basis. 

Solvent effects were also noted in the 1,3-cyclooctadiene-MA-acry-lonitrile polymerizations.The acrylonitrile content of the polymer was dependent on the solvent employed, showing increasing incorporation in the order 2-butanone > benzene > chloroform. However, the ratio of 1,3-cyclo-octadiene to MA remained 1 1 and did not depend on acrylonitrile content. The donor tendency of the solvents, specific monomer-solvent interactions, etc., could also have caused variation of acrylonitrile in the polymers. 

Evidence for radical formation was given by acrylonitrile polymerization, and hydrogen-bonding and solvent effects are considered important in the formulation of the transition-state complexes. In the reaction of olefins with aliphatic ketones in the presence of oxidizing agents e.g. Ce, Cu )... 

SAN resins themselves appear to pose few health problems in that they have been approved by the EDA for beverage botde use (149). The main concern is that of toxic residuals, eg, acrylonitrile, styrene, or other polymerization components such as emulsifiers, stabilizers, or solvents. Each component must be treated individually for toxic effects and safe exposure level. 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

Polymerization of acrylonitrile in polar solvents. Polar solvents are expected to interfere with the association of nitrile groups in pairs and to replace the nitrile-nitrile association complex by a nitrile-solvent association. Under such conditions structures such as IV should no longer arise and the "matrix effect" should disappear. 

However, DMF is a solvent for polyacrylonitrile and the polymerization occurs in a homogeneous medium for solutions containing 30 per cent monomer or less. This reduces the value of these experiments as an argument to show the influence of a matrix effect. Indeed the fact that auto-acceleration disappears when DMF is added to acrylonitrile was considered as a proof for the fact that precipitation of the polymer was the cause of autoacceleration. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

The trapped radicals stabilized by the gel effect can be used as post-polymerization initiators. During the post-polymerization of MMA in the presence of the residual MMA, enriched with MMA or acrylonitrile 1511 (Tables 3.6 to 3.8) a threshold in the polymer/solvent (alcohol — water — monomer) ratio has been observed, under which... 

And yet the polymerization rate of acrylonitrile in alcohols as solvent is comparable to that in dimethylformamide One has to conclude that the mere fact of this type of interaction alone can certainly not account for the water effect in AN polymerization. 

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