Acrylonitrile polymerization, polar solvents

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. 

Further experiments were therefore carried out with polar solvents which do not dissolve the polymer. Most striking results were obtained with trichloroacetic acid. The polymerization of acrylonitrile in this solvent was found to proceed under precipitating conditions at all concentrations. In spite of this, the conversion curves were perfectly linear in solutions containing 60 volume per cent monomer or less (18). Moreover, these systems exhibit marked post-polymerization showing the presence of long-lived radicals. 

Exercise 10-29 Propenenitrile (acrylonitrile, CH2=CHCN) will polymerize readily at —50° in a polar solvent [e.g., dimethylmethanamide, HCON(CH3)2] under the influence of sodium cyanide, NaCN, Show the initiation and propagation steps of this reaction, and predict the structure of the polymer. Why is a polar solvent necessary Why does this polymerization proceed but not that of propene under the same conditions ... 

This technique allows the formation of many different types of block copolymers. Lithium metal can be used to initiate polymerizations in solvents of varying polarity. Monomers, like styrene, a-methylstyrene, methyl methacrylate, butyl methacrylate, 2-vinylpyridine, 4 vinylpyridine, acrylonitrile, or methyl acrylate, can be used. The mechanism of initiation depends upon the formation of ion radicals through reactions of lithium with the double bonds ... 

TERP, SBRP, and BIRP are routinely carried out without solvent (bulk polymerization), but several solvents have also been used. Polar solvents, such as iV,N-dimethylformamide (DMF) and tetrahydrofuran (THF), were used for the polymerization of N-isopropylacrylamide (NfPAM), acrylonitrile (AN), N-vinyl carbazole (NVC), and acrylic acid (AA). °TERP of NIPAM was also carried out in a DMF/water mixture at 20 °C under condition C7 TERP has been also applied to emulsion polymerization in an aqueous dispersed media (see below). The results dearly indicate the high compatibility of these methods for polar functional groups and solvents. 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

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. 

Quinine-acrylonitrile copolymer. Cinchona alkaloids can be copolymerized with another vinyl monomer such as acrylonitrile with AIBN as initiator. The highest yield of polymer in the case of quinine is obtained when the quinine/acrylonitrile ratio is 1 20. This method was used to obtain a polymeric form of the alkaloid in which the crucial part of the molecule for asymmetric reactions—the amino alcohol unit—is free. The polymers are stable, light yellow solids, soluble in polar aprotic solvents (DMF and DMSO), but insoluble in common organic solvents. 

Monomers containing active hydrogen (e g., adds and hydroxy eompoimds) are not suitable for GTP, but aeiylates, acrylonitrile, and Af,Af-dimethyl aerylamide can be polarized quite readily. The reaction is very susceptible to impurities, and all reagents and solvents must be scrupulously dried. Although both low- and high-temperature polymerization reactions are possible, a range of 270 to 320 K is preferred. 

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