MXPA97002983A - Procedure for preparing perfluoropolieteres with extreme groups containing bromo or and - Google Patents

Abstract

A process for preparing perfluoropolyethers with bromine or iodine-containing end groups, respectively, by subjecting a peroxidic perfluoropolyether to heat treatment, in the absence of ultraviolet radiation, at temperatures of 120 ° C to 280 ° C in the presence of bromine or iodine containing compounds respectively having the following general formulas: wherein T, T ', T' ', equal or different from each other, are selected from Br, F, H or R'f, where R'f is a perfluoroalkyl chain of 1 to 8 carbon atoms; R R'R''CI where R, R ', R ", equal to or different from each other may be I (iodine), H, F or R'f where R f has the meaning indicated above

Description

PROCEDURE TO PREPARE PERFLUQRQPOLIETERES WITH EXTREME GROUPS CONTAINING BROMINE OR IODINE MEMORIR DESCRIPTIVE The present invention relates to a method for preparing perfluoropolyethers with bromine or iodine-containing end groups, respectively. As it is known, these products can be used as additives in rubber mixtures curable with peroxides and as replenishers of fluoroelastomer molds in the case of products containing bromine, and as useful intermediates for the synthesis of resins containing fluorine, elastomers. containing fluorine and surfactants containing fluorine in the case of products containing iodine. It is known in the art that it is possible to prepare perfluoropolyethers with bromine-containing end groups in good yields and with controlled molecular weight using perfluoropolyethers having a straight and branched chain by subjecting a peroxidic perfluoropolyether to photolysis with UV radiation in the presence of bromine. temperatures between -40 and + 130 ° C in the presence or absence of an inert solvent. See, for example, USP 5,155,282. In this patent it is specified that if one works in the absence of UV radiation in a wide range of temperatures, low yields are obtained in product containing bromine and without any control of molecular weight. The preparation process indicated in the aforementioned patent thus involves the use of UV radiation and therefore costly procedures and costly plants. Other methods for preparing products with halogen-containing end groups are used in the art. For example, end groups are obtained wherein the halogen can be bromine, iodine or chlorine starting from the silver salt of the perfluoropolyether acid. See, for example, USP patent 3,810,874. In EP 151,877 there is described a method for preparing perfluoropolyethers with chlorine or bromine-containing end groups, starting from perfluoropolyethers with hydrogen-containing end groups by reaction with chlorine or bromine to obtain chlorine or bromine-containing derivatives. In patent EP 348,948, starting from acyl fluoride of the perfluoropolyether, the salt is prepared by reaction with a carbonate and by further reaction with iodine to obtain a perfluoropolyether with iodine-containing end groups. In EP 472,423 a process similar to that of EP '948 is described wherein the acyl fluoride is reacted with a metal iodide to prepare acyl iodide and subsequent treatment with UV radiation to obtain perfluoropolyether with iodine-containing end groups.

