CA1224900A

CA1224900A - Batch polymerization process

Info

Publication number: CA1224900A
Application number: CA000409762A
Authority: CA
Inventors: Richard A. Morgan; Ausat A. Khan
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1981-08-21
Filing date: 1982-08-19
Publication date: 1987-07-28
Also published as: JPS5838706A; BR8204789A; US4380618A; EP0073121A1; EP0073121B1; JPS6143364B2; DE3267591D1

Abstract

ABSTRACT
Polymerization Process In the batch process for copolymerizing tetrafluoroethylene and selected comonomers to prepare a copolymer thereof, the improvement which comprises employing a perfluoroalkylethane sulfonic acid or its salts or a perfluoroalkyl ethane sulfate salt as the dispersing agent.

Description

TITLE
Batch Polymerization Process BACKGROUND
The batch copolymerization of 5 tetrafluoroethylene (TF~) and copolymerizable fluorinated ethylenically unsaturated comonomers in the presence of a dispersing agent is known.
However, it is desirable to impr4ve upon this copolymerization by increasing the reaction rate and 10 by incorporating greater amounts of comonomer into the copolymer.
Due to the higher reactivity of tetrafluoroethylene (T~E) compared to some comonomers, it has been difficult to achieve high 15 levels of comonomer incorporation into melt-processible copolymers with TFE at rea~onably rapid batch polymerization rates. Accordingly, the ability to incorporate more of such comonomers, particularly the comonomer hexafluoropropylene, into 20 TFE copolymers is advantageous since certain resin physical properties are a function of comonomer content. Furthermore 7 it is desirable to produce TEE
homopolymer and TFE copolymers of small particle sizes for certain applications such as coatings and 25 the like.
SUMMARY OF_THE INVENTION
It has now been discvvered that if a selected perfluoroalkyl ethane sulfonic acid or its salt or a perfluoroalkyl ethane sulfate salt is 30 employed as the dispersing agent, the total monomer conversion and the productivity in batch polymerization of the TFE/comonomer polymerization to make melt-processible copolymers is increased and the amount Qf comonomer incorporated into TFE copolymer is increased, particularly when the comonomer is hexafluoropropylene.
In addition, it has now been found that when a selected perfluoroalkyl ethane ~ulfonic: acid or its salt is employed as the dispersing agent, smaller siæe polymer particles are formed when employed to make T~E homopolymer or nonmelt-processible TFE
copolymers.
Specifically, one aspect of this invention can be described as follows: In the batch process for preparing ~elt-processible copolymers of tetrafluoroethylene and at least one copolymerizable fluorinated ethylenically unsaturated comonomer by polymerizing tetrafluoroethylene and at least one said comonomer in an aqueous polymerization medium containing a free-radical initiator and 0.01-0.5 peroent by weight dispersing agent, preferably 0.05-0~1 percent, based on weight of ~queous medium, the improvement which comprises employing as the dispersing agent (l) a mixture of compounds of the formula F~F2-CF2~nCE~2 CH2 wherein n is a cardinal number of between 2-8 and the average value of n is between 3 and 6 or (2) a compound of said formula wherein n is a cardinal number selected from between 2-6; and Y is -S03M or -OSO3M' wherein M is a cation having a valence of l, and M' is an alkali metal cation or ammonium.
Another aspect of this invention can be described as follows: In the batch process for preparing tetrafluoroethylene homopolymer or nonmelt-processible copolymers of tetrafluoroethylene and at least one copolymerizable fluorinated ethylenically unsaturated comonomer by fe~ding tetrafluoroethylene alone or with at least one said comonomer into an aqueous polymerization medium containing a free-radical ini~iator and 0~01 to 0.5 percent dispersing agent based on weight of aqueous medium, the improvement which comprises employing as the dispersing agent (1) a mixture of compounds o the formula F~CF2-CF;~nCH2-CH2-S()3M
wherein n is a cardinal number of between 2-8 and the average value of n is between 3-6, or (~) a compound of said formula wherein n is a cardinal number selected from 2~6; and M is a cation having a valence of 1.
D'9Ci~ Oi Or 111~ on ~C 2 2~nC 2 CB2 Y
dispersing agent employed in the reaction is most readily available as a mixture of compounds in which n is a cardinal number of 2-8 with an average value of about 4. The average can be between 3-6, but 4 is most commonly available. The cation M employed in the functional group denoted as Y in the formula is preferably hydrogen, ammonium or an alkali metal, and most preferably is H , NH4, Na , Li or K , while the cation M' is preferably NH~, Na , Li , or K .
Representative copolymerizable fluorinated ethylenically unsaturated comonomers are represented by the formulas F Cl H ~
C=CF2 C=CF2 or C=CH2 Rl F R2 wherein Rl is -Rf, -Rf,X, -O-R~ " or -O-Rf,X in which R~ is a perfluoroalkyl radical of 1-12 carbon atoms, Rf, is a linear perfluoroalkylene diradical of 1-12 carbon atoms in 3t~¢~

