IL25460A - Preparation of perfluoroolefins - Google Patents

Description

S □ PATENTS FORM NO. 3 PATENTS AND DESIGNS ORDINANCE SP E CI FIC A T ION 'PREPARATION OF PERFLUOROOLEFINS" We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, a British Company of Imperial Chemical House, Millbank, London, S. W.1. , England DO HEREBY DECLARE the nature of this invention and in vh at manner the same is to be performed, to be particularly described and ascertained in and by the following statement : - This invention relates to a method of making perfluoroolefins using tetrafluoroethylene as a starting point, particularly branched-chain Internally unsaturated perfluoroolefins that are oligomers of tetrafluoroethylene.

Polyfluorinated organic compounds whose molecules are made up of chains of -^CF2 groups terminating in other halogenated groups or halogens, for example — CF3, —CC13, — S-?6 and CI are useful in various ways, for example as thermally stable solvents, dielectric fluids and evaporative coolants. Related compounds, in which the chains terminate in functional groups, for example carboxyl, and their derivatives are useful for their surface-active properties.

If it were possible to prepare from tetrafluoroethylene perfluoro- several atoms ^ olefins containing carbon chains of f¾af=e^ ¾oT¾=g'&6¾i5= one would have available compounds potentially useful as thermally stable liquids , dielectric fluids and evaporative coolants and as starting materials for preparing perfluoroacids and other related compounds having potentially valuable properties. The object of the present invention is to prepare such olefins from tetrafluoroethylene.

Thus according to our invention we provide a process for making branched, internally unsaturated perfluoroolefins that are oligomers of tetrafluoroethylene comprising contacting tetrafluoroethylene with one or more fluorides of potassium, rubidium, caesium or of a quaternary ammonium radical under anhydrous conditions and recovering said branched, internally unsaturated perfluoroolefins.* The invention also provides novel branched and internally un-saturated perfluoroolefins that are oligomers of tetrafluoroethylene having the general formula (CaF )n where n is an integer equal to at least 4.

The reaction is conveniently carried out at temperatures from °C to 170°C in the presence of an inert solvent, preferably from 50°C to 0¾ depending on the fluoride and solvent employed. The V less conveniently, for example at temperatures up to 700°C with potassium fluoride or caesium fluoride or at temperatures from about 150°C to 200°C with tetraethylammonium fluoride. It is advantageous to include a small proportion of a free-radical inhibitor in the reaction system, for example 0.1 to 0.50 by v/eight of the tetrafluoro-ethylene of σ-pinene, in order to inhibit free-radical polymerisation of the tetrafluoroethylene.

In this specification the term fluoride of potassium, rubidium or caesium is meant to include not only the normal fluorides F, RbF and CsP but also acid fluorides of potassium, rubidium and caesium, for example KB1.HP, CsF.HF, and fluorosalts that are sources of fluoride ions for example fluorosulphinates S03P and CsS02]?.

Examples of quaternary ammonium fluorides are tetraethyl mmonium fluoride and tetramethylammoniuEi fluoride. Tetraalkylammoniua fluorides containing different alkyl groups in the same molecule can be used, also quaternary ammonium fluorides containing both alkyl and aryl groups. The proportion of fluoride generally employed in the reaction is from 0. 0 to i i¾ by weight of the tetrafluoroethylene.

Suitable solvents include dimethylformamide , N-methyl pyrrolidone, hexamethylphosphoramide, dimethyl sulphoxide, the dimethyl ethers of ethylene glycol and of diethylene glycol. The quantity of solvent is not critical but is preferably sufficient to maintain the fluoride in a well-agitated suspension. The fluoride should be dry and those that are insoluble are preferably in a finely divided state such as that produced by ball-milling.

