OTHER TYPES OF POLYMERIZATION
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Polymerization of acrylonitrile may also occur in homogeneous medium using solutions of sodium thiocyanate or zinc chloride as solvents.
This process is most commonly used in the production of PAN for the manufacture of carbon precursor fibers. Solution polymerization cannot be performed in organic solvents such as DMF and DMAc which have higher chain transfer constant than the polymer. The main advantage of Solution Polymerization over Suspension Polymerization
is that as the PAN is formed it remains dissolved in the solvent and upon reaching a certain concentration in the range of 20% to 30%, the polymer solution can be spun directly. This eliminates the PAN filtration, drying and preparation steps, as occurs in the Suspend Polymerization process.
Another polymerization process known as Emulsion Polymerization
was used in the past to produce anti-flame fibers produced by Union Carbide under the Dynel brand, which had 60% vinyl chloride and 40% acrylonitrile. Its advantage is that it allows the production of copolymers of higher molecular weight than the other two processes mentioned. In this process a small amount of the monomer is dispersed in the medium containing catalysts and an emulsifying agent such as sodium lauryl sulfate, at a concentration of 1014 to 1020 micelles per liter. The radicals formed in aqueous medium are rapidly captured by the micelles and continue to grow inside them until they reach the desired molecular weight, as they exit the reactor and the polymerization is stopped. The emulsion containing the PAN needs to be coagulated for removal of the polymer, which is done by adjusting pH or acid salts such as aluminum sulfate. Thereafter the PAN is dried and dissolved in a solvent suitable for spinning. Due to its low productivity and the need for additional operations to remove the emulsion PAN, this process no longer has industrial applications.
Another polymerization process of PAN, known as Bulk Polymerization
, can be carried out with some advantages over water processes, mainly for the production of ultra-high molecular weight polymers. But it can only be carried out on a small scale, very difficult because of the difficulty of removing the large amount of heat released in the polymerization process which is of the order of 320 cal / g and the insolubility of the polymer formed in the monomer. When catalysts such as benzoyl peroxide are employed, the reaction is autocatalytic and the temperature can easily rise above 200 to 300 ° C, leading to boiling of the monomer and cyclization of the obtained polymer. No commercial application of this process is known so far for PAN production.