AU591219C - Copolymer of difluoromethylene oxide and tetrafluoroethylene oxide - Google Patents

Description

COPOLYMER OF DIFLUOROMETHYLENE OXIDE AND TETRAFLUOROETHYLENE OXIDE

Field of the Invention

This invention is in the field of fluorine chemistry and more particularly in the field of direct fluorination.

Background

Perfluoroalkylpolyethers are of current inter-'^" est for many new material applications due to their lack of chemical reactivity and their outstanding thermal stability. Their remarkable stability, comparable to that of perfluoroalkanes, along with their interesting surface properties, viscosities and broad liquid ranges make saturated perfluoro- polyethers attractive solvents, hydraulic fluids, heat transfer fluids, vacuum pump oils, lubricants, and grease base stocks. Very high molecular weight perfluoropolyether solids have potential uses as sealants, elastomers, and plastics. See Paciorek, K.J.L, Kaufman, J. , Nakahara, A., Journal of Fluo¬ rine Chemistry, 10, 277 (1977); cGrew, F.C. , Chemical Engineering News, 45, 18 (August 7, 1967); Eleuterio, H.S., Journal of Macromolecular Science- Chemistry, A6, 1027 (1972). Several synthetic methods exist for preparing saturated perfluoropolyethers. The anionic polymeri¬ zation of perfluoroepoxides, particularly hexafluoro- propylene oxide and tetrafluoroethylene oxide, have been used with success. See Hill, J.T., Journal of Macrσmolecular Science-Chemistry, A6, 1027 (1972) . The preparation of perfluoropolyethers via this method first involves the oxidation of a perfluoro- olefin to a perfluoroepoxide , followed by an ionic polymerization of the epoxide to an acyl fluoride terminated'perfluoropolyether and conversion of the acyl fluoride end-groups to unreactive end-groups by decarboxylation reactions. The inability to form very high molecular weight polymers, the lack of stability of many perfluoroepoxides, and the extreme difficulty encountered when attempting to polymerize substituted perfluoroepoxide have been cited as drawbacks associated with this art. Additionally, anioniσ polymerization of perfluoroepoxide does not lend itself well to the manufacturing of perfluoro copolymers since perfluoroepoxide vary widely in reactivity.

An alternative synthetic method for the pro¬ duction of perfluoropolyethers involves the UV photolysis of tetrafluoroethylene and/or hexafluoro- propylene in an inert solvent in the presence of oxygen. This multistep process yields an acyl fluoride terminated polymer containing both the -CF₂-, -CF₂-CF₂-CF₂-CF₂-, (CF₂-CF₂"0) , and (CFfCF- - CF_-0) repeating units as well as unstable peroxidic oxygen linkages (CF₂-0-0-CF₂) . Treatment of the polymer at elevated temperatures and with fluorine gas gives a stable polymer containing perfluoroalkyl ends groups. See U.S. Patents Nos. 3,665,041; 3,847,978; 3,770,792; and 3,715,378. Although this process can be used successfully to prepare copolymers, the process is completely random with little control of the kinds and numbers of repeating units. Undesirable linkages such as the peroxidic oxygen and the poly(difluoromethylene) portions of the polymer are unavoidable and give the polymer undesirable properties for many applica¬ tions. The formation of by-product polytetrafluoro- ethylene and the need for fairly exotic solvents adds significantly to the production costs of the '- polymer.

Disclosure of the Invention

This invention comprises substantially 1:1 random and 1:1 alternating copolymers of difluoro- methylene oxide and tetrafluoroethylene oxide. The perfluoroethers are formed by controlled direct perfluorination of methylene oxide/ethylene oxide copolymers.

Starting copolymers can be synthesized by ring-opening polymerization of 1,3-dioxolane.