In the patent USP 4,853,097 the name of the applicant begins to be made of acyl fluoride of a perfluoropolyether, which is reacted with ammonia and then with hypobroputo to obtain a derivative with a bromine-containing end group. In all of these patents of the prior art 1, either UV radiation involving expensive plants has been used or starting from functionalized perfluoropolyethers that require transformation reactions for their production. Also in this case, the industrial procedures that can be carried out require additional steps and therefore higher production costs. There was a need to have available a process for preparing perfluoropolyethers with end groups containing bromine or iodine respectively, which would eliminate the disadvantages of the known art and would make available a simplified and therefore less expensive procedure. The Applicant has now unexpectedly and surprisingly found that it is possible to obtain perfluoropolyethers with bromine or iodine-containing end groups in good yields and at the same time obtain molecular weight control with the process of the invention described below starting from peroxidic perfluoropolyethers without use ultraviolet radiation. An object of the invention is a process for preparing perfluoropolyethers with end groups containing bromine or iodine, respectively, by subjecting a peroxidic perfluoropoiether to thermal treatment, in the absence of ultraviolet radiation, at temperatures between 120 ° C and 280 ° C in the presence of compounds containing bromine or iodine having the following general formulas: T T'T "C Br where T, T ', T" and equal to or different from each other, are selected from Br, F, HR%, where R'F is a perfluoroalkyl chain the 8 carbon atoms, case in which T = Br and T'y T ", equal to or different from each other, are Br, F, H R'r, preferred; a still more preferred particular case is one where T = T '= T "= Br; R R'R" CI where R, R', R ", equal to or different from one another can be I (iodine), H , F R'f, where R '? Has the meaning indicated above preferably R = R' = R "= I, R = R '~ H and R" = I, R = H and R' = R "= I . In particular, the perfluoropolyether comprises repeating units statistically distributed along the selected chain of: -CF20-, -CF-zCFaO-, -CF2CF (CF3) 0-, -CF (CFa) 0-, -CF-a (CFa)? CF20-, -CF2CF (0R ^) O-, -CF (OR ^) 0-, where z = lo 2, R? is -CFa, -C2Fj3, or -C3Fv. In the case of perfluoropolyethers with iodine-containing end groups, the end groups are selected from: -craI, -CFaCFal -CFICFa, -CFIOR *; where -R, is defined as before; instead, in the case of perfluoropolyethers containing bromine, the bromine-containing end groups are selected from -CF ^ Br, -CFaCF.üBr -CFBrCF3 and -CFBrOR ^ wherein R is defined as above. Optionally, in the products obtained with the process of the invention, both in the case of bromine-containing end groups in iodine-containing end groups, perfluoropolyether products having end groups selected from: I) ~ CF3, ~ Ca > Fs and "Car?, II) -CF2COF, -OCOF, -CF2C0CF3; III) C1CFSCF (CF3) ~, CFaCFC1CF2-, CICFaCFs.-, C1CF2-; In the case of bromine-containing end groups, the preferred reaction temperature it is comprised between 160 ° C and 230 ° C, in the case of end groups containing iodine between 130 ° C and 180 ° C. The average molecular weight of the product obtained with the process of the present invention can be changed depending on the The starting peroxidic perfluoropolyether, in particular, the higher the content of the peroxidic units of the starting product, the lower the average molecular weight of the bromine derivative or iodine derivative obtained.This makes it possible to adjust the molecular weight obtained by changing the characteristics of the product. The content of peroxidic units is usually expressed as PO, defined as PO = g of active oxygen / 100 g of polymer, understood by active oxygen oxygen atoms bound to another oxygen atom. The number-average molecular weight One of the iodine or bromine-containing derivatives according to the present invention is generally between 400 and 4000. In particular, perfluoro-oxyoalkylenes can be selected from the following classes: (a) Tl-0 (CF52CF (CFa) 0) "(CFX0) le > -T2 where: TI and T2 equal to or different from one another are extreme groups containing bromine of the type -CFaBr, -CFBrCF3 or end groups containing iodine of the type -CFs »I, -CFICF3 and optionally also end groups of the type I), II), III) indicated above; X is F or CF3; a and b are integers in such a way that the molecular weight is included in the aforementioned scale; a / b is between 10 and 100. (b) T3 -0 (CFaCFa0) c (CF2u) ct (CFa (CFs,) JCCFs >) - T4 where T3 and T4, equal to or different from each other, are end groups containing bromine of the type -CFaBr, -CF ^ CFaBr, or end groups containing iodine of the type -CFaI, -CFS? CF? ZI, and optionally also end groups of types I), II), III) , previously indicated; c, d and h are integers in such a way that the molecular weight is included in the aforementioned scale; c / d is between 0.1 and 10; h / (c + d) is between 0 and 0.05. (c) T5 -0 (CF2CFs, (CF3) 0) e, (CF5.CF20) f. (CFXO) ß-rñ where T5 and T6, equal to or different from each other, are bromine-containing end groups of the type -CF ^ Br, -CFa.CFaBr, -CFBrCF3 or end groups containing iodine of the type -CF2I, -CFÍZCFS »! -CFICF3, and optionally also end groups of the types I), II), III) indicated above; X is F or CFa; e, f, g are integers such that the molecular weight is included in the scale indicated above; e / (f + g) is between 0.1 and 10, f / g is between 2 and 10. (d) T7-0 (CF! 20) (CFa.CFtOR. ^ O) V (CF (0R) 0 ) -T8 where: RF is -CF3, C2FS, -C3F-7-; T7 and T8, equal to or different from each other, are bromine-containing extreme groups of the type -CF ^ Br, CFaBr, -CFBrOR, or end groups containing iodine of the type -CF2I, -CF ^ CFal, -CFIORf; and optionally also end groups of the types I), II), III) indicated above; j, k, 1 are integers such that the molecular weight is included in the scale indicated above; +1 and j + k + 1 are at least equal to 2, k / (j + l) is between 0.01 and 1000, 1 / j is between 0.01 and 100. The peroxidic perfluoropolyethers used as starting material and which comprise the above-mentioned repeating units are well known in the art, for example, see USP 3,665,041, 2,242,218, 3,715,378 and EP Patents 239,123, EP 344,547, USP 5,144,092. Peroxidic perfluoropolyethers generally have a number average molecular weight of from 1, 600 to 100, 000, preferably from 3, 000 to 50, 000; PO is generally comprised between 0.2 and 4.0, preferably between 2 and 3.5. With the process of the invention in the case of bromine-containing end groups, the yields are very high, of the order of 50-90% by weight, very generally of the order of 80-90% by weight. -In the case of extreme groups containing iodine, the yields are approximately 50-70% by weight. As already stated above, to obtain perfluoro-polyethers with bromine-containing end groups by the process of the present invention, the peroxidic perfluoropolyether is reacted with bromine-containing compounds of the general formula TT "T" C Br as defined above . As stated above, to obtain perfluoro-polyethers with iodine-containing end groups by the process of the present invention, the peroxidic perfluoropolyether is reacted with iodine-containing compounds having the formula RR 'R "CI as defined above. products obtained by the process of the present invention generally comprise mixtures of alpha, omega-diiodo and monoiodoperfluoropolyethers in the case of preparation of perfluoropolyethers with iodine-containing end groups; or alpha, omega-dibro operfluoropolyethers and monobrornoperfluoropolyethers in the case of preparation of perfluoropolyethers with bromine-containing end groups. The functionality f of the final product in extreme groups containing bromine or iodine is defined as: f = 2 (end groups containing iodine or bromine) / total of end groups. The functionality t will have the value of 2 when -all the end groups are end groups containing iodine or bromine, flpar of the reaction mixture, if desired, it is extremely easy to carry out the separation of the product containing diiodo or dibromo , for example by chromatographic separation on silica gel, from the optional prefluoropolyethers containing fluoroacrylic end groups. Therefore, it is possible to obtain a product containing diiodine or dibromo with high functionality with overall yields that depend on the relative percentage of end groups containing iodine or bromine and end groups of fluoroacyl obtained during the synthesis. For example, as regards the iodine-containing products obtained after the thermal treatment of the peroxy fluoropolyoxy-alkylene peroxide in the presence of the iodine-containing compounds of the present invention, they are formed of alpha, omega-diiodoperfluoropolyethers in admixture with alpha-iodoperfluoropolyethers and with perfluoropolyethers that do not contain iodine. The end groups formed during the treatment of the peroxidic perfluoropolyether with the iodine-containing compound of the present invention are in fact composed only of end groups containing iodine and end groups of the acyl fluoride type (-C0F). Obviously, the original end groups of the starting peroxidic perfluoropolyether, which may be end groups of -type I), II) or TTI), as defined above, are also present in the final product. Therefore, if it is desired to prepare a product of high functionality in iodine-containing end groups, it is sufficient to start from a perfluoropolyoxyalkylene peroxide having an average molecular weight high enough that the original end groups of the perfluoropolyoxyalkylene peroxy are of no value. . Subsequently, it suffices to subject the mixture to chromatographic separation on silica gel or another stationary phase which may contain the products having an acid character. In particular, the end groups -COF can be hydrolyzed to end groups -C00H, for example by reaction with water according to the methods described in USP 3,810,874, then dissolving the mixture in a solvent and percolating the solution in a column packed with gel of silica to obtain a product consisting substantially of alpha, or ega-diiodoperfluoropolyethers and then having high functionality (defined as above) in end groups containing iodine. Similarly, this is also true for the bromine-containing groups obtained by the process of the present invention. In this case, both the yield as the selectivity in alpha, or ega-dibromo derived with the heat treatment process in the presence of the bromine-containing compound of the present invention is very high, as well > 90% As already stated, the bromine-containing end groups products obtained according to the present invention can be used as removers of the fluoroelastomer molds. The processing time of the fluoroelastomers is thus reduced and the overall productivity of the processing capacity of the fluoroelastomers is therefore increased. The product obtained with iodine-containing end groups according to the present invention is used, as already mentioned, as an intermediary according to what was previously reported. The following examples are given for illustrative purposes and are not limiting of the scope of the present invention.

