Thiocarbonyl fluoride

Perfluoropyridine gives the usual replacement of the 4-fluonne on reactions with either aromatic or aliphatic mercaptides [55, 36] The reaction of perfluoropyridine with cesium tnfluoromethyl mercaptide, generated from thiocarbonyl fluoride and cesium fluonde, shows temperature dependence of selectivity in fluonne displacement [55 36] (equation 24)... 

Dimeric and trimeric thiocarbonyl fluorides react in the same manner (53) ... 

The effect of halogens on reactivity is also seen for fluorothiocarbonyl monomers. Thiocarbonyl fluoride is polymerized at —78°C by a trace of mild base such as dimethylforma-mide. The polymerization of hexafluorothioacetone is an extreme example of the effect of... 

As indicated above, the most important of the thiocarbonyl compounds is thiocarbonyl fluoride, CF2=S. It has been synthesized by a number of routes (43), the most convenient of which for laboratory investigation is dimerization of thiophosgene followed by conversion of the dimer to 2,2,4,4-tetrafluoro- i, 3-dithietane (I) and thermal cracking of the dithietane (44). 

A detailed procedure for preparing thiocarbonyl fluoride by this route has been published by Sharkey and Jacobson (45). 

A number of routes to thiocarbonyl fluoride that do not involve tetrafluoro-dithietane have been developed. In one (50), phosgene is chlorinated to give tri-chlorosulfenyl chloride, which is converted to chlorodifluorosulfenyi chloride by reaction with antimony trifluoride, and the fluorinated compound is then dehalogenated by reaction with tin. 

Thiocarbonyl fluoride has also been prepared by reaction of bis(trifluoromethyl-thio)merkury and iodosilane (5i). The first product formed is trifluoromethyl-thiosilane, which spontaneously decomposes to thiocarbonyl fluoride and fluorosilane. 

A direct route to thiocarbonyl fluoride is reaction of chlorodifluoromethane with sulfur in a hot tube at 700-900° C (52). This reaction gives very high yields and is amenable to preparation of... 

Another direct route is reaction of tetrafluoroethylene with gaseous sulfur in a hot tube (53). The main product is thiocarbonyl fluoride. Lesser amounts of trifluorothioacetyl fluoride and bis(trifluoromethyl)disulfide are also formed. 

Very high molecular weight polymers are formed by anionic polymerization of thiocarbonyl fluoride at low temperatures (58). These products are thioacetals that come about through addition polymerization of the 0=S bond. 

Above 175° C, poly(thiocarbonyl fluoride) decomposes (63) to regenerate thiocarbonyl fluoride monomer. Chains capped with CF3 are stable to 300° C when heated in nitrogen, though they decompose at 200° C or below when heated in air. Chains so capped have been made by reaction of the polymer with antimony pentafluoride. 

Study of infrared and NMR spectra have established that the end groups in poly(thiocarbonyl fluoride) are CF3S—, —SCF=S, and —SCF=0. All three end groups are present in polymer initiated by dimethylformamide, though —SCF=Q appears to be present in smaller amounts than the other two. Identification of these groups is as follows (64) ... 

The anionic polymerization of thiocarbonyl fluoride appears to procede by the scheme given below in which Be denotes the anion supplied by the initiator. 

Because of its chemical inertness, no direct way of curing poly(thiocarbonyl fluoride) has been found. However, creep has been reduced and strength at elevated temperatures improved by milling into the polymer a free-radical generator, such as dicumyl peroxide or azobisisobutyronitrile, and a free-radical acceptor, such as N,N -m-phenylenebismaleimide or triacryloylhexahydro-s-triazine, and curing with heat and pressure (65). A better method is to mill in divinylbenzene and a small amount of benzoyl peroxide and cure with heat and pressure (66). The divinylbenzene forms a crosslinked matrix that mechanically traps poly(thio-carbonyl fluoride) molecules. Since the elastomer is in effect filled with poly(di-vinyl benzene), the final composition is less resilient than untreated poly(thio-carbonyl fluoride). 