1,3-dioxolane can be polymerized to give a σopolymer of methylene oxide and eth lene oxide. Strict head-to-tail polymerization gives a 1:1 alternating copolymer while random head-to-tail/ head-to-head polymerization gives a 1:1 random copolymer as depicted below; H(-OCH₂OCH₂CH₂)_nOH Alternating

H(-0CH 2.0CH 2_.CH~__)n—(OCH z-CH- 2.OCH- 2.)mOH Random

When treated with elemental fluorine in a controlled manner, the following perfluorocarbon Q polymers are formed:

F₂/N₂ Alternating copolymer ■—^ (-OCF₂OCF_CF₂) OY

Random copolymer ^ X(-OCF₂OCF₂CF₂) —(OCF₂CF₂OCF₂)_mOY

wherein X and Y may be the same or different and are -CF₃, -C₂F , -COF, -CF OCF.,, -CF₂COF, -COOH, or

_]__Q -CF COOH and n and m are integers greater than 1.

The molecular weight of the perfluoropolyethers can range from about 500 to about 200,000amu; the lower molecular weight polymers are fluids; the higher molecular weight polymers are solids.

15 The perfluoropolyether fluids of this invention are useful as hydraulic fluids, heat transfer media or as bases for high performance greases which require fluids having a wide liquid range. The perfluoropolyether solids are useful as moldable

20 elastomers or grease fillers. In addition, the solid polymers can be broken down, for example by pyrolysis at 600°C, to produce low molecular weight fluids. Best Mode of Carrying Out the Invention

The difluoromethylene oxide/tetrafluoroethylene oxide polymers are produced by reacting elemental fluorine with a hydrocarbon polymer containing both ethylene oxide and methylene oxide repeat units. The preferred method of synthesizing the starting- polymers is by polymerization of 1,3-dioxolane. The ring-opening polymerization of 1,3-dioxolane using a highly selective (i.e., sterospecific) catalyst such as ZnBr₂ triethylaluminum gives a strictly alterna-<- ting copolymer containing approximately equal numbers of ethylene oxide and methylene oxide repeating units. Polymers prepared from 1,3-dioxo¬ lane using less sterospecific catalysts such as strong acids can be used to prepare random copoly¬ mers. Polymers prepared by other synthetic tech¬ niques containing alternating or randomly distri¬ buted methylene oxide and ethylene oxide units along the polymer chain can be fluorinated to give a polymer similar to perfluoropolyethers prepared using polydioxolane.

The perfluoropolyethers of this invention are compounds, or mixtures thereof, having the following average formula:

X-(OCF₂OCF₂CF₂) -(OCF₂CF₂OCF₂)^-OY

wherein X and Y are may be the same or different and are select from -CF,, -C₂F₅, -COF, -CF₂OCF_ , -CF₂C0F, -COOH, or -CF₂COOH. Subscripts n and are average indicia of composition such that when n and m are both greater than 1 and are approximately equal, a random copolymer is defined and when either n or m approaches zero in value, the polymer is referred to as an alternating copolymer which can be represented as follows:

X-(0CF_o 20CF_ 2CF 2_n)nOY

wherein X and Y may be the same or different and are -CF₃, -C₂F-., -COF, -CF₂OCF₃, -CF₂COF, -COOH, or -CF_COOH and wherein n is an integer greater than 1-. Polymers containing intermediate values for n and m can be made, thus giving rise to properties common to both the random and alternating structures.

Because of the reactive nature of elemental fluorine, the LaMar process is the preferred fluori¬ nation technique. See R. J. Lagow and J. L. Mar- grave Progress in Inorganic Chemistry, 26, 161