E3EIP 01 In a flask equipped with a refrigerant and a magnetic stirrer, 20.8 grams of perfluoropolyether peroxidic having the structure TO (CFaCFa0) or (CF20) c (0) PT are introduced, where p is an integer such as to give an average peroxide power PO = 1.93 (per PO are the grams of active oxygen, 15 amu, for 100 grams of polymers); c and d are integers such that the average ratio c / d = 0.92 and such as to give a number average molecular weight equal to 10,000; T equals CF ^, COF. Then 8.2 grams of diiodo methane (CH »I_2) is added, kept under stirring and heated gradually to 170 ° C. It is maintained at 170 ° C for 2 hours and then at 180 ° C for 5 hours. The higher clear phase is separated (17 g), the yodométpco analysis, shows PO = 0 and the anabolic & amp; -NM mutantrap to the following structure: Ts-occF-iCFaO) (CF ¡aQ c_ ~ i 4 where T3 and T4 are -CFs »I, -CFaCFaI, -COF, -CFaC0F, and traces of CF3 end groups, the ratio of acid end groups / end groups containing iodine is equal to 1.5. c / d is equal to 0.62 and the number average molecular weight is equal to 1280 amu The product thus obtained is hydrolyzed with water to transform the fluoroacrylic end groups into carboxylic acid end groups, then diluted in 1,1,2- tnclorotrifluoroethane is percolated on a column packed with silica gel, to separate the products containing alpha, omega-diiodo from the products of onoacid or diacid.The products containing alpha, omega-diiodo are therefore quantified after the distillation of the solvent and turn out to be 15% by weight with respect to the initial mixture. Analysis of 9R-Nf1R shows a complete absence of acid end groups, the ratio of c / d equal to 0.71 and the number average molecular weight being equal to 1,300. The percentage of diiodo-containing compound in the reaction mixture results in a strict agreement with the value determined based on the assumption that the end groups are statistically distributed among the various polyrne chains. The value determined based on the probabilities results to be: alpha, or ega-diiodinated (0.4) 2 x 100 = 16% alpha, diazyl fluoride (0.6) 2 x 100 = 36% alpha-iodized, omega- acyl fluoride 2 x 0.4 x 0.6 x 100 = 48% Therefore, perfluoropolyethers comprising external iodine-containing groups turn out to be 64%.

