TECHNOLOGY
Heterofluoropolymers Aim for Commercial Use Sulfur, oxygen, or nitrogen give properties useful in elastomers Du Pont chemists are studying three classes of fluoropolymers possessing unusual properties which could lead to their commercial use in specialty elastomers. The polymers have backbones containing carbon atoms bonded to sulfur, oxygen, or nitrogen atoms. The fluoropolymers are polythiocarbonyl fluoride, polyaziridines based on perfluoroazirines, and a variety of copolymers based on vinyl monomers and hexafluoroacetone ( H F A ) . Du Pont's V. A. Engelhardt has summarized recent work on the polymers by eight chemists at the company's Central Research Department (Wilmington, Del.) before the Third International Symposium on Fluorine Chemistry in Munich, West Germany. Dr. Engelhardt says that the sulfurcontaining polymer, polythiocarbonyl fluoride, is a dense, exceptionally resilient elastomer, surprisingly resistant to chemical attack. The oxygen-containing polymer, hexafluoroacetone, confers desirable properties, such as dyeability, to vinyl copolymer formulations. The nitrogen-containing polyaziridines are still in such early stages of study that their commercial potential can't be assessed. Du Pont chemists find that thiocarbonyl fluoride can be made conveniently through a three-step process based on thiophosgene or by the reaction of sulfur and tetrafluoroethylene ( T F E ) . The fact that sulfur, a lowcost raw material, accounts for 39% (by weight) of the thiocarbonyl fluoride might make it possible for the elastomer to compete in price with polymers made from vinyl fluoride and tetrafluoroethylene. In the thiophosgene route, the first step is dimerization with ultraviolet light irradiation. Fluorination of the dimer with antimony trifluoride gives 80
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2,2,4,4 - tetrafluoro - 1,3 - dithietane. Pyrolysis of the dithietane at 475° to 500° C. in an inclined platinum tube yields thiocarbonyl fluoride in nearly quantitive amounts. The molecular weight of thiocarbonyl fluoride polymers is greatly affected by polymerization temperature. Low temperatures favor high molecular weight. At —78° C , polymers with inherent viscosities of 4 to 6 are obtained by anionic polymerization in anhydrous ether with dimethylformamide initiator. At —50° C , products that have inherent viscosities of about 2 are obtained. The polymer is linear. Resists. Resiliency of polythiocarbonyl fluoride is 9 5 % compared to natural rubber's 8 5 % . It also resists boiling fuming nitric acid and boiling 10% sodium hydroxide solutions for short periods. On prolonged exposure, the elastomer degrades. However, polythiocarbonyl fluoride elastomers are rapidly attacked by organic bases that wet the surface of the polymer. Triethylamine or pyridine, for example, causes rapid and complete degradation at room temperature. Evidence exists that reactive end groups of thiocarbonyl fluoride probably account for this sensitivity. Thiocarbonyl fluoride elastomers can be used at up to 175° C. Above this temperature, the polymer unzips to regenerate thiocarbonyl fluoride. Du Pont chemists also credit reactive end groups for this degradation. The elastomers of thiocarbonyl fluoride have an exceptionally low glasstransition temperature ( — 118° C ) . The crystalline-amorphous transition for polythiocarbonyl fluoride is 35° C. Oriented crystalline films have tensile strengths of about 11,000 p.s.i. and ultimate elongation of roughly 90%. Thiocarbonyl fluoride has been polymerized and copolymerized under free-radical conditions. It has been copolymerized with thioacid fluorides, olefins, vinyl compounds, allylic compounds, and acrylic esters. The oxygen-containing class of new REACTION. Du Pont's Dr. V. A. Engelhardt explains one of three basic reactions for making new heterofluoropolymers
fluoropolymers is based on polymerization of hexafluoroacetone (HFA) through the carbonyl group. The presence of HFA in vinyl fluoride and ethylene polymerizations adds desirable properties to the resulting copolymers. Copolymers based on HFA are dyeable, have high solubility, compatibility, and adhesive properties, compared with these properties of polyethylene and polyvinyl fluoride, he explains. Dr. E. Howard of Du Pont was the first to observe that hexafluoroacetone readily polymerizes, Dr. Engelhardt notes. For example, Dr. Howard found that methyl radicals add to HFA to give the corresponding tertiary alcohol. The free-radical reaction of HFA with cyclohexane gives two products, the major one an alcohol and the other an ether. As temperature increases, more ether forms. "In view of the fact that the literature on the polymerization of ketones is virtually nonexistent, (the only exception being a report by Prof. Kargin and his associates in the U.S.S.R., who obtained polyacetone) the ease with which HFA polymerizes is somewhat
