Tetrafluoroethylene oxide and its derivatives 

Report 3. Properties and application of tetrafluoroethylene oxide derivatives

A high and steady interest of researchers and experts in organofluorine compounds is explained by a great progress in chemistry of organofluorine compounds and, on the other hand, by extremely valuable characteristics of articles produced ( high thermal and frost resistance, chemical stability to different aggressive compounds, etc.).

Derivatives of tetrafluoroethylene oxide (TFEO) are distinguished among other fluoropolymers by a complex of attractive properties caused by high strength of the C-F, C-C and C-O bonds, flexibility of chains and a small value of the interaction forces between the chains.

The specific properties of heterochain fluoropolymers are to a great degree associated with a specific nature of the fluorine atom and its bonds with carbon in these compounds. The highest electronegativity (4) among the elements of the periodic system and van-der-Waals radius comparable with that of hydrogen (1.25A in comparison with 1.1A for hydrogen) cause a high energy of the C-F bond and comparatively small differences in steric interactions of fluorine atoms in comparison with hydrogen atoms in compounds of similar structure: the radii of fluorine and hydrogen atoms are less than the lengths of the C-C bond. Thus, replacement of hydrogen atoms with fluorine atoms in the mentioned polymers does not bring any noticeable steric limitations at conformational transitions despite a significant change in bond electron characteristics. This specific nature of the C-F bonds allows to use the fluorine atoms in natural polymers for a directed change of their properties without a significant disturbance of stereochemical interactions causing a wide variety of functions of biological systems (2,3).

In the report presented, properties of oligomers based on TFEO depending on the way of their production and also application of TFEO oligomers, products of their stabilization and functional derivatives on their base are under consideration.

Properties and application of TFEO oligomers.

The properties of TFEO oligomers depend to a great extent on the synthesis conditions.

In solid phase polymerization, a chain growth of the crystal lattice is mostly influenced , consequently polymers with greatest molecular mass are formed (4). Such polymers possess thermal resistance even higher than fluoroplast: the activation energy of thermal destruction is 98 and 86 kcal/mol and maximum decomposition rate is attained at a temperature of 628^oC and 568^oC respectively.

Investigations of fluoropolymers produced under ionization radiation have shown, that their molecular mass is 100, 000-170,000, they do not contain functional end groups and possess the crystal structure with a partial macromolecular orientation. Such a product is a sole representative of high-molecular perfluoropolyethers and is remarkable for extremely high chemical and thermal stability (6).

TFEO oligomers of comparatively high molecular mass have been produced on activated carbon (7), some of their properties are given in the Table below:

Production of liquid TFEO oligomers has been described in details in patents (8,9).

In this case a mixture of oligomers of the general formula

CF

with a polymerization degree of up to 50 is formed. The boiling point of TFEO oligomers with a polymerization degree up to 10 is given in the Table below:

In the photooxidation of TFE in gas phase a mixture of oligomers of the general formula

CF₃O-(CF₂O)n - (CF₂CF₂O)m - C(O)F

is formed at n=12 and a ratio of n/m=10(10).

Polymers of a similar structure were produced also in TFE oxidation by ozone, in this case a number of polyether units -CF

To use TFEO oligomers as chemically and thermally stable liquids and oils, the reactive fluoroanhydride groups are replaced with more stable ones (1,12-14). Thus, inert polyfluoropolyethers with enhanced thermal stability are produced by fluoro- anhydride group replacement with the fluorine atom. For that, perfluoropolyoxaacyl fluorides are transformed to acids, then treated with fluorine at a temperature of 120-250^oC (15,16).

As a result of decarboxylation and fluorination, inert products of the following structure

are produced.

The replacement of fluoroanhydride group with the fluorine atom can be executed also by fluorination at a reduced temperature (50-350^oC) in the presence of catalyst (magnesium, aluminum, zinc, lead, bismuth etc.) (17).

Heating perfluoropolyoxaethylencarbonic acids in an aqueous medium in the presence of potassium persulfate at a temperature of 50-100^oC results in the formation of perfluorinated ethers (18):

The end fluoroanhydride group can be replaced with a hydrogen atom by heating an alkali metal salt of the appropriate perfluoropolyoxaethylencarbonic acid in ethylene glycol (19):

Rf=CF₃CF₂O(CF₂CF₂O)n-

An exposure of TFEO oligomers to UV-rays ( the wave length from 2500 to 3700A) in a nitrogen medium at a temperature from –80^oC to 200^oC results in the detachment of the fluoroanhydryde group and the formation of inert perfluoropolyethers with the doubled molecular weight(20):

The same result is attained using plasma instead of UV-irradiation. Thus, placing perfluoropolyoxaacyl fluoride in a plasma furnace at a residual pressure of 1-3 mm of mercury column at a temperature of decarboxylation leads to the formation of perfluoropolyether with the twofold weight in a yield up to 94% (21).