(1979) . When using such techniques, low concen¬ trations and small quantities of fluorine are introduced initially in the fluorination reactor. Typically, fluorine gas is diluted with nitrogen; however, other diluents such as helium work equally as well. As the fluorination proceeds, higher fluorine concentrations and greater flows can be utilized without significant fragmentation of the polymer. Due to the extreme exothermic nature of the reaction, the fluorination must be carried out slowly unless provisions have been made for removing the heat of reaction. Submersion of the reactor in a cooled liquid bath or the use of an internal Freon cooling coil can satisfactorily remove the heat. Fluorine gas is the preferred fluorinating agent and is available commercially at sufficient purity levels. Other fluorinating agents such as chlorine trifluoride or bromine trifluoride can be used; however, some chlorine or bromine substitution on the polymer generally will take place when these agents are used. The physical form of the polymer fluorinated is not critical; however, the fluorina¬ tion of fine powders work especially well. The fluorination can be carried out by passing- dilute fluorine over the polymer in a stationary reactor, in a rotating drum reactor, in a fluidized bed reactor or in a solvent reactor. The polymer may be soluble in the solvent (which must be inert to fluorine gas) or it may be present as a slurry.

Although a powdered polymer can be fluorinated in the neat form or in a solvent, the method of choice is to fluorinate the polymer in the presence of a hydrogen scavenger such as sodium fluoride (NaF) to adsorb the by-product hydrogen fluoride. The fluorination of ethers in the presence of hydrogen fluoride scavengers is described in U.S. Patent Application Serial Number 796,623, filed November 8, 1985 entitled "Perfluorination of Ethers in the Presence of Hydrogen Fluoride Scavengers", filed concurrently herewith, the teachings of which are incorporated by reference herein. A 5:1 ratio of NaF to polymer is preferred; however, a 4:1 ratio also works well. Higher concentrations of NaF do not show a significant additional positive effect. The LaMar direct fluorination of a polyether containing both ethylene oxide and methylene oxide units can be illustrated as follows: F₂/He -(CH₂CH₂-0-CH₂-0)- ^ X-(CF₂CF₂-0-CF₂)_n~O

1. T=amb

2. _ _.

wherein X and Y may be the same or different and are defined as -CF,, -C₂F₅, -COF, -CF₂0CF_, -CF₂COF, COOH, or CF^COOH and n is an integer greater than 1.

Q5 Perfluoroethers of a broad range of molecular'^" weights (500-200,000 amu) can be prepared depending upon the molecular weight of the starting hydro¬ carbon material and the fluorination conditions used. High fluorine concentrations, fast flow rates

10 and elevated temperatures each favor fragmentation, thus lower molecular weight products are obtained. Milder fluorination conditions designed to prevent fragmentation lead to an extremely stable high molecular weight perfluoropolyether.

15 When mild fluorination conditions are used to fluorinate a high molecular weight polymer (greater than 20,000 amu), a white solid is typically obtained. Several schemes can be employed to prepare interme¬ diate molecular weight fluids. One scheme is to

20 perfluorinate a low molecular weight polymer using mild fluorination conditions. Treating a higher molecular weight polymer with slightly harsher fluorination conditions can lead to fluids when the conditions are chosen to give a controlled amount of chain cleavage. "Perfluorination" of a high mole¬ cular weight polymer using mild conditions can be used to replace a specified number of hydrogens with fluorine. A second step is designed to promote fragmentation. Elevated temperatures and high fluorine concentrations are used to give the per¬ fluoropolyether fluid.

An alternate scheme, and possibly the method of choice for preparing a wide range of molecular weights involves the fluorination of a high mole- ^ι- cular weight polymer using mild fluorination condi¬ tions to give a high molecular weight solid contain¬ ing both the perfluoro alkyl and acyl fluoride end groups. Treatment of the polymer with pure fluorine at elevated temperature ( 100°C) gives a polymer containing only perfluoro alkyl end groups. The resulting high molecular weight solids can be broken down to lower molecular weight components by pyrolysis. This procedure is described in United States Patent Application Serial No. 796,624, filed November 8, 1985 entitled "Pyrolysis of Perfluoropolyethers" filed concurrently herewith, the teachings of which are incorporated by reference herein. Pyrolysis of the solid in the presence of nitrogen, air or fluorine gives lower molecular weight polymers. By selecting the proper pyrolysis temperature (400-500°C) and by carrying out the pyrolysis in a distillation- type apparatus, a well-defined boiling point range can be collected while less volatile components are returned to the high temperature portion of the apparatus to be further fragmented. If the pyrolysis is not carried out in the presence of fluorine, an additional fluorination at elevated temperatures is needed to remove the acyl fluoride terminal groups. Various terminal groups are obtained in the fluorination and pyrolysis reactions. For many applications where an inert material is required, it is desirable to remove acid and acyl fluoride end groups. This is best accomplished by treating the polymer with pure F_ at a temperature greater than *. 100°C. Some of the reactions occurring are re¬ presented by the following equations where P- corresponds to a perfluorinated polyether chain.