surprising," Dr. Engelhardt adds. The Du Pont chemists have successfully grafted HFA on polymeric substrates such as polyethylene. In contrast to linear polyethylene, the grafted copolymer is soluble in hot benzene, and is soft, flexible, and clear. It has a specific gravity of 1.21 versus 0.95 for linear polyethylene. The HFA-grafted copolymer has improved impact strength, is dyeable, and adheres better to metals than does the polyethylene. HFA polymerizes with ethylene at 600 atm. under free-radical conditions. Deuterium exchange experiments indicate that nearly all (85 to 100%) of the HFA is present in the chain as pendent hexafluoroisopropanol groups. However, the properties of HFAgrafted polyethylene and those of HFA-ethylene copolymer are quite different. The infrared spectrum of the grafted polymer has sharper bands, indicating a more orderly structure. The copolymer is very soluble in cold benzene, but the grafted polymer is insoluble. To account for these differences, the Du Pont group concludes that the copolymer is a highly branched polymer which forms by chain transfer. Hydrogen is extracted from the chain by back biting of the hexafluoroisopropanol groups. Hydrogen extraction creates a new radical site from which another chain can grow, result-
ing in a highly branched polymer structure quite different from the linear structure of the grafted polymer. When HFA copolymerizes with monomers having hydrogen that is less easily extracted, the HFA ends up predominantly in chain ether groups. For example, with vinyl fluoride, only about half the HFA is present as pendent alcohol groups. The Du Pont chemists conclude that the ratio of alcohol to ether formation in HFA copolymer chains depends on the availability and extractability of the hydrogen in the polymer chain. Copolymerization with tetrafluoroethylene gives a product with no hexafluoroisopropanol groups, Dr. Engelhardt explains. Third Class. The third class of fluoropolymers being evaluated is polyaziridines [/. Am. Chem. Soc, 87, 3716 (1965)]. In these polymers, a carbon-nitrogen double bond provides the unsaturation for anionic polymerizations. The polymer backbone is composed of three-membered rings. Polyaziridine synthesis begins with the reaction of hexafluoropropene and triethylammonium azide in sym-tetrachloroethane at —5° C. to give perfluoropropenyl azide. Dr. I. L. Knunyants and E. G. Bykhovskaya in the U.S.S.R. were the first to perform this reaction. The azide evolves nitrogen at 25° to 40° C. and forms 2,3 - difluoro - 2 - trifluoromethyl - 2H-
azirine in 25% yield. Under conditions conducive to anionic polymerizations (catalytic amount of base and low temperature), the azirine polymerizes to a colorless, transparent elastomer in almost quantitative conversion. If 2,3 - difluoro - 2 - trifluoromethyl2H-azirine is converted with hydrogen fluoride to its isomer, 2,2-difluoro-2trifluoromethyl-2H-azirine, the isomer also polymerizes anionically. Properties of the two azirine polymers are not the same. While both are low-molecular-weight polymers, no solvent has been found to dissolve the elastomer made with the 2,2-difluoro isomer. One potential drawback to the polymers is that they are attacked by water. The monomers also hydrolyze readily. Heat stability of the polyaziridines also poses a problem. Heating the elastomer causes a sudden exothermic reaction which occurs with almost no evolution of volatiles and produces a yellow, waxy oil. With the polymer based on the 2,3-difluoro monomer, the exothermic reaction occurs at 140° C. For the 2,2-difluoro isomer, reaction takes place at only 40° C. The oils that are obtained are still polymers. Du Pont chemists believe that heating the polymers causes an unusual exothermic bond migration in which the aziridine rings open up and azomethine links form.
Perfluoro monomers having a carbon doubly bonded to a sulfur, oxygen, or nitrogen atom lead to new fluoropolymers
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