which the attachin~ valences are at each end of 'che linear chain, and X is ~ or Cl; and ~2 is Rf or R~,X.
Representative copolymerizable fluorinated ethylenically unsaturated comonomer includes hexaf luoropropylene, perfluorohexene-l, perfluorononene-l, perfluoro(methyl vinyl ether), perfluoro(n-propyl vinyl ether), perfluoro(n-heptyl v inyl e the r ), per f luorome thyl e thylene, 10 perfluorobutyl ethylene, ~-hydroperfluoropentene-l, 3-hydroperfluoro(propyl vinyl ether), and the like, or mixtures thereof such as a mixture of hexafluoropropylene and perfluoro(propyl vinyl ether). Preferably the comonomers are selected from perfluoro(alkyl vinyl ethers) of the formula Rf-O-CF=CF2; ~r perfluoro(terminally unsaturated olefins) of the formula R~-CF=CF2; or perfluoroalkyl ethylenes of the formula Rf-CH=CH2, wherein Rf is alkyl of 1-5 carbon 20 a toms .
Comonomer content in the TFE/comonomer copolymers can range from O.OOS mole percent up to about 20 mole percent, and more than one comonomer can be present~ Thus the TFE/comonomer copolymers comprise both melt-processible TFE copolymer and nonmelt-processible TFE copolymer classes. The comonomer content is low ~nough that the copolymers are plasti¢s rather than elastomers, i.e., they are partially crystalline and after extrusion do not exhibit a rapid retraction to original length from a stretched condition of 2X at room temperature.
The aqueous batch dispersion polymerization of TFE by itself or with various comonomers is well known. The reaction medium consists of water, monomers, a dispersing agen~/ a free-radical 3()~

