Acrylonitrile, polymerisation

The previous examples regarding nitrile-alcohols corresponded to this classification. However, since their danger is related to acrylonitrile polymerisation they were classified in the previous paragraph. The three examples below exclude this type of interpretation. 

Castaneda Hernandez, H. R. et al Chem. Abs., 1987,107, 78274 Acrylonitrile polymerises violently in contact with strong bases, whether stabilised or unstabilised [1]. Alkaline hydrolysis of acrylonitrile is exothermic and violent, especially when the temperature is above 60° C, the pressure is above atmospheric, and when heating at 60°C is prolonged above 10 mins. Polymerisation does not induce a violent reaction at 40-50°C at a concentration of 3% of sodium hydroxide in water [2],... 

Werner JB, Carter JT. 1981. Mortality of United Kingdom acrylonitrile polymerisation workers. Br J Ind Med 38 247-253. 

Imperial Chemical Industries Ltd. Acrylonitrile Polymerisation Products, Brit. Pat. 715,194 (September 8, 1954). Polymerisation Process, Brit. Pat. 733,093 (July 6, 1955). 

Significantly endothermic AHf (1) 147 kJ/mole 2.8 kJ/g. The monomer is sensitive to light, and even when inhibited (with aqueous ammonia) it will polymerise exother-mally at above 200°C [1]. It must never be stored uninhibited, or adjacent to acids or bases [2]. Polymerisation of the monomer in a sealed tube in an oil bath at 110°C led to a violent explosion. It was calculated that the critical condition for runaway thermal explosion was exceeded by a factor of 15 [3]. Runaway polymerisation in a distillation column led to an explosion and fire [4]. Another loss of containment and fire resulted from acrylonitrile polymerisation in a waste solvent tank also containing toluene and peroxides (peroxides are polymerisation initiators) [5]. Use of the nitrile as a reagent in synthesis can lead to condensation of its vapour in unseen parts of the equipment, such as vent-pipes and valves, which may then be obstructed or blocked by polymer [6]. 

A radical polymerisation can be carried out with a range of polymerisation techniques. Those with only a single phase present in the system are bulk and solution polymerisations, involving the monomer, a solvent if present and the initiator. By definition, the formed polymer in a bulk or solution polymerisation remains soluble (either in the monomer or the solvent). A precipitation polymerisation is one in which the system starts as a bulk or solution polymerisation, but the polymer precipitates from the continuous phase to form polymer particles which are not swollen with monomer. A precipitation polymerisation when the polymer particles swell with monomer is called dispersion polymerisation apart from polymerisation in the continuous phase, the polymer particles have an additional locus of polymerisation, and the particles in these systems are colloidally stabilised. Precipitation polymerisation is often performed in an aqueous medium (e.g. acrylonitrile polymerisation in water). Dispersion polymerisation is usually performed in organic solvents that are poor solvents for the formed polymer (supercritical or liquid carbon dioxide may also be used as a continuous medium for dispersion polymerisation). 

A third source of initiator for emulsion polymerisation is hydroxyl radicals created by y-radiation of water. A review of radiation-induced emulsion polymerisation detailed efforts to use y-radiation to produce styrene, acrylonitrile, methyl methacrylate, and other similar polymers (60). The economics of y-radiation processes are claimed to compare favorably with conventional techniques although worldwide iadustrial appHcation of y-radiation processes has yet to occur. Use of y-radiation has been made for laboratory study because radical generation can be turned on and off quickly and at various rates (61). 

The principal use of the peroxodisulfate salts is as initiators (qv) for olefin polymerisation in aqueous systems, particularly for the manufacture of polyacrylonitrile and its copolymers (see Acrylonitrile polymers). These salts are used in the emulsion polymerisation of vinyl chloride, styrene—butadiene, vinyl acetate, neoprene, and acryhc esters (see Acrylic ester polymers Styrene Vinyl polymers). 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

The butadiene-acrylonitrile rubbers were first prepared about 1930 about five years after the initial development of free-radical-initiated emulsion polymerisation. Commercial production commenced in Germany in 1937, with the product being known as Buna N. By the late 1980s there were about 350 grades marketed by some 20 producers and by the early 1990s world production was of the order of 250000 tonnes per annum, thus classifying it as a major special purpose rubber. 

The common feature of these materials was that all contained a high proportion of acrylonitrile or methacrylonitrile. The Vistron product, Barex 210, for example was said to be produced by radical graft copolymerisation of 73-77 parts acrylonitrile and 23-27 parts by weight of methyl acrylate in the presence of a 8-10 parts of a butadiene-acrylonitrile rubber (Nitrile rubber). The Du Pont product NR-16 was prepared by graft polymerisation of styrene and acrylonitrile in the presence of styrene-butadiene copolymer. The Monsanto polymer Lopac was a copolymer of 28-34 parts styrene and 66-72 parts of a second monomer variously reported as acrylonitrile and methacrylonitrile. This polymer contained no rubbery component. 

To produce the Type 2 polymers, styrene and acrylonitrile are added to polybutadiene latex and the mixture warmed to about 50°C to allow absorption of the monomers. A water-soluble initiator such as potassium persulphate is then added to polymerise the styrene and acrylonitrile. The resultant materials will be a mixture of polybutadiene, polybutadiene grafted with acrylonitrile and styrene, and styrene-acrylonitrile copolymer. The presence of graft polymer is essential since straightforwsird mixtures of polybutadiene and styrene-acrylonitrile copolymers are weak. In addition to emulsion processes such as those described above, mass and mass/suspension processes are also of importance. 

