Acrylonitrile and Methacrylonitrile Polymers

Polymers from acrylonitrile are used in synthetic fibers, in elastomers, and in plastic materials. The monomer can be formed by dehydration of ethylene cyanohydrin:

Other commercial processes exist, like condensation of acetylene with hydrogen cyanide, or ammoxidation of propylene:

Acrylonitrile polymerizes readily by free-radical mechanism. Oxygen acts as a strong inhibitor. When the polymerization is carried out in bulk, the reaction is autocatalytic [242, 243]. In solvents, like dimethylformamide, however, the rate is proportional to the square root of the monomer concentration [242]. The homopolymer is insoluble in the monomer and in many solvents. Acrylonitrile polymerizes also by anionic mechanism. There are many reports in the literature of polymerizations initiated by various bases. These are alkali metal alkoxides [246], butyllithium [247, 248], metal ketyls [249, 250], solutions of alkali metals in ethers [251, 252], sodium malonic esters [232], and others. The propagation reaction is quite sensitive to termination by proton donors. This requires use of aprotic solvents. The products, however, are often insoluble in such solvents. In addition, there is a tendency for the polymer to be yellow. This is due to some propagation taking place by 1,4 and by 3,4 insertion in addition to the 1,2 placement [253, 254]:

Another disadvantage of anionic polymerization of acrylonitrile is formation of cyanoethylate as a side reaction. It can be overcome, however, by running the reaction at low temperatures. An example is polymerizations initiated by KCN at -50C in dimethylformamide [254], or by butyllithium in toluene at -78C[255]. Both polymerizations yield white, high molecular weight products that are free from cyanoethylation. It was suggested that the terminations in anionic polymerizations of acrylonitrile proceed by proton transfer from the monomer. This, however, depends upon catalyst concentrations [256, 257]. At low concentrations, the terminations can apparently occur by a cyclization reaction [257] instead:

Industrially, polyacrylonitrile homopolymers and copolymers are prepared mainly by free-radical mechanism. The reactions are often conducted at low temperatures, in aqueous systems, either in emulsions or in suspensions, using redox initiation. Colorless, high molecular weight materials form. Bulk polymerizations are difficult to control on a large scale. Over half the polymer that is prepared industrially is for use in textiles. Most of these are copolymers containing about 10% of a comonomer. The comonomers can be methyl methacrylate, vinyl acetate, or 2-vinylpyridine. The purpose of comonomers is to make the fibers more dyeable. Polymerizations in solution offer an advantage of direct fiber spinning. Polyacrylonitrile copolymers are also used in barrier resins for packaging. One such resin contains at least 70% acrylonitrile and often methyl acrylate as the comonomer. The material has poor impact resistance and in one industrial process the copolymer is prepared in the presence of about 10% butadiene–acrylonitrile rubber by emulsion polymerization. The product contains some graft copoly mer and some polymer blend. In another process the impact resistance of the copolymer is improved by biaxial orientation. The package, however, may have a tendency to shrink at elevated temperature, because the copolymer does not crystallize. It is possible to form clear transparent polyacrylonitrile plastic shapes by a special bulk polymeri zation technique [258, 259]. The reaction is initiated with p-toluenesulfinic acid–hydrogen peroxide. Initially, heterogeneous polymerizations take place. They are followed by spontaneous transformations, at high conversion, to homogeneous, transparent polyacrylonitrile plastics [260]. Amajor condition for forming transparent solid polymer is continuous supply of monomer to fill the gaps formed by volume contraction during the polymerization process [261]. Methacrylonitrile, CH₂=C(CH₃) CN, can also be prepared by several routes. Some commercial processes are based on acetone cyanohydrin intermediate and others on dehydrogenation (or oxydehydrogenation) of isobutyronitrile. It is also prepared from isobutylene by ammoxidation:

Just like acrylonitrile, methacrylonitrile does not polymerize thermally but polymerizes readily in the presence of free-radical initiators. Unlike polyacrylonitrile, polymethacrylonitrile is soluble in some ketone solvents. Bulk polymerizations of methacrylonitrile have the disadvantage of long reaction time. The rate, however, accelerates with temperature. The polymer is soluble in the monomer at ambient conditions [262]. Emulsion polymerization of methacrylonitrile is a convenient way to form high molecular weight polymers. With proper choices of emulsifiers, the rates may be increased by increasing the numbers of particles in the latexes. At a constant rate of initiation, the degree of polymerization of methacrylonitrile increases rather than decreases as the rate of polymerization rises [263]. Methacrylonitrile polymerizes readily in inert solvents. The polymer, depending on the initiator and on reaction conditions, is either amorphous or crystalline. Polymerizations take place over a broad range of temperatures from ambient to 5C, when initiated by Grignard reagents, triphenyl ethylsodium, or sodium in liquid ammonia [264]. The properties of these polymers are essentially the same as those of the polymers formed by free-radical mechanism. The homopolymer, prepared by polymerization in liquid ammonia with sodium initiator at -77C, is insoluble in acetone, but it is soluble in dimethylformamide [265]. When it is formed with lithium in liquid ammonia, at -75C, the molecular weight of the product increases with monomer concentration and decreases with initiator concentration. If, however, potassium initiates the reaction rather than lithium, the molecular weight is independent of the monomer concentration [266, 267]. Polymethacrylonitrile prepared with n-butyllithium in toluene or in dioxane is crystalline and insoluble in solvents like acetone [268]. When polymerized in petroleum ether with n butyllithium, methacrylonitrile forms a living polymer [269]. Highly crystalline polymethacry lonitrile can also be prepared with beryllium and magnesium alkyls in toluene over a wide range of temperatures.

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Poly (vinyl chloride)

Miscellaneous Fluorine Containing Chain-Growth Polymers

Copolymers of Fluoroolefins

Poly chloro tri fluoro ethylene

Poly tetra fluoro ethylene

Polyacrylamide, Poly(acrylic acid), and Poly(methacrylic acid)

Thermoplastic and Thermoset Acrylic Resins

Acrylic Elastomers

Polymerizations of Acrylic and Methacrylic Esters

Polymers of Acrylic and Methacrylic Esters

ABS Resins

High-Impact Polystyrene