EXAMPLE 2 In a flask equipped with a coolant and a magnetic stirrer, 25.6 g of perfluoropolyether peroxidic having the structure T0 (CF52CF: 20) c (CFaO cj (O p> T, PO = 3.18, ratio of c / d = was introduced. 3.5 and number average molecular weight = to 43,000; T has the meaning indicated in example 1. Then 13.1 g of iodine form (CHIa) is added, kept under agitation and gradually heated to 150 ° C and maintained for 2 hours at this temperature.

It is cooled and the upper phase, equal to 19.1 g, is separated. The iodometric analysis shows P0 = Q and the analysis of a ^ F-RUN shows a structure similar to that of Example 1, the ratio of end groups being acid / end groups containing iodine equal to 1.2. The ratio of c / d is equal to 2.23 and the number average molecular weight is equal to 980. The mixture is subjected as in Example 1 to hydrolysis and chromatographic separation, obtaining an equal amount of alpha, ornega-d-iodoperf1 oro-ethers at 20% by weight with respect to the initial mixture, which has a number average molecular weight equal to 950.

E3E11PLQ 3 In a flask equipped with a refrigerant and a magnetic stirrer, 503 g of per-oxidic perfluoropolyether having the structure TO (CFs > CFs »0) and (CFa0) C? (0) e > T, PO = 3.18 (therefore, the peroxidic perfluoropolyether contains one mole of peroxidic bonds -00-), ratio of c / d = 3.5 and number average molecular weight equal to 43,000; T has the meaning indicated in example 1; the kinematic viscosity at 20 ° C equal to 17,600 cSt. Then 101 g of carbon tetrabronide (0.30 mmoles of CB) are introduced and then kept under stirring. It is placed in an oil bath at 160 ° C for 3 hours, at 170 ° C for 5 hours, at 180 ° C for 8 hours and at 190 ° C for 10 hours. During this treatment, the progressive decrease of the solid reagent (CBr \ v) is observed until it disappears completely; at 190 ° C a remarkable reflux of liquid occurs in the coolant, so it is not possible to increase the temperature in the reactor to eliminate residual traces of peroxide. The refrigerant after being replaced by a Claisen flask and the temperature is increased to 240 ° C and maintained for 4 hours, distilling 68.7 g of product. The residue is equal to 338 g. By iodometric analysis, the PO of the distillate and that of the residue gives 0. The analysis of «, -RMN shows that both the distillate and the residue have the following structure: T30- (CFsCFa0) c: (CF20) ciT4. wherein T3 and T4 are -CF_2.Br, -CF_2 CFS! Br and t races of -CF2C0F; the relation between the extreme groups CFs.Br/CF:2CF52B is equal to 90: 10. The number average molecular weight of the distillate is equal to 530, with a c / d ratio of 2.49 and functionality and end groups containing bromine f = 1.92; the residue has a number average molecular weight equal to 1370, the ratio of c / d is equal to approximately 2.09 and the functionality in bromine-containing end groups is equal to 1.91. The total yield in product with bromine-containing end groups is 81% by weight.