A set of various bond characteristics in polyether molecules together with inertness of the end trifluoromethyl groups determine  high thermal and thermal oxidation stability, low glass transition temperature, exceptional resistance to strong oxidizers and many other unique properties of stabilized perfluoropolyethers based on TFEO (1, 22-24).

A consequence of chemical resistance of perfluoropolyethers is their incombustibility and biological inertness (22,25). Polyethers stabilized by one of the mentioned methods ( fluorination, irradiation, alkali treatment ) can be applied as high-temperature motor oils, hydraulic liquids, heat carriers, high stable plastic lubricants and etc. (1,20,28-39).

More over, TFEO oligomers are applied for modification of surface properties of polymeric materials to give them oil-, water-, mud proof properties (18).

Synthesis and properties of TFEO oligomer derivatives containing various functional groups.

The fluoroanhydryde group of TFEO oligomers is easy to transform by known chemical conversions to other functional groupings depending on the direction of further application of the perfluoropolyether (1,12,13).

Thus, treatment with water transforms TFEO oligomers into the appropriate perfluoropolyoxaethylencarbonic acids:

Hydrolysis of fluoroanhydrides is carried out either directly by water or by aqueous solutions of salts (CaCl) or alkalis followed by acidation of the hydrolysis products with hydrochloric acid (7,16,40-41).

There is no data in literature on the direct application of perfluoropolyetheric acids based on TFEO, as a rule, they serve to produce other derivatives.

Different salts are easy to form from perfluoropolyoxaethylencarbonic acids and from TFEO oligomers having the end fluoroanhydride groups. Salts of alkali metals, ammonia and substituted amines are recommended to use as surface-active substances and dispersing agents (7,42).

A treatment of perfluoropolyoxaammonium carbonic acids or acylfluorides with alcohols results in the appropriate esters (8,43). Methyl esters of perfluoroetheric acids are easy to saponify with alkalis to form salts. Mainly, methyl esters are used as intermediate products for synthesis of other derivatives.

Starting from methyl esters of perfluoroacids, the appropriate amides, nitriles and amidines are formed (43):

Similarly dinitriles based on TFEO copolymers and difluoroanhydride of glutaric acid were produced with the following structure (23,43-45):

NC-CF₂-(OCF₂CF₂)n-O-(CF₂)₅-O-(CF₂CF₂)mCF₂-CN;

NC-(CF₂)₄-O-(CF₂CF₂O)pCF₂-CN

The dinitriles produced are the starting compounds for synthesis of polyperfluorotriazine polymers (43,45):

The presence of hinged oxygen atoms in the main chain of triazine polymers essentially improves their low-temperature elastic properties and slightly increase their heat resistance (43).

Besides, interaction of  dinitriles based on TFEO oligomers with alcohols or thiols results in the formation of compounds of the following formula:

used as monomers in the synthesis of polymers resistant to oxidation and hydrolysis (23,44).

Diphosphatriazines of the formula

are used as additives, corrosion inhibitors, to oils and lubricants based on Krytox perfluoropolyethers (46).

Reduction of perfluoropolyoxaacylfluorides by boron hydride of an alkali metal in an inert solvent (dioxane) leads to the formation of the appropriate perfluoropolyether alcohols of the formula (9,47):

Interaction of the perfluoropolyether alcohols of the mentioned type with pyromellitic anhydride or cyanuric chloride is used for production of lubricants possessing high thermal stability (8).

Synthesized diethers of phosphoric acid on the basis of perfluoropolyetheric alcohols of the formula:

XCF₂CF₂O(CFXCF₂O)nCFXCH₂O]₂PO (OM)

X=F,CF₃ ; n=1-8; M = H, Li, K, Na, NH₄

possess good oil-resistant properties (9).

Thus, on the basis of TFEO oligomers, alcohols of the formula:

CF₃O(CF₂CF₂O)nCF₂CH₂OH , n =1-5

were produced. They are oilproof and have a low glass transition temperature: