Acrylonitrile, heats polymerization

Acrylonitrile will polymerize violendy in the absence of oxygen if initiated by heat, light, pressure, peroxide, or strong acids and bases. It is unstable in the presence of bromine, ammonia, amines, and copper or copper alloys. Neat acrylonitrile is generally stabilized against polymerization with trace levels of hydroquinone monomethyl ether and water. 

Suspension polymerization is frequently employed as the second stage following a preliminary bulk polymerization, such as in the manufacture of some HIPS and ABS polymers. Polybutadiene or another elastomer is dissolved in liquid styrene, and this monomer or a mixture of styrene and acrylonitrile is polymerized in a batch kettle. The syrup produeed at 30-35% conversion is too viscous for effective mixing and heat transfer. It is therefore dispersed in water, and the polymerization is finished as a suspension reaction. 

Conditions contributing to instability Acrylonitrile will polymerize when hot. and the additional heat liberated by the polymerization may cause containers to explode. Pure AN may self-polymerize, with a rapid build-up of pressure, resulting in an explosion hazard. Inhibitors are added to the commercial product to prevent self-polymerization. 

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Acrylonitrile-butadiene rubber (also called nitrile or nitrile butadiene rubber) was commercially available in 1936 under the name Buna-N. It was obtained by emulsion polymerization of acrylonitrile and butadiene. During World War II, NBR was used to replace natural rubber. After World War II, NBR was still used due to its excellent properties, such as high oil and plasticizer resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients. 

Incineration or heating to decomposition releases toxic nitrogen oxides (Sittig, 1985) and cyanides (Lewis, 1990). Wet oxidation of acrylonitrile at 320 °C yielded formic and acetic acids (Randall and Knopp, 1980). Polymerizes readily in the absence of oxygen or on exposure to visible light (Windholz et al., 1983). If acrylonitrile is not inhibited with methylhydroquinone, it may polymerize spontaneously or when heated in the presence of alkali (NIOSH, 1997). 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

Because of the highly exothermic nature of acrylonitrile polymerization, bulk processes arc not normally used commercially. Howevei. a commercially feasible process lor bulk polymerization in a continuous stirred lank reactor has been developed. The heat nl reaction is controlled hy operating at relatively low conversion levels and supplementing the normal jacket cooling with reflux condensation of umcaclcd monomer... 

Vinyl-type addition polymerization. Many olefins and diolefins polymerize under the influence of heat and light or in the presence of catalysts, such as free radicals, carbomum ions or carbamons. Free radicals are particularly efficient in starting polymerization of such important monomers as styrene, vinylchloride, vinylacetate, methylacrylate or acrylonitrile. The first step of this process—the so-called initiation step—consists in the thermal or photochemical dissociation of the catalyst, and results in the formation of two free radicals-. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

Bulk polymerization was also studied briefly (Table II), adapting a technique useful in production of impact styrene (I). Solid polybuta-diene was dissolved in styrene, mixed with acrylonitrile and/or methyl methacrylate, charged into a 12-ounce crown-cap bottle, flushed with nitrogen, and tumbled end-over-end at 42 rpm in a constant-temperature water bath for 16 hours at room temperature, 48 hours at 80 °C, and 48 hours at 90°C, then heated in an oven 24 hours at 110°C and 24 hours at 150°C. ABS polymerization produced a grainy, inhomogeneous, light-... 

Film Formation. Starch-g-PAN samples used for this study were prepared as described above, except that the small amounts of PAN homopolymer were not removed by DMF extraction. For the graft polymerization with gelatinized starch, the starch-water slurry was heated for 30 min at 85°C before reaction at room temperature with acrylonitrile and ceric ammonium nitrate. Saponifications in these experiments were carried out by stirring. 55 g of graft copolymer with 450 ml of 0.711 sodium hydroxide solution in a sigma mixer for 2 hr at 90-100°C. 

Application To produce a wide range of styrene acrylonitrile (SAN) copolymer with excellent chemical resistance, heat resistance and suitable property for compounding with ABS via the continuous bulk polymerization process using Toyo Engineering Corp. (TEC)/Mitsui Chemicals Inc. technology. 

Description Styrene monomer, acrylonitrile, a small amount of solvent and additives are fed to the specially designed reactor (1) where the polymerization of the fed mixture is carried out. The polymerization temperature of the reactor is carefully controlled at a constant level to maintain the desired conversion rate. The heat of the polymerization is easily removed by a specially designed heat-transfer system. At the exit of the reactor, the polymerization is essentially complete. 

Economically produced block copolymers containing acrylonitrile or methacrylo-nitrile as the principal component have been prepared that are heat resistant, weath-erable, and oil and flame resistant. These materials were prepared using reversible addition-fragmentation chain transfer polymerization. 

Miscellaneous Reactions. Sodium bisulfite adds to acetaldehyde to form a white crystalline addition compound, insoluble in ethyl alcohol and ether. This bisulfite addition compound is frequently used to isolate and purify acetaldehyde, which may be regenerated with dilute acid. Hydrocyanic acid adds to acetaldehyde in the presence of an alkali catalyst to form cyanohydrin the cyanohydrin may also be prepared from sodium cyanide and the bisulfite addition compound. Acrylonitrile [107-13-1] (qv) can be made from acetaldehyde and hydrocyanic acid by heating the cyanohydrin that is formed to 600—700°C (77). Alanine [302-72-7] can be prepared by the reaction of an ammonium salt and an alkali metal cyanide with acetaldehyde this is a general method for the preparation of a-amino acids called the Strecker amino acids synthesis. Giignard reagents add readily to acetaldehyde, the final product being a secondary alcohol. Thioacetaldehyde [2765-04-0] is formed by reaction of acetaldehyde with hydrogen sulfide thioacetaldehyde polymerizes readily to the trimer. 

Peng has studied the kinetics of the crosslinking process for HIPS in the absence of initiator [40]. In the absence of styrene and oxygen, polybutadiene cannot be crosslinked by heating alone. For HIPS most of the crosslinking takes place at high conversion. In the presence of acrylonitrile (ABS), the onset of crosslinking starts earlier in the polymerization. 

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