polymerization initiator, and, optionally, an unreactive fluorocarbon phase to promote monomer diffusion or to solubilize the initiator and a chain transfer agent such as a low molecular weight hydrocarbon. A high molecular weigh~ hydrocarbon wax of very low water solubility is sometimes used to reduce coagulation of the dispersion during polyme r i z a ~ ion .
Polymeriza~ion temperatures between 20~140~C
m2y be employed and pressures of 1.4-7.9 MPa are ordinarily used. Generally higher temperatures arld pressures are employed to promote polymerization rate, especially if a comonomer is unreactive relative to TFE. The TFE and ~ometimes the Gsmonomer are fed continuously to the reaction vessel ~o maintain reaction pressure, or in some instances the comonomer is all added initially and pressure is maintained with TFE feed only D The monomer~s) are fed until the desired final dispersion solids level (15-50%) is achieved. The agitator speed in the reaction vessel may be held constant during polymerization or it may be varied to control diffusion and thus polymerization rate.
Initiators commonly employed are free-radical initiators such as ammonium or potassium persulfate or disuccinic acid peroxide. The dispersing agent will be present in an amount between 0.01~0.5 percent based on weight of aqueous medium and preferably between 0.05-0.1 percent.
By the term "melt-processible" it is mean~
that the copolymer can be processed ( i . e., fabr icated into shaped articles such as films, fibers, tubes, wire coatings and the like) by conventional melt extruding means. Such requires that the melt 35 viscosity at the processing temperature be no more than 107 poise. Preferably it is in the range of 103 to 107 poise, and most preferably 104 to 106 poise.
Melt viscosities of the melt-processible 5 polymers were measured according to American Society for Testing and Materials test ~-1238-52T, modified as follows: The cylinder, orifice and piston tip are made of a corrosion-resistant alloy, Haynes Stellite 19*, made by Haynes Stellite Co. The 5.0 ~ sample is 10 charged to the 9.53 mm (0.375 inch) inside diameter cylinder, which is maintained at 372~C ~ 1C. Five minutes after the sample is charged to the cylinder, it is extruded through a 2010 mm (0.08~i inch) diame~er, ~.00 mm ~0.315 inch) long squareedge orifice under a load (piston plus weight) of 5000 grams. This corresponds to a shear stress of 44.8 XPa ( 6 . 5 pou nd s pe r s~ua re i nch). The melt viscosity in poises is calculated as 53170 divided by the observed extrusion rate in grams per minute.
The relatively low molecular weight melt-processible copolymers require that the comonomer contents be high enough to ass~re good resin physical properties such as flexural strength Generally, for perfluoro(alkyl vinyl ethers), this 2mount will be at least 0.4 mole percent of the copolymer, and can be up t.o about 3.6 percent.
Preferably the amount will be about 1.0-1.6 percent and the ether will be perfluoro(propyl vinyl ether) (PPVE). Generally, for the perfluoro(terminally unsaturated olefin), the amount will be at least about 5 mole percent of the copolymer, and can be up to about 15 mole percent. Preferably the amount will be about 6-9 mole percent and the olefin will be hexafluoropropylene (HFP).

*denotes trade ma~k .............. . .. .................... ........... ..... .. . . . ... .............................. .....