The reaction may be carried out at any convenient pressure, preferably above atmospheric pressure but not necessarily so. The reaction is normally complete in from 1 to 2 hours at the higher end of the preferred temperature range, but at the lovrer end it may be extended well beyond 12 hours. The shorter reaction times are V as oligomers of tetrafluoroethylene, are not straight-chain compounds and do not possess terminal unsaturation. They are on the contrary branched and internally unsaturated. They have the empirical formula (Cal14)n, where n is an integer equal to at least 4· Formed in the highest yield by the process of the invention are those where n is from to 7) that is to say the tetramer, pentamer, hexamer and heptamer, but products for which n is greater than 7 are also formed though in lower yield. Available experimental evidence does not so far enable one to present precise structural formulae for each of the oligomers, but it has shown that they a re internally unsaturated and contain a high proportion of C¾ groups from which it is certain that the molecules are branched. The nain components of the tetra er, pentamer, hexamer and heptamer fraction of the reaction products have been separated therefrom by vapour-phase chromatography and investigated. Elementary analysis has shown each to have the empirical formula (θ2Ϊ4)η and vapour density determinations have shown n to be j 5> 6 and 7 respectively. Mass-spectrographic analysis of each oligomer has shown the presence of a high proportion of CF3 groups and only ?/eak mass-peaks for the parent ion have been observed. Nuclear magnetic resonance spectra observations have confirmed that the molecules are branched. The oligomers resist oxidation by aqueous potassium or sodium permanganates but are oxidised by potassium permanganate in acetone to give small fragments such as CP3H and CP3C¾C00H.

The reaction mechanism leading to the formation of the oligomers is not yet fully understood but it is possible that tetrafluoroethylene reacts with a pentafluoroethyl anion to give a further anion which isomerises and then undergoes further reaction with tetrafluoroethylene* G-rowing in this way the anion can lose a fluoride ion at an appropriate stage to yield an oligomer. Thus the structure of the pentamer could V · which is not incompatible with the chenical and physical evidence available and can be arrived at by anion foraation and isonerisation as hereinbefore postulated.

The invention is illustrated by Examples 1 -15 and 1 7-1 · Example 6 is a comparison experiment in which the reaction was carried out without any fluoride present and it can be seen that no oligomers were forced. Examples - 6 are summarised in the Table.

Tetrafluoroethylene (20 g.) was distilled into an autoclave containing anhydrous diaethylfornanide (50 ml.) and anhydrous potassium fluoride (2 g.). The autoclave was then heated to 150°C with stirring. After 3 hours the internal pressure had dropped fron 350 to 25 lb/ sq, inch gauge. The contents of the vessel were then separated into a pale -yellow lower layer and a dark upper layer. The lower layer (l 3 gt) was washed with water, dried and distilled to give a mixed perfluoroolefin fraction boiling mainly between 1 0° and 130°C, and exhibiting unsaturation in the infra-red region at 6.1 nicrons ( 6-+.0 en,"1).

Exanple 2 An autoclave containing anhydrous dinethylfornanide (200 nl.) and anhydrous K (6 g.) was pressurised, at roon temperature, with tetrafluoroethylene to 200 lb/sq. inch gauge. The mixture was then heated, with stirring, to 30°C. After 2¾- hours the pressure in the systen had fallen from 350 to 80 Ib/sq. inch gauge. At this stage the system was repressurised with tetrafluoroethylene to 350 lb/sq. inch gauge and the reaction continued. After a further 3 hours the systen was again repressurised to 3 0 lb/sq. inch gauge and after a further 3 hours the reaction was terminated. The lower layer was then separated fron the diaethylfornanide layer. After washing with water and drying 60 g. of perfluoroolefins remained, boiling mainly within the range 110°-1300C. slurry was then transferred to a dry 500 nls. stainless steel stirred autoclave which was then swept out with dry nitrogen. The reaction mixture was stirred and heated to 60°C and the autoclave pressurised with tetrafluoroethylene to 50 Ib/sq. inch gauge. An exothermic reaction took place and raised the temperature to 70 °C whereupon the pressure fell to approximately atmospheric. The reaction system was re-pressurised repeatedly with tetrafluoroethylene each time the rate of the exothermic reaction decreased until the equivalent of 350 lb/ sq. inch was being charged to the autoclave at each re-pressurisation. After hours of this procedure, by which time the accumulated pressure drop had reached 1 200 lb/sq. inch, the autoclave was cooled and the products discharged. The oligomer layer was separated from the solvent, washed with a little dimethylformanide , then with water and finally dried over anhydrous sodium sulphate.