F ^£ 2

P_f-0-CF₂-COF ^ P_f-0-CF + COF

^F2 P--CF.--0-CF -O-COF ^P -CF, + 2COF- + 1/2 0_

£ 2 2 ' t 2 2

^F2 P_f-0-CF₂-CF₂-0-COF ^ P_f-0-CF₂-CF₃ + C0F₂ + 1/2 0₂

F ^S 2

P_f-0-CF₂-C00H ^P_f-0-CF₃ + C0₂ + HF

In addition to reactions of this type which relate exclusively to the terminal groups of the polymer, fluorine can react at elevated temperatures with stray hydrogens left on the polymer resulting in chain cleavage at that point. However, at 100°C, approximately 80% of the remaining hydrogens can be replaced with fluorine without chain degradation providing that fewer than 1% of the hydrogens remain in the polymer. Typically, upon completing the fluorination at elevated temperatures, the hydrogen content of the polymer is below 5ppm as determined by Fourier transform infrared spectroscopy.

The perfluoropolyether fluids of this invention have distinct advantages over the existing fluid, •- namely Fomblm Z TM fluids. Fomblin ZTM fluids have a widely varying structure containing repeating units such as polydifluoromethylene, -CF-CF-CF,,- and

-CF₂CF₂CF₂CF₂- which can increase the viscosity of the fluid at low temperatures. F NMR analysis of Fomblm Z TM fluids shows that the fluid structure is less random than previously thought and that the ethylene oxide and methylene oxide units tend to be present in blocks. Three or more sequential methy- lene oxide units act as a weak point in the polymer chain and limit the thermal and oxidative stability of Fomblm Z TM fluids. Perfluoropolyethers of this invention contain either 1 or 2 methylene oxides in a row depending upon the starting material used. Like Fomblin Z TM fluids, the polymers contain difluoromethylene oxide units (for good low tempera¬ ture properties) and tetrafluoroethylene oxide (for improved high temperature stability) .

The invention is illustrated further by the following examples: Example 1

1,3-dioxolane was polymerized by placing 250g of the dried material in a nitrogen-purged 1 L flask. 1.6g of ZnBr and 3.5 cσ of a 5% triethyl- aluminum in toluene solution was added to the flask. After 3 days the polymerization was complete. The solid polymer was ground to 50 mesh or smaller using liquid nitrogen in a blender.

2g of the sieved polydioxolane powder were mixed with lOg of 100 mesh NaF powder in a nickel ,. boat which was placed in an 18" long reactor con¬ structed from 1 1/2" nickel pipe containing Teflon O-ring sealed flanged ends. The assembled apparatus was flushed with lOOcc/min of N for several hours before beginning the fluorination. The nitrogen flow was monitored with a glass rotameter while the fluorination flow rate was controlled with a Monel needle valve and monitored with a Hastings mass flow transducer, Type F-50M. The fluorine, supplied by Air Products, was used without further purification. The fluorine flow was set at 2cc/min while the N_ flow was maintained at lOOcc/min for 2 days. After 48 hours of relatively mild conditions, pure fluo¬ rine was used for 5 hours followed by 5 hours of exposure to pure fluorine at 110°C to remove any acyl fluoride terminal groups. Upon completing the fluorination at elevated temperatures, the apparatus was again flushed with lOOcc/min N₂ for approxi¬ mately one hour. The solid reaction product was stirred with 75ml of Freon 113 for approximately 1 hour. Upon removing the solid by filtration, 1.9g of a low volatility, low viscosity oil was recovered from the Freon. The oil, when placed in a freezer held at -50°C, continued to flow well. The Freon insoluble portion was washed with approximately 300cc of distilled water to dissolve away the NaHF leaving behind 0.8g of a white free flowing powder which is a higher molecular weight version of the oil obtained (Total yield: 54.9%). The fluid was characterized by 19F NMR. Each '- of the individual spectral lines were assigned to a structure by comparison with the spectra of known perfluoro compounds. Spectral data for the fluid is summarized in the table below:

Table

Chemical Relative

Structure Shift (ppm) Intensity (%)

-CF₃CF OCF₂0- 50.0 1.9

-OCF₂CF₂OCF₂OCF₂CF₂0- 53.2 24.0

CF₃OCF₂-0- 55.5 3.6

CF_₃OCF₂CF₂0- 57.3 5.2

CF₃OCF₂0- 59.2 4.5

CF₃CF₂0- 89.0 2.9

CF₃CF₂0- 90.3 1.9

-OCF₂OCF₂CF₂0- 92.5 51.3 On the basis of the NMR spectroscopic analysis, the average structure was the fluorocarbon analogue of the hydrocarbon starting material polydioxolane. -14-

Example 2

300g of polydioxolane powder was dissolved in 500ml of methylene chloride and mixed with 1200g NaF powder. The solvent was evaporated and the resul- ting solid was ground cryogenically to give a powder which will pass a 50 mesh screen. The powder was placed in a 9" ID x 2' long aluminum drum reactor which rotates at 5 rev./min/ The reactor was flushed with nitrogen for several hours prior to beginning the fluorination. A gas flow of 30.0 cc/min fluorine and 2 L/min nitrogen was maintained for 36 hours. The nitrogen was decreased to 1 L/min for an additional 12 hours. The polymer is treated with pure fluorine for several hours to insure perfluorination. A reactor temperature between 0°C and +20°C was desirable for best results. A final fluorination at 110°C for 4 hours was used to replace any residual hydrogen with fluorine and to convert reactive acetyl fluoride end groups to inert trifluσromethyl or pentafluoroethyl terminal groups. Extraction of the powder with 2 liters of Freon 113 gave 370g of the desired difluoromethylene oxide- tetrafluoroethylene oxide copolymer. An additional 160g of a Freon insoluble solid was also obtained which can be converted to a fluid via pyrolysis.

Elemental analysis for solid: calculated (C J,Fo_02)n:

C, 19c80; F, 62.63, found: C, 18.11; F, 62.53.

Example 3

Two grams of polydioxolane were placed in a nickel boat along with lOg of NaF pellets (1/8" mesh) . The boat was placed in a 1 1/2" nickel tube reactor and flushed with lOOcc/min N₂ prior to beginning the fluorination. The fluorine and nitrogen flow rates were set at 2cc/min and lOOcc/min, respectively. After 48 hours had elapsed, the sample was treated for 12 hours with pure fluorine at 100°C. Extraction of the product mixture with Freon 113 gave 1.5g of a clear, low viscosity, nonvolatile oil. The NaF/NaHF-. pellets were screened from the sample leaving behind 0.4g of a white solid (Total yield: 38.6%). Infrared analysis and the NMR spectra of the oil were very similar to that observed for the oil prepared according to Example ^ι- 1.