The HFP content in the melt~pr~essible TFE/HFP copolymers described herein was determined by measurement o~ the ratio of the IR absorbance at 10.18 ~m to the absorbance at 4.25 ~m. This ratio is referred to as the ~FP index or HFPI. Standard samples of known HFPI values were also run and a small correction was made, if necessary, to the test sample HFPI value. The mole percent ~FP present is equal to 2.1 times the 8FPI. Approximately 0O05 mm 10 thick compression molded films were scanned under a nitrogen atmosphere.
The PPVE content in the melt-processible TFE/PPVE copolymers described herein was also determined by Infrared Spectroscopy. The ratio of absorbance at 10.07 ~m to that at 4.25 ~m was determined using approxi.mately 0.05 mm thick compression molded films. This ratio was then used to determine percent PPVE by means of a calibration curve established with standard films of known PPVE
content.
The analysis for PPVE and ~FP in TFE/PPVE/HFP terpolymer is similar to that described above for the copolymers. However, the HFP and PPVE
absorbances severely overlap so that computerized deconvolution is necessary to estimate the individual absorbances. The deconvoluted absorbances are then used to determine comonomer concentrations similarly to that desc r i bed abov e .
By the term "nonmelt-processible" is meant a tetrafluoroethylen~ polymer or copolymer whose melt viscosity is so high that the polymer cannot be easily extruded by melt fabrication techniques.
Generally the higher the molecular weight of the polymer, the higher the melt viscosity. A melt viscosity above which tetrafluoroethylene polymers or copolymers are nonmelt-process ible is 1 x 109 poises. The ~elt viscosities of nonmelt-pxocessible polymers are so high that molecular weights are usually measured indirectly by a procedure which 5 gives the standard specific gravity (SSG) of the resin. The SSG of the resin varies inversely with molecular weight: as the molecular weight increases, ~he numer ical value of ~he SS~ decreases. Th~ SSG
values reported for the examples herein were deter~ined by the procedure described in U.S. Patent No. 3,142,665 except that 12 gram, instead of 3.5 gram, voidfree chips of the same diameter were employed.
The presence of comonomer in a PTFE
copolymer ten~s to depress the resin melting point and thus its use temperature. The nonmelt-processible copolymers contain only small a~ounts of comonomer to assure maintenance of a high melting point. Preferedly the level of comonomer in these nonmelt-processible copolymers is 0.01 to 0.30 mole percent.
The analysis of the very low HFP levels in the nonmelt-processible copolymers is accomplished by determining the ratio of IR absorbance at 10.18 ~m to that at 10.70 ~. This ratio is then multiplied by 0.21 to obtain the mole percent HFP in the resin.
According to the present invention, higher levels of comonomer incorporation into the TFE
copolymer are possible at a constant polymerization rate when the employed dispersing agent is a selected perfluoroalkylethane sulfonic acid or its salts or a perfluoroalkyl ethane sulfate salt.
Nonmeltproc~ssible PTFE homopolymers are generally used as coatings. A small raw dispersion particle size is advantageous in certain coRtings applications. The use of perfluoroalkyl ethane sulfonic acid (or salts thereof) as the dispersing agent in the aqueous dispersion polymerization of PTFE homopolymer affords smaller raw dispersion particle sizes than do heretofore employed ~urfactants. The raw dispersion particle sizes (average particle diameters) were measured by the light scattering procedure disclosed in U.S. Patent No. 3,391,099.
EXAMPLES_1 AND 2 AND COMPARISONS
A cylindrical, horizontally disposed, water-jacketed, paddle-st irred, stainless steel reactor having a length to diameter ratio of about 1.5 and a water capacity of 80 parts was charged with 55 parts of water and the desired type and level of dispersing agent shown in Table I. The mixture wa~
heated to 65C and then the reactor was evacuated and purged with TFE. The reactor ~empera~ure was then raised to g5C and agitation begun at 40-42 rpm. The reactor was pressured to the desired level (370-420 psig or 2.6-2.9 MPa) with HFP and then to 600 psig (4.1 MPa) with TFE. A freshly prepared solution (1.32 parts) of 0.0175M ammonium persulfate initiator was added to the reactor at the rate of 0.11 parts/minute to initiate polymerization kickoff and then a 0.023 to 0.024M potassium persulfate initiator solution was added at the rate of 0.022 parts/minute for the remainder of the batch. After polymerization starts, as indicated by an 0,07 MPa (10 psig) pressure drop, additional TFE was added to the reactor at the rate of 0.09 parts/minute until completion of the polymeri7ation (final solids levels of 21-24 percent were obtained). The agitator speed was varied as required to maintain a constant 600 psig t4.1 MPa) pressure level.

At the end of the reaction, the TFE feed and the agitator were turned off. Cooling water was circulated through the reactor jacket and the reactor was vented. The addition of initia~or solution was stopped, the reactor was purged of any residual monomer with nitrogen, and the aqueous copolymer dispersion discharged. The dispersion was coagulated by vigorous stirring to obtain a TFE/HFP copolymer fluff which was dried before analyses were carried out, Examples 1 and 2 and comparisons A and B are summarized in Table I. The higher HFP content obtained using the sulfa~e and sulfona~e dispersing agents of this invention (relative to the use of 15 anunonium perfluorocaE~rylate) are demonstrated in the Table O