Whilst the ASA materials are of European origin, the AES polymers have been developed in Japan and the US. The rubber used is an ethylene-propylene terpolymer rubber of the EPDM type (see Chapter 11) which has a small amount of a diene monomer in the polymerisation recipe. The residual double bonds that exist in the polymer are important in enabling grafting with styrene and acrylonitrile. The blends are claimed to exhibit very good weathering resistance but to be otherwise similar to ABS. 

The molecules join together to form a long chain-like molecule which may contain many thousands of ethylene units. Such a molecule is referred to as a polymer, in this case polyethylene, whilst in this context ethylene is referred to as a monomer. Styrene, propylene, vinyl chloride, vinyl acetate and methyl methacrylate are other examples of monomers which can polymerise in this way. Sometimes two monomers may be reacted together so that residues of both are to be found in the same chain. Such materials are known as copolymers and are exemplified by ethylene-vinyl acetate copolymers and styrene-acrylonitrile copolymers. 

The very fast oxidation of the radical precludes its detection and identification by esr however, reacting mixtures are capable of initiating polymerisation of acrylonitrile. The oxidations of allylic alcohols by V(V) perchlorate are ca. thirty times faster than those of saturated alcohols. This is supporting evidence for radical intermediates in view of the expected delocalisation of the free electron... 

The reductions by ferrous ion and mono- and bis-bipyridyl complexes of Fe(II) are also simple second-order with (for the Fe reaction at zero ionic strength ). 2 = TO X 10 exp(—12.1 X 10 /Rr) l.mole . sec . This reaction generates an intermediate capable of oxidising ethanol but the effect is suppressed by addition of Cl , Br and acrylonitrile, the latter being polymerised. 

Traces of mineral acids are sufficient to cause the very vioient polymerisation of acrylonitrile. 

Acrylonitrile came into contact with silver nitrate and was kept in this way for a long time. It gave rise to a violent detonation thert was put down to nitrile polymerisation, which formed successive layers of pilymer at the surface of the salt particles the temperature rise that was caused accelerated the polymerisation gradually. 

Bromine was added to acrylonitrile in small portions at 0°C and then by heating to 20°C between each portion. After adding half the amount of bromine, the temperature reached 70°C and the container detonated. The accident was explained by a violent polymerisation, catalysed by traces of hydrogen bromide that were the result of the following substitution reaction ... 

In the presence of bas, amines or mineral acids, ethylenecyanohydrin dehydrates into acrylonitrile, which then polymerises violently. 

A polymer is produced by the emulsion polymerisation of acrylonitrile and methyl methacrylate in a stirred vessel. The monomers and an aqueous solution of catalyst are fed to the polymerisation reactor continuously. The product is withdrawn from the base of the vessel as a slurry. 

In a process for the production of acrylic fibres by the emulsion polymerisation of acrylonitrile, the unreacted monomer is recovered from water by distillation. Acrylonitrile forms an azeotrope with water and the overhead product from the column contain around 5 mol per cent water. The overheads are condensed and the recovered acrylonitrile separated from the water in a decanter. The decanter operating temperature will be 20 °C. 

The reaction between acrylonitrile and formaldehyde (as paraformaldehyde or tri-oxane), under strong acid catalysis (usually sulphuric) and most often in presence of catalytic quantities of acetic anyhydride, to produce triacrylohexahydrotriazine, is inclined to violent exotherm after an induction period. The runaway can be uncontrollable on sub-molar scale. It may be due to acrylate polymerisation or to increasing reactivity of the formaldehyde equivalent due to progressive de-oligomerisation. Procedures claimed to prevent the risk have been described in the literature but do not seem reliable. 

Bromine was being added in portions to acrylonitrile with ice cooling, with intermediate warming to 20°C between portions. After half the bromine was added, the temperature increased to 70°C then the flask exploded. This was attributed either to an accumulation of unreacted bromine (which would be obvious) or to violent polymerisation [1], The latter seems more likely, catalysed by hydrogen bromide formed by substitutive bromination. Chlorine produces similar phenomena, even if the flask stays intact. The runaway is preceded by loss of yellow colouration and accompanied by formation of 3-chloroacrylonitrile and derivatives. It can be suppressed by presence of bases [2],... 

At pressures above 6000 bar, free radical polymerisation sometimes proceeded explosively [ 1 ]. The parameters were determined in a batch reactor for thermal runaway polymerisation of acrylonitrile initiated by azoisobutyronitrile, dibenzoyl peroxide or di-/er/-butyl peroxide [2],... 

The molar heat of formation of this endothermic compound (+230-250 kJ, 4.5 kJ/g) is comparable with that of buten-3-yne (vinylacety lene). While no explosive decomposition of the isocyanide has been reported, the possibility should be borne in mind [1], It is stable at — 15°C, but isomerises to acrylonitrile and polymerises at ambient temperature [2],... 

Reacts violently with mineral acids, amines or inorganic bases, probably because of dehydration to acrylonitrile and subsequent catalysed polymerisation of the latter. 

After refluxing for 24 horns at 105°C, a mixture exploded, possibly due to dehy-drohalogenation of the bromonitrile to acrylonitrile, and polymerisation of the latter. 

Second step polymerisation of styrene (to get HIPS) or styrene and acrylonitrile (to get ABS) with partial grafting of PS or SAN (styrene/ acrylonitrile) sequences onto the PB chains. 

Transparency is often required. This is achieved by arranging that the particle size of the modifier to be below that of the wavelength of visible light (0.4-0.8 pm). This can normally be achieved by emulsion polymerisation, e.g., polybutadiene, polystyrene. Adhesion and surface compatibility between the polymer and modifier can be achieved by surface grafting of polar groups, e.g., acrylonitrile, various acrylates, onto the impact modifier surface before blending. 

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