science
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Monomer casting method gives polychloral itiators for chloral polymerization. A Distribution of anionic disadvantage of these initiators is that they give polymers of low thermal initiator in the monomer stability, the Du Pont workers point is key to control of out. Tetraalkylammonium chlorides polymerization in method such as tetrabutylammonium chloride trialkylsulfonium chlorides also indeveloped by Du Pont group or itiate chloral polymerization. TriUsing a monomer casting technique, chemists at Du Pont's central research department in Wilmington, Del., have prepared films, sheets, and complicated shapes of polychloral. The chloral polymers have mechanical properties typical for engineering plastics. Dr. Otto Vogl (now at the University of Massachusetts, Amherst) and Du Pont's Dr. Henry C. Miller and Dr. William H. Sharkey have found that in addition, the polymers are nonflammable and are insoluble in most solvents. In fact, unusually good flame resistance is one of the big incentives for working on the materials. Polychloral can be prepared in predetermined, coherent shapes by distributing an anionic initiator in the monomer under conditions where polymerization will not occur and then changing conditions to bring about polymerization. The Du Pont group has done this by adding initiator above the ceiling temperature of the polymerization (58° C), transferring the warm, initiated monomer into a mold, and polymerizing by cooling, preferably to room temperature or below. They call this monomer casting technique cryotachensic polymerization. The polymer forms as fast as the heat of polymerization can be removed. The ceiling temperature of polymerization varies with concentration and is reduced by adding an inert diluent. Initiators. Alkali metal salts containing nucleophilic anions are good initiators for chloral polymerization, the Du Pont chemists find. Most of their work has been carried out with the initiator, lithium tert-butoxide. They also find that many other alkali and alkaline earth salts are initiators for chloral polymerization. Tertiary amines such as pyridine and trialkylamines are also good in-
phenyl phosphine reacts instantaneously and quantitatively with chloral to give triphenyl(2,2-dichlorovinyloxy) phosphonium chloride. Like triphenylphosphine, the phosphonium chloride can also initiate chloral polymerization and gives high yields of polychloral with good thermal stability. Polymerizing chloral with chloride-donating Lewis acids such as aluminum chloride or antimony pentachloride gives polychloral, probably by chloride initiation. Polymerization of pure chloral monomer always yields 75 to 80% polychloral, the Du Pont group finds. Baking at 100° to 120° C. for one to four hours removes a portion of the monomer. To remove all the monomer, the polymer must be extracted with solvents such as methanol or acetone. This can be done without appreciable change in dimension of the cast object. A 95% conversion of
chloral monomer to polymer can be achieved if 20% of unreactive diluent is added before polymerization. Chloral copolymerizes easily with isocyanates or ketenes by cryotachensic polymerization, the Du Pont chemists say. Compositions of high chloral content similar to chloral homopolymer are insoluble. Effective initiators for the copolymerization include most initiators used for chloral homopolymerization. Properties. Since chloral polymers can't be molded or otherwise processed, test pieces must be machined or stamped out of thin sheets. Polychloral resists impact better than does polymethyl methacrylate, according to the Du Pont group. Polychloral does not support combustion in oxygen, even at 1.1 atm. The polymer does not melt or drip but decomposes to gaseous products in a hot flame. Polychloral is stable to acids, even to fuming nitric acid. It is less stable toward organic bases and is easily degraded with hot secondary amines. The thermal degradation of polychloral begins noticeably at 200° C. The polymer degrades quantitatively to the monomer, especially under reduced pressure.
Du Pont's H. C. Miller (left) and W. H. Sharkey discuss chloral polymerization MARCH 20, 1972 C&EN 41