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EX~MPLE 3 Into a clean, stainless steel, horizontal, agitated autoclave having a volume of 36 liters were placed 21.8 kg of demineralized water, 50.0 g of ammonium carbonate buffer and 45 g of a mixture of ammonium perfluoroalkyl(C4-C16)ethane sulfonates (Ave-C~). The autoclave was closed, evacuated, purged three times with tetrafluoroethylene (TFE) and evacuated again. Ethane was introduced into the clave until a 16.9 kPa rise in pressure occurred.
Then 165 mL of perfluoro(propyl vinyl ether) (PPVE) and 345 mL of Fr~eoff~ F-113 (CC12FCC1~2) were ~ucked into the clave. The agitator was turned on and the contents were heated to 80C. The autoclave 15 was pressured to 2.1 MPa with TFE and simultaneously a solution of 1.5 9 of ammonium persulfate (APS) dissolved in 500 mL of demineralized water was pumped into the clave. After polymerization kickoff had occurred (0.07 MPa drop in pressure), additional PPVE
and a solution of 1.2 g of APS in 1000 mL of water were pumped to the clave for the remainder of the pslymerization at the rates of lo 19 mL/minute and 10 mL/minute, respectively. The agitator speed was varied to control the reaction so that 50 g per minute of additional TFE was needed to maintain a constant 2.2 MPa pressure. After 7 kg of TFE had been added (measured after kickoff), the TFE and PPVE
feeds were stopped ~nd the agitator was turned off.
The initiator solution tAps) continued to be pumped 30 until the clave was vented of unreacted monomer. The coagulated polymer was dried at 150C. Analysis by infrared spectroscopy showed it contained 1.17 mole percent PPVE. Its melt viscosity at 372C was 4.0 x 103 poise.
. 35 The identic:al proeedure to the above was followed except that the perfluoroalkylethane sulfonate was replaced by the same weight o~ ammonium perfluorocaprylate~ In this case, polymer PPVE
5 content was found to be only 0.61 mole percent and the mel~ viscosity was 6.8 x 104 poise, Into a clean, stainless steel, horizontal, agitated autoclave having a volume of 36 liters was 10 charged ~4.5 kg of demineralized water. The autoclave was closed, evacuated, purged three times with TFE and evacuated again, Ethane was introduced into the clave until a 30O4 kPa rise in pressure occurred. The vacuum in the clave was then used to 15 suck in 45 mL of PPVE, 350 mL of Ereor~ F-113, and a solu~ion of 40 9 of a mixture of potassium perfluo;roalkyl(C4-C16)ethane sulfonates (Ave-CB) dissolved in 500 mL of water. Then 1 kg o~ ~FP was pressured into the clave and the mixture was heated to 80C with the agitator running at 50 rpm. After the temperature had lined out at 80C, the clave was pressured to 2.8 MPa with TFE. A
solution of 1.7 9 of APS in 300 mL of water was pumped into the clave over a 10 minute period and 25 then a solution of 2.0 g/liter of APS in demineralized water was pumped to the clave at the rate of 9 mL/minute for the remainder of the re ac tion . When the pumping o th e secc~nd ~PS
~olution was begun~ a pump was also activated to add PPVE to the clave at the rate of 0.7 mL/minuteO
After polymerization kickoff had occurred, additional TTE was added to the clave to maintain the 2.8 MPa pressure. The agitator speed was adjusted so as to react 79 g of TFE per minute. ~fter 8.7 kg of TFE
(measured after kickoff) had reacted, the TFE and PPVE feeds were cut off. The agitator and .inîtiator feeds were left on until the clave pressure dropped to 1.7 MPa. The feeds were then shut off and the c lav e wa s v en ted . Th e coag ula ted and d r ied polyme r 5 contained 2.38 mole percent ~FP, a PPVE content of 0.34 mole percent, and had a mel~ viscosity of 6.6 x 104 poise.

A horizontally disposed, water/steam jacketed, cylindrical stainless~steel autoclave lc~ated in a barr icade and having a capacity of 36 liters and a length-to-fliameter ratio of about 1.5 to 1, and provided with a 4-bladed cage-type agitator running the length of the autoclave, was evacuated and then charged with 3d,0 g of paraffin wax, 21.8 kg of demineralized water, 27 9 of a mixture of C4~C16 perfluoroalkyl ethane sulfonic (Ave-C8) acids dissolved in 198 g of water, 0~044 g of iron powder, and 0.044 g of copper powder. The autoclave 20 was the~ heated to 6SC, evacuated and purged wi~h tetrafluoroethylene~ after which 15 9 disuccinic acid peroxide dissolved in about 100 mL water and then 25 mL of hexafluoropropylene were added. The autoclave temperature was set at 88C and the agitator was turned on. When the temperature reached the set point, the autoclave was pressured up with TFE to 350 psig (2.4 MPa~. After "kickoff" (0.57 MPa drop in pressure), the clave temperature and pressure were raised to 90C and 2.8 MPa, respe~tively. TFE was then fed to the autoclave to maintain the reaction pressure at 2 .8 MPa until a total of 11. 8 kilograms of TFE had been added to the autoclave. The TFE feed was then cut off and the pressure was allowed to decrease to 1.2 MPa before the agitator was stopped 35 and the vapor space of the reactor was vented. The