AXIS? PLQ *.

With the same procedure that was used in example 3, 219 g were introduced. of peroxidic perfluoropolyether in the reactor, having the structure TOICF'aCFaOcíCFaO) < a (0) "T, with PO = 2.04 and kinematic viscosity at 20 ° C = 38,000 cSt, the ratio of c / d is 1.7, T has the meaning of example 1. 67 g of CBr« was added. (0.20 moles) and heated under stirring at 180 ° C at 190 ° C for 11 hours and at 200 ° C for 4 hours. It is cooled and the solid residue is separated by filtration. The product is then treated at 230 ° C for 3 hours to remove traces of peroxide. 177 g of product having P0 = 0 and structure similar to that of Example 3 are obtained. The number-average molecular weight determined by c, F-NMR is equal to 1900, the ratio of c / d is equal to 1.13 and the Functionality in extreme groups containing bromine is equal to 1.90.

EXAMPLE 5 With the same procedure as that used in Example 3, 521 g of the peroxidic perfluoropolyether used in Example 4 are introduced into the reactor. 91.5 g of CBr *. they are added and heated under agitation at 180 ° C for 4 hours, at 190 ° C for 4 hours, at 200 ° C for 4 hours more and then at 210 ° C for 4 hours more. The solid residue is separated by filtering and treated at 200 ° C for 4 hours to remove traces of residual peroxide. 480 g of product are obtained (yield 92% by weight), having PO = 0 and the same structure as the from example 3. The number average molecular weight (through (Xc, F-NMR) is equal to 1650 and the ratio between the end groups of the type -CF2Br and CFaCFzBr equals 942.6, the ratio of c / d is equal to 0.99 and the functionality in the extreme groups containing bromine is equal to 1.90.

With the same procedure as that of example 3, 20.7 g of peroxidic perfluoropolyether are introduced into the reactor, having the structure TO (CFaCF20) c < CFS20) c, (0) l_T, with PO = 3.7, ratio of c / d = 4.98 and number average molecular weight equal to 9800, T being equal to CF and CaFs- 5.3 g, of CHBr3 are added and heated under stirring at 160 ° C for 6 hours, at 165 ° C for 15 hours, 180 ° C for 4 hours, at 190 ° C for 3.5 hours, at 200 ° C for 3.5 hours and at 230 ° C for 4 hours. The two phases are separated and discharged 10 g of product having PO = 0 and a structure similar to that of example three, with T3 and 14 = CFaBr, - CFaCFaBr, - CF2C00H and traces of -CFa and -CFaCFa (derived from starting peroxidic perfluoropolyether), the ratio between the bromine-containing end groups and the end groups -CFaCOOH being equal to 4.1. The number-average molecular weight determined by NMR is equal to 900, the ratio between the end groups -CFaBr and -CFaCFaBr equals 86:14, the ratio of c / d = 3.5 and the functionality in brominated end groups f = 1.4 .

Claims (8)

NQVEPñD PE Lñ INVENCIÓN CLAIMS

1. - A process for preparing perfluoropolyethers with bromine or iodine-containing end groups, respectively, by subjecting a peroxidic perfluoropolyether to heat treatment, in the absence of ultraviolet radiation, at temperatures of 120 ° C to 280 ° C in the presence of bromine-containing compounds or iodine having respectively the following general formulas: T T'T "C Bren where T, T ', T", equal to or different from each other, are selected from Br, F, H or R'? j wherein R 'is a perfluoroalkyl chain of 1 to 8 carbon atoms; RR 'R "CI where R, R', R", equal to or different from each other, can be I (iodine), H, F or R 'where R' * has the meaning indicated above.

A process for preparing perfluoro-polyethers with bromine or iodine-containing end groups, respectively, according to claim 1, further characterized in that in the bromine-containing compound T = Br and T 'and T ", the same or different ones of others are Br, F, H or R '? in the case of end groups containing iodine because in the compound containing iodine R = R'-R "= I.

