Preparation of hexafluoropropene oxide polymers

Details Preparation of hexafluoropropene oxide polymers Preparation of hexafluoropropene oxide polymers

2000-12-05

2002-07-16

Truong, Duc (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From fluorine-containing reactant

C528S220000, C528S275000, C528S425000, C528S491000, C524S755000, C524S757000, C524S795000, C524S845000

Reexamination Certificate

active

06420515

ABSTRACT:

This invention relates to a method for preparing hexafluoropropene oxide (referred to as HFPO, hereinafter) polymers, and more particularly to a method for preparing essentially difunctional HFPO polymers having a minimized content of monofunctional HFPO polymer.
BACKGROUND OF THE INVENTION
One prior art method for preparing difunctional HFPO polymers is described in U.S. Pat. No. 3,250,807. This method is to react FOC—(CF
2
)
n
—COF wherein n is from 0 to 6 with HFPO in an aprotic polar solvent in the presence of a catalyst such as an alkali metal fluoride or activated carbon, thereby forming difunctional HFPO polymers, as shown by the following reaction scheme.
An attempt to add HFPO to previously furnished —COF groups as above, however, often gives rise to the problem that chain transfer side reaction occurs to form a HFPO polymer having a hexafluoropropyl group at one end (monofunctional HFPO polymer) as shown by the following scheme.
An improved method for preparing essentially difunctional HFPO polymers while preventing such chain transfer is disclosed in U.S. Pat. No. 3,660,315 or JP-B 53-5360. This method involves mixing the compound of the formula:
FOCCF(CF
3
)OCF
2
CF
2
OCF(CF
3
)COF   (3)
with cesium fluoride in tetraethylene glycol dimethyl ether to form the compound of the formula:
CsOCF
2
CF(CF
3
)OCF
2
CF
2
OCF(CF
3
)CF
2
OCs   (4),
and removing the excess of cesium fluoride from the solution, thereby forming a uniform solution, which is used as an initiator for the polymerization of HFPO. Specifically, after the excess of cesium fluoride is separated off, polymerization is effected at a low temperature of −60° C. to −30° C., thereby forming pure difunctional HFPO polymers having a number average molecular weight of about 50.
However, it is described in J. Macromol. Sci. Chem., A8(3), 499 (1974) that if the molar ratio of HFPO to the initiator is increased in order to produce difunctional HFPO polymers having a higher degree of polymerization, the side reaction to produce a monofunctional HFPO polymer increases and the purity of difunctional HFPO polymers lowers.
U.S. Pat. No. 4,356,291 or JP-A 57-175185 describes that a HFPO polymer having a number average molecular weight of 445 is obtained using highly purified HFPO in addition to the above initiator. It is pointed out that HFPO generally contains impurities such as hydrogen fluoride, acid fluorides and water, which limit the maximum degree of polymerization of polymers resulting from polymerization of HFPO. Then by subjecting highly purified HFPO to polymerization, a HFPO polymer having a high molecular weight is produced. However, no reference is made therein to the by-produced monofunctional HFPO polymer and the purity of the desired difunctional HFPO polymer.
Understandably, the prior studies on difunctional HFPO polymers placed a main focus on the reduction of undesired monofunctional HFPO polymers resulting from chain transfer and the formation of HFPO polymers having a high degree of polymerization.
All these methods, however, have the drawback that the compound of formula (3) itself contains monofunctional impurities. More particularly, the compound of formula (3) is generally prepared by the following method.
Upon reaction of oxalic fluoride with HFPO, there are produced not only the end compound of formula (3), but also HFPO oligomers as shown by formulas (3′) and (3″). A precise distillation operation is necessary to separate these oligomers from the end compound. Still worse, the end compound purified by such a precise distillation operation yet contains about 4 to 6% by weight of the monofunctional component having a cyclic structure shown by the above formula (5). Undesirably, since this by-product of formula (5) has the same molecular weight as the end compound of formula (3), it is almost impossible in practice to separate the by-product by further distillation. Use of the fraction resulting from distillation as the initiator means that the monofunctional component already exists prior to the polymerization of HFPO.
On the other hand, known perfluorodicarboxylic fluorides include perfluoroadipic fluoride, perfluoroglutaric fluoride and perfluorosuccinic fluoride. If a polymerization initiator is prepared from these compounds in the same manner as the compound of the above formula (3), side reaction such as esterification can take place, failing to obtain an alcoholate equivalent to the perfluorodicarboxylic fluoride added. If polymerization of HFPO is carried out using this polymerization initiator, there are produced polymers having a wide molecular weight distribution because of the increased content of low molecular weight components. This polymerization initiator is inadequate.
Under the circumstances, it is desired in the polymerization of HFPO to prepare a polymerization initiator using a starting reactant which is available at a relatively low cost, which quantitatively forms an alcoholate with an alkali metal fluoride in an aprotic polar solvent and which is free of monofunctional impurities.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for preparing difunctional HFPO polymers having a minimized content of monofunctional HFPO polymer, using a polymerization initiator prepared from a starting reactant which is available at a relatively low cost.
We have found that when a perfluorodicarboxylic fluoride or perfluorodiketone of the general formula (1) or (2) shown below is mixed with an alkali metal fluoride in an aprotic polar solvent, there is obtained a uniform solution in which a quantitative amount of an alcoholate is formed. Subsequent polymerization of HFPO using this solution as a polymerization initiator results in a difunctional HFPO polymer which is substantially free from the terminal ether structure based on the compound of the above formula (5), has a narrow molecular weight distribution, and has a minimized content of monofunctional HFPO polymer.
Accordingly, the invention provides a method for preparing a hexafluoropropene oxide polymer comprising the steps of mixing a perfluorodicarboxylic fluoride or perfluorodiketone of the following general formula (1) or (2) with a metal fluoride in an aprotic polar solvent, and feeding hexafluoropropene oxide to the resulting solution.
Herein Rf is a perfluoroalkylene group which may be separated by an oxygen atom, and R
1
, which may be the same or different, is a perfluoroalkyl group of 1 to 8 carbon atoms.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The method for preparation of HFPO polymers according to the invention uses as the polymerization initiator a solution which is prepared by mixing a perfluorodicarboxylic fluoride or perfluorodiketone of the general formula (1) or (2) with an alkali metal fluoride in an aprotic polar solvent. More particularly, the polymerization initiator is prepared by suspending an alkali metal fluoride in an aprotic polar solvent, adding a perfluorodicarboxylic fluoride or perfluorodiketone to the suspension, and agitating the mixture.
The alkali metal fluoride used herein is preferably cesium fluoride. Examples of the aprotic polar solvent include glymes such as monoglyme, diglyme, triglyme and tetraglyme, tetrahydrofuran and 1,4-dioxane, with the glymes being especially preferred.
With respect to the perfluorodicarboxylic fluoride or perfluorodiketone used herein, an inexpensive hydrocarbon dicarboxylic acid or hydrocarbon diester of the following general formula (6) or (7) is fluorinated by well-known fluorinating methods (including direct fluorination and electrolytic fluorination) to form a corresponding perfluoro compound of the general formula (1) or (2).
In the formulas, R
2
is an alkylene group which may be separated by an oxygen atom, Rf is a perfluoroalkylene group obtained by substituting fluorine atoms for all the hydrogen atoms in R
2
. R
3
is an alkyl group of 1 to 8 carbon atoms, the R
3
groups may be the same or different, and R
1
is a perfluoroalkyl group of 1 to 8 carbon atoms obtained by substituti

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