3(~
polymerization time from Icickoff to the time feed was turned of was 87 minu'ces.
The resulting dispersion was discharged from the autoclave and cooled, after which the supernatan'c 5 solid paraffin wax was r~moved. The dispersion contained about 3S.6 percenl: solids and had a raw dispersion particle size of O.lû5 llm on average. The coa~ulated polymer had a SSG value of 2.215 and a HFP
~ content of 0.078 mole percent. A control using 10 ammonium perfluo~oc~p~ryla~e as the dispersing a~ent contained 0 .07 4 mole percent HFP .

A horizontally ~isposed, water/steam jacketed, cylindrical stainless~steel autoclave 15 lc>cated in a barricade and having a capacity of 36 liters and a length-to-diameter ratio of about 1. 5 to 1, 2nd provided with a 4-bladed cage-type agitator running the length of the autoclave, was evacuated and then charged with 855 g of paraffin wax, 22.0 ks 20 sf demineralized water, 30 g of potassium perfluoroalkyl ethane sulfonate IAve-C8) dispersing agent, and 0.044 9 of iron powder. The autoclave was then heated to 65~C, evacuated and purged with tetrafluoroethylene, after which 15 9 disuccinic acid peroxide dissolved in about 100 ml water and 0.6 g of methanol in 100 ml o~ water were added. The autoclave temperature was then set to 90~C and the agitator speed was star~ed at 46 rpm. The autoclave was pressured to 380 psig (2.6 MPa) ~ith TFE starting when the temperature was BO~C. The TFE was added slowly t3-5 minutes) so that the temperature was just 90~C when the pressure-up was completed. Reaction occurred and after the pressure had dropped to 300 psig ~2.1 MPa) (nkick-off") r TFE was fed to the autoclave to maintain the reaction pressure at ............................................................................................................................ .. .. .

335 psig (2.3 MPa) until a total of 11.8 kilograms of TFE had been added ~o the autoclave. The TFE feed was then cut off and the pressure was allowed to decrease to 175 psig (1.2 MPa~ before the agitator 5 was stopped and the vapor space of the reactor was vented. The polymerization time from kickoff to the time feed was turned off was 103 minu'ces.
The re~ulting dispersion was discharged from the autoclave and cooled, after which the supernatant 10 solid paraff in wax was removed . The dispersion contained about 35.0 percent solids and had a raw dispersion particle size o 0.150 ~m on average. The coagula~ed polymer had an SS~: value of 2.218. A
control which employed a~unonium perflurocaprylate as 15 the dispersing agent produced a dispersion having a raw dispersion particle size of 0.175 ~m on average.

2~

Claims

CLAIMS:

1. In the batch process for preparing melt-processible copolymers of tetrafluoroethylene and at least one copolymerizable fluorinated ethylenically unsaturated comonomer of the formula wherein R1 is -Rf, -Rf,X, -O-Rf,, or -O-Rf,X in which Rf is a perfluoroalkyl radical of 1-5 carbon atoms, -Rf, is a linear perfluoroalkylenedi-radical of 1-5 carbon atoms in which the attaching valences are at each end of the linear chain, and X
is H or C1; and R2 is -Rf or Rf,X, by polymerizing tetrafluoroethylene and at least one said comonomer in an aqueous polymerization medium containing a free-radical initiator and 0.01-0.5 percent dispersing agent, based on weight of aqueous medium, the improvement which comprises employing as the dispersing agent 1) a mixture of compounds of the formula F?CF2-CF2?nCH2-CH2-Y
wherein n is a cardinal number of 2-8 and the average value of n is between 3 and 6 or 2) a compound of said formula wherein n is a cardinal number selected from between 2-6, and Y is -SO3M or -OSO3M' wherein M is a cation having a valence of 1 and M' is an alkali metal cation or ammonium.