3. - A process for preparing perfluoro-poly ethers with bromine or iodine-containing end groups, respectively, in accordance with claim 1, further characterized in that in the bromine-containing compound t = T "= T" = Br; the compound containing iodine R = R '= H and R "-I, or R = H and R' = R" = I.

4.- A process for preparing perfluoro-polyethers with end groups containing bromine or iodine, respectively , according to claim 1-3, further characterized in that the perfluoropolyether comprises repeat units statistically distributed along the chain, selected from: -CF20-, -CFaCFaO-, -CF »CF (CF3) 0-, - CF (CF3) 0-, CFa (CFa) zCFa0-, -CFaCF (0Rf.) 0 ~, -CF (ORf) 0-, where z = 1 or 2, R is -CF3, -CaFa, or -C3F

5. A process for preparing perfluoropolyethers with bromine or iodine-containing end groups, respectively, according to claim 4, further characterized in that in the case of iod-end groups or, the iodine-containing end groups are selected from -CF2I, -CFaCF2I, -CFICF3, -CFIOR-r, wherein R is defined as above; in the case of bromine-containing end groups, the bromine-containing end groups are selected from ~ CF2Br, -CFaCFaBr, -CFBrCF3 and -CFBrOR.r, where defined as above.

6. A process for preparing perfluoro-polyethers with bromine or iodine-containing end groups, respectively, according to claim 1-5, further characterized in that in the case of exo groups containing bromine the reaction temperature is between 160 ° C and 230 ° C; in the case of extémos groups that contain iodine between 130 ° C and iBOßC.

7. A process for preparing perfluoro-polyethers with bromine or iodine-containing end groups, respectively, according to claims 4-6, further characterized in that the number-average molecular weight Pfln of the iodine-containing or bromine-containing derivatives is from 400 to 4000, and are selected from the following classes: (a) Tl-0 (CFaCF (CF3) 0) «(CFX0) to-T2 where: TI and T2 equal to or different from each other, are extreme groups that contain bromine of the type -CFaBr, -CFBrCF =, or end groups containing iodine of the type -CFaI, -CFICF3 and optionally also end groups of the type: I) -CF3, -CaF »and -C3F-r, II) -CFaCOF , -OCOF, -CF2COCF3; III) ClCFaCF (CF3) -, CF3CFClCFa-, ClCF2CFa-, ClCFa-; X is F or CFa; a and b are integers such that the molecular weight is included in the scale indicated above; a / b is between 10 and 100; (b) T3 0 (CF2CF2?) C (CFaO) c (CFa (CFa) CFa) r, -4, where T3 and T4, equal to or different from one another are bromine-containing end groups of the type -CFaBr, -CFs, CFaBR, or end groups containing iodine of the type -CFal, -CFaCFal, and optionally also end groups of the types I), II), III) indicated above; c, d And h are integers such that the molecular weight 00 it is included in the scale indicated above: c / d is between 0.1 and 10; h / (c + d) is included in -t re 0 and 0.5; (c) T5 -0 (CFaCF (CF3) 0) ß (CFaCFaO) f. (CFXO) SJ-T6, where T5 and T6, equal to or different from each other, are bromine-containing end groups of the type -CF2Br, -CF2CFaBr, ~ CFBrCF3 end groups containing iodine of the type -CFaI, -CFaCF »! -CFICFa, and optionally also end groups of types I), II), III) indicated above, X is F or CF3; e, f, g are integers such that the molecular weight is included in the scale indicated above; e / (f + g) is between 0.1 and 10, f / g is between 2 and 10; (d) T7-0 (CFa0) j (CFaCF (0Rf) 0) (CF (OR ^) 0) -T8, wherein: R 'is -CF3, -CZFS, -C3Ft; T7 and TB, equal to or different from each other, are extreme end groups containing bromine of the type -CFaBr, -CFsCF2Br, -CFBrOR-r, or end groups containing iodine of the -CFal type, -CF2CF-2I, -CFIORf.; and optionally also end groups of the types I), II), III) indicated above, j, k, l are integers such that the molecular weight is included in the scale indicated above; k + 1 and j + k + 1 are at least equal to 2, k / (j + l) is between 0.01 and 1000, 1 / j is between 0.01 and 100. 8.- A procedure for preparing perfluoropolyethers with end groups containing bromine or iodine, respectively, according to claim 7, further characterized in that the product obtained is subjected to a chromatographic separation on silica gel.