Copolymerization of thiocarbonyl fluoride and perfluorothioacetyl fluoride, CF3CF==S, have been done in ether at —80° C using N-methylmorpholine and tetraisopropyl titanate as initiators. The products, which contain 13-40% of the acid fluoride, are of relatively low molecular weight, do not melt very much lower than the homopolymer, and are much less elastic than the copolymer. 

Copolymers with hexafluorocyclobutanone have been made in ether at low temperatures using cesium fluoride as an initiator. They are of low molecylar weight and are more resistant to amine degradation than the homopolymer. Copolymerization of perfluoroacetahlehyde and perfluoropropionaldehyde with thiocarbonyl fluoride has been accomplished by initiation with complex compounds of the type Me(PR3)BB (67) in which Me is a group VIII metal and B is P(C6Hs)3 or a halogen. The products have excellent resistance to acids. 

Copolymerization of fluorothioacyl fluorides with thiocarbonyl fluoride has just been discussed. These compounds also homopolymerize. The procedure used for polymerization of thiocarbonyl fluoride, which is dimethylformamide initiation in ether at — 78° C, is effective for chlorofluorothioacetyl fluoride, difiuorothioacetyl fluoride, trifluorothioacetyl fluoride, chlorodifluorothioacetyl... 

The low temperature peroxyborane system is very effective for converting thiocarbonyl fluoride to homopolymer. The product is comparable to those formed by anionic polymerization. Since polymerization of thiocarbonyl fluoride is substantially slower than that of the chlorofiuoride, this monomer copolymerizes with exceptional ease with a large number of vinyl compounds to give products that appear to be random copolymers. 

Monomers that copolymerize with thiocarbonyl fluoride include olefins, vinyl halides, vinyl esters, ally esters, acrylates, vinyl ethers, and vinyltrichloro-silane. Nonconjugated diolefins lead to crosslinked products. Conjugated dienes inhibit polymerization. 

Propylene copolymerization is a remarkable case. Copolymers with molecular weights as high as 800000 are readily obtained. The copolymerization tends toward a product containing two molecules of thiocarbonyl fluoride for each propylene molecule. Compositions with higher thiocarbonyl content can be obtained by use of less propylene in the starting monomer mixture. Products containing approximately 2 1 thiocarbonyl fluoride/propylene ratios are soft, pliable elastomers that retain flexibility to — 55° C. 

Ethylene, isobutylene, tert-butylethylene, and other olefins also copolymerize with thiocarbonyl fluoride. What is astonishing is that tetramethylethylene, which is so sterically hindered as to be unreactive in olefin polymerizations, copolymerizes readily with thiocarbonyl fluoride. Most of these comonomers do not behave in the same way as propylene. They give products with compositions more closely approximating the monomer mixture used. 

Rationalization of these facts appear to require an abnormal addition. It has been proposed (39) that a free-radical adds to thiocarbonyl fluoride first to give... 

Copolymers offer the twin advantages of reduced crystalline melting temperature and introduction of crosslinking sites. One of the best for. both of these purposes that has been reported so far is the thiocarbonyl fluoride-allyl chloro-formate copolymer (39). Products containing as little as 2-3 mole % of allyl chloroformate melt below 0°C and are readily crosslinked by incorporation of zinc oxide followed by heating. 

Thietanes are formed by intramolecular cyclization of thiocarbenes (110) (75BCJ1490), on pyrolysis of the copolymer of sulfur, tetrafluoroethylene and thiocarbonyl fluoride (111) (69CC1274), and upon electrolysis or base treatment of dithione (112) (67JOC1562,78JOC1980, 79JCR(S)320>. 

Propene 1-(2-Hydroxy-ctliylthio)-3.8,3-trifluoio- ElOb,. 622 (FjC-CaCII 1 IIS-C,-OH) Thiocarbonyl Fluoridate S-(2.2-Di-fluoro-1-methyl-propyl)- F 10b,. 189 (CO -> CF2 CCI2 - CO)... 

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