2. The process of Claim 1 wherein Y is -OSO3M'.

3. The process of Claim 1 wherein Y is -SO3M.

4. The process of Claims 1, 2 or 3 wherein M is H+, NH+ , Na+, Li+ or K+.

5. The process of Claims 1, 2 or 3 wherein the comonomer is a perfluoro(alkyl vinyl ether).

6. The process of Claims 1, 2 or 3 wherein the comonomer is a perfluoro(terminally unsaturated olefin).

7. The process of Claims 1, 2 or 3 wherein the comonomer is a perfluoroalkyl ethylene.

8. The process of Claims 1, 2 or 3 wherein the comonomer is hexafluoropropylene.

9. The process of Claims 1, 2 or 3 wherein the comonomer is perfluoro(propyl vinyl ether).

10. The process of Claims 1, 2 or 3 wherein the comonomer is a mixture of hexafluoropropylene and perfluoro(propyl vinyl ether).

11. In the batch process for preparing tetrafluoroethylene homopolymer or nonmelt-processible copolymers of tetrafluoroethylene and at least one copolymerizable fluorinated ethylenically unsaturated comonomer of the formula wherein R1 is -Rf, -Rf,X, -O-Rf,, or -O-Rf,X in which R is a perfluoroalkyl radical of 1-5 carbon atoms, -Rf, is a linear perfluoroalkylenedi-radical of 1-5 carbon atoms in which the attaching valences are at each end of the linear chain, and X
is H or C1; and R2 is -Rf or Rf,X, by polymerizing tetrafluoroethylene alone or with at least one said comonomer in an aqueous polymerization medium containing a free-radical initiator and 0.01-0.5 percent dispersing agent, based on weight of aqueous medium, the improvement which comprises employing as the dispersing agent 1) a mixture of compounds of the formula F?CF2-CF2?nCH2-CH2-S03M
wherein n is a cardinal number of between 2-8 and the average value of n is between 3-6, or 2) a compound of said formula wherein n is a cardinal number selected from 2-6, and M is a cation having a valence of 1.

12. In the batch process for preparing homopolymers of tetrafluoroethylene and copolymers of tetrafluoroethylene and at least one copolymerizable fluorinated ethylenically unsaturated comonomer of the formula wherein Rl is -Rf, -Rf,X, -O-Rf,, or -O-Rf,X in which Rf is a perfluoroalkyl radical of 1-5 carbon atoms, -Rf, is a linear perfluoroalkylenedi-radical of 1-5 carbon atoms in which the attaching valences are at each end of the linear chain, and X
is H or Cl; and R2 is -Rf or Rf,X, by polymerizing tetrafluoroethylene alone or with at least one said comonomer in an aqueous polymerization medium containing a free-radical initiator and 0.01-0.5 percent dispersing agent, based on weight of aqueous medium, the improvement which comprises (A) when the process is for the preparation of melt-processible copolymers of tetrafluoro-ethylene and at least one said comonomer, employing as the dispersing agent 1) a mixture of compounds of the formula F?CF2-CF2?nCH2-CH2-Y
wherein n is cardinal number of 2-8 and the average value of n is between 3 and 6 or 2) a compound of said formula wherein n is a cardinal number selected from between 2-6, and Y is -SO3M or -OSO3M' wherein M is a cation having a valence of 1 and M' is an alkali metal cation or ammonium, and (B) when the process is for the preparation of homopolymers of tetrafluoroethylene or nonmelt-processible copolymers of tetrafluoro-ethylene and at least one said comonomer, employing as the dispersing agent 1) a mixture of compounds of the formula F?CF2-CF2?nCH2-CH2-SO3M
wherein n is cardinal number of between 2-8 and the average value of n is a cardinal number selected from 2-6, and M is a cation having a valence of 1.