The oligomer mixture (250 g.) was distilled and the following fractions were collected (i) b.pt. < 90°C 2.3 g. (ii) 90 - 92°C 27.Ο (iii) 92 - 1 30°C 22.0 (iv) 1 30 - 1 32°C 11 5.0 (v) > 1 32°C 80.0 Fraction (ii) was mainly the tetramer (C2F4)4; fraction (iv) mainly the pentamer (C2F4)e; whilst fraction (iii) was a mixture of tetramer and pentamer. Fraction (v) was mainly the hexa er (C2 4)6, heptamer (C2F )7 and higher oligomers.

Examples - .6 The results of the experiments on which these Examples are based are summarised in the Table. The reactants were charged into a 5OO mis. stainless steel autoclave and the reaction carried out in a manner similar to that described in Example 3· In all the in experiments 10 and 1 where the volune of solvent -was 100 nls. Diglyne, the solvent used in experiiaent 14 is the dimethylether of diethylene glycol. Extensive side reactions occurred between tetra- fluoroethylene and diglyne and dine thoxye thane in experiments 14 and 15 respectively.

Table In the above Table DMF represents dine thylf oraanide NMP N-aethyl pyrrolidone DMSO dimethyl sulph oxide HMP hexamethyl hosphoramide DG diglyne DUE 1 ,2-dinethoxyethane This illustrates a reaction at atnospheric pressure. Tetra fluoroethylene was bubbled at atmospheric pressure through a liquid collected as a lower layer, and was identified by vapour-phase chromatography and infra-red absorption spectra as a mixture of oligomers (C2F4 )n where n is 1+ to 6* Example -)8 Tetrafluoroethylene was passed over a statio bed of caesium fluoride contained in a stainless steel tube at 700°C. Among the reaction products were mixed erfluorobutenes formed by pyrolysis of tetrafluoroethylene and a small amount of a liquid whose main constituent i? s identified by mass-spectrograph measurements as the tetramer (C2-? )4.

Example This illustrates a reaction under pressure in the absence of solvent. Tetrafluoroethylene (20 g. ) was distilled on to dry tetra-ethylaononiu fluoride (2.5 g.) contained in a dry 50 ml. autoclave at -80°C . The reaction system was then slowly heated to 1 0°C, and maintained at this temperature for i+ hours. After cooling to room temperature unreacted tetrafluoroethylene was vented to atmosphere. Approximately 2.0 g. of liquid products remained in the autoclave and were shown to be a mixture of tetrafluoroethylene oligomers consisting mainly of etramer and pentamer.

Claims (1)

1. Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed^ we declare that ^ What we claim is A process for making branched, internally unsaturated perfluoro- olefins that are oligomers of tetrafluoroethylene comprising contacting tetrafluoroethylene under anhydrous conditions with at least one fluoride source selected from fluorides of potassium, rubidium and caesiun and quaternary ammonium radicals, acid fluorides of potassium, rubidiun and caesiun, and fluorosalts that are sources of fluoride ions, and recovering said branched, internally unsaturated perfluoroolefins. A process as clained in claim in which the reaction is carried out at a tenperature of from 20°G to 70°C in the presence of an inert solvent, A process as claimed in claim 2 in which the inert solvent is selected from dinethylformamide , dimethyl sulphoxide, N-nethyl pyrrolidone, hexaiiethylphosphorami.de, dimethyl ethers of ethylene glycol and of diethylene glycol, A process as claimed in any of claims 1 to 3 in which the proportion of fluoride source in the reaction system is from 0.10}Ό to Ifj^ weight of the tetrafluoroethylene, A process for making branched, internally unsaturated perfluoroolefins comprising contacting tetrafluoroethylene at a tenperature from 50¾ to 150°C with fron 0.10 to 5 by weight of caesium fluoride in the presence of dimethylformamide. A process as claimed in any of claims 1 to 5 in which the reaction is carried out at above atmospheric pressure, A process for making branched, internally unsaturated perfluoroolefins that are oligomers of tetrafluoroethylene substantially as hereinbefore described and with reference to Examples to 5 an