Example 4

Fluorination of polydioxolane using the very mild conditions as described in Examples 1 and 2 gives a perfluoro product with a minimal amount of chain degradation occurring during the fluorination reaction. The oil present in the sample results from the direct fluorination of lower molecular weight chains in the hydrocarbon starting material. The oil to solid ratio of the final product can be increased by employing a two-step direct fluorina¬ tion process. In the initial phase, dilute fluorine is passed over the sample to replace the majority of the hydrogen. The second step, perfluorination of the sample with pure fluorine at elevated tempera¬ ture, give a product with a lower average molecular weight. The exothermicity of the reaction with elemental fluorine results in some chain fragmen¬ tation. Two grams of polydioxolane was mixed with lOg of NaF powder. The reactor was purged with lOOcc/min N₂ for 1 hour, followed by reaction of the polymer with 2cc/min F₂ diluted with lOOcc/min N₂ for 48 hours. Next, the polymer was subjected to pure fluorine at 100°C for 8 hours at which time some chain cleavage occurred. Using this procedure, 2.4g of oil and O.lg of solid material are obtained (50.8% total yield) .

Industrial Applicability

The difluoromethylene oxide/tetrafluoroethylene oxide fluids of this invention are useful as oils, hydraulic fluids or as bases for high performance greases which require fluids having a wide liquid range. The fluids can be prepared in the molecular weight range desirable for a particular use. For example for vacuum pump oils, fluids ranging in molecular weight from about 5,000 to about 20,000 amu are desirable. Fluids ranging from about 750-2,000 amu are useful as vapor phase soldering fluids and those ranging from about 1,000-3,000 as hydraulic fluids. The perfluoropolyether solids are useful as moldable elastomers or grease fillers. In addition, the solid polymers can be broken down, for example by pyrolysis, at 500-600°C to produce low molecular weight fluids. The perfluormethylene oxide/ethylene oxide polymers of this invention have both very good thermal stability and excellent low temperature properties. They are devoid of particular molecular structures believed to be associated with poor thermal stability and high fluid viscosity. Equivalents

Those skilled in the art will recognize, or be able to ascertain using no more than routine experi¬ mentation, many equivalents to the specific e bodi- ments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims (7)

1. Perfluoropolyethers of the formula:

   X-(OCF,2OCF2,CF2,)nOY

   wherein X and Y may be the same or different 5 and are -CF-, -C₂F₅, -COF, -CF₂OCF₃, -CF₂COF, -COOH, or -CF₂COOH and wherein n is an integer greater than 1; the perfluoropoly ethers having a molecular weight of about 500 to about 200,000 amu.

   Q
2. 2. Perfluoropolyether fluids of the formula-:

   wherein X and Y may be the same or different and are -CF, or ^_C₂F and wherein n is an integer greater than 1 such that the fluids 5 range in molecular weight from 750-20,000 amu.
3. 3. Perfluoropolyethers having a chain structure consisting essentially of the repeating unit OCF^OCF-CF- and having terminal groups selected from the group consisting of -CF,, -C₂F_, -COF, -CF OCF , -CF₂COF, -COOH and -CF₂COOH, the perfluoropolyethers having a molecular weight of about 500 to about 200,000 amu.
4. 4. Perfluoropolyethers of claim 3, wherein the terminal group is -CF₃ or -C₂F₅.
5. 5. A method of preparing perfluoropolyethers having a chain structure consisting essentially of the repeating unit OCF₂OCF₂CF₂ and having terminal groups selected from the group con¬ sisting of -CF₃, ~C₂F₅, -COF, -CF₂OCF₃, -CF₂C0F, -COOH and -CF COOH, the perfluoro¬ polyethers having a molecular weight of about ^l- 500 to about 200,000 amu, comprising the steps of: a. providing a copolyether consisting essen¬ tially of methylene oxide and ethylene oxide units in a molar ratio of about 1:1; b. perfluorinating the copolyether by: i) exposing the copolymer to a mixture of an inert gas and fluorine gas, the fluorine concentration being from about 1 to about 10%; ii) gradually increasing the concentra¬ tion of fluorine gas until the polymer is exposed to pure fluorine gas thereby perfluorinating the copolyether to produce a perfluoro- polyether.
6. 6. A method of claim 5, wherein the fluorination is accomplished in the presence of sodium fluoride.
7. 7. A method of claim 5, further comprising: iii) treating the polymer with fluorine gas at an elevated temperature sufficient to convert any reactive end groups to perfluoroalkyl groups.