Process for the preparation of perfluoropolyoxyalkylenes...

Details Process for the preparation of perfluoropolyoxyalkylenes... Process for the preparation of perfluoropolyoxyalkylenes...

2000-12-29

2003-01-21

Davis, Brian J. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

C568S614000, C568S615000, C568S677000, C568S683000, C568S864000

Reexamination Certificate

active

06509509

ABSTRACT:

The invention relates to a process for the preparation of perfluoropolyoxyalkylenes functionalized with hydroxyl end groups.
Specifically the invention relates to a process for the preparation of monofunctional and bifunctional perfluoropolyoxyalkylenes having end groups of —CH
2
OH type starting from the corresponding perfluoropolyoxyalkylenes having —COOR end groups (R=C
1
-C
5
alkyl). Said process has a higher productivity since in the reduction reaction a lower amount of reducing agent is used. Further a lower hydrogen development is noticed during the reaction. This makes the process better controllable from the industrial safety point of view and economically cheaper.
Monofunctional and bifunctional perfluoropolyoxyalkylenes having hydroxyl end groups of the —CH
2
OH type are known in the prior art.
These compounds are described, for example, in U.S. Pat. No. 3,766,251 and U.S. Pat. No. 3,810,874 and they can be used for example as lubricants for magnetic discs or as macromonomers for the preparation of polyurethane, polyester, polyether copolymers, epoxy resins, etc.
The perfluoropolyethers with hydroxyl end groups can be prepared by reduction of the corresponding ester precursors, i.e. starting from perfluoropolyethers having one or two end groups —COOR wherein R is a linear or branched C1-C5 alkyl. Said process is described for example in the patents GB 1,309,401, U.S. Pat. No. 3,847,978.
Generally in the known processes, the reduction reaction is carried out by using reducing agents in inert solvents. Among the reducing agents metal hydrides, such as for example lithium-aluminum hydride (LiAlH
4
), dissolved in inert solvents such as tetrahydrofuran, diglyme or dioxane can be mentioned; preferably as reducing agent sodium-boron hydride (NaBH
4
) in the above mentioned inert solvents or in protic solvents such as methanol, ethanol, is used.
When NaBH
4
is used as reducing agent, the stoichiometry of the reduction reaction of the ester precursor to the corresponding perfluoropolyether with hydroxyl end groups CH
2
OH, implies the use of two equivalents of ester by mole of NaBH
4
and it does not involve hydrogen formation.
The process can be schematized with the following reactions, by using sodium-boron hydride as reducing agent and a bifunctional ester with both end groups —COOC
2
H
5
as precursor:
1) 4[Rf(COOCH
2
CH
3
)
2
]+4NaBH
4
→[R
f
(CH
2
O)
2
]
3
B
2
+R
f
(CH
2
ONa)
2
++2(CH
3
CH
2
O)
4
B
−
2) [R
f
(CH
2
O)
2
]
3
B
2
+R
f
(CH
2
ONa)
2
+2(CH
3
CH
2
O)
4
B
−
+4HCl+12H
2
O→4R
f
(CH
2
OH)
2
+8CH
3
CH
2
OH+4H
3
BO
3
+4NaCl.
However tests carried out by the Applicant show (see the comparative Examples) that said reduction reaction in order to reach an almost total conversion (higher than 99%), requires a large excess of reducing agent NaBH
4
, generally higher than 60% with respect to the stoichiometric value. Another drawback is that during the reduction step a remarkable hydrogen amount develops, in the range 1.5-2 moles of hydrogen for each equivalent of ester. The large excess of reducing agent necessary for the reduction reaction and the remarkable volume of hydrogen developed during the reaction, result critical both from an economic point of view, due to the cost of the reducing agent, and from a point of view of to the process safety in an industrial plant. In fact the gaseous effluents must be diluted with large volumes of inert gas in order to bring the composition of the gaseous mixture, in particular the hydrogen concentration, under the inflammability and explosiveness limits (lower than 4% by volume).
The need was therefore felt to prepare functional perfluoropolyethers having —CH
2
OH end groups by a cheaper process from the economic point of view, furthermore characterized by a greater production combined with a greater reliability from the industrial safety point of view.
The Applicant has surprisingly and unexpectedly found that the reduction reaction to obtain perfluoropolyethers having hydroxyl —CH
2
OH end groups, starting from the corresponding ester precursors, can be advantageously carried out by adding in the reaction mixture besides the reducing agent, an organic base. It has been unexpectedly found that the presence of this organic base allows to carry out the reducing reaction by using a small excess of the reducing agent with respect to the stoichiometric amount, generally lower than 15%, and surprisingly that it inhibits the hydrogen development during the process.
It is therefore an object of the present invention a process for obtaining monofunctional and bifunctional perfluoropolyoxyalkylenes having hydroxyl end groups with structure:
T
1
—CFW
1
—O—R
f
—CFW
2
—T
2
  (I)
wherein:
T
1
, T
2
, equal to or different from each other, are selected from end groups of the —CH
2
OH, or —F, —CF
3
, —CF
2
CF
3
type with the proviso that at least one of T
1
and T
2
is —CH
2
OH;
W
1
and W
2
, equal to or different from each other, are F, CF
3
;
R
f
is a perfluoropolyoxyalkylene chain having a number average molecular weight 500-10,000 comprising one or more units of the —CF
2
CF
2
O—, —CF
2
O—, —C
3
F
6
O—, —CF
2
(CF
2
)
z
CF
2
O—, —CR
4
R
5
CF
2
CF
2
O— type;
wherein: z is an integer equal to 1 or 2;
R
4
and R
5
, equal to or different from each other, are selected from H, Cl, linear or branched perfluoroalkyl groups having a number of C atoms from 1 to 4;
said units being statistically distributed along the perfluoropolyoxyalkylene chain;
said process comprising the following steps:
a) adding, under inert gas atmosphere, an ester precursor having structure (I) and T
1
, T
2
end groups, equal to or different from each other, selected from —F, —CF
3
, —CF
2
CF
3
, —COOR (R=linear or branched alkyl group having a number of C atoms from 1 to 5), with the proviso that at least one of T
1
and T
2
is —COOR, in a reaction mixture formed by:
an inert or protic reaction solvent;
a reducing agent selected from the metal hydride class;
a non reactive organic base with respect to the ester;
b) hydrolysis of the product obtained in step a) for obtaining the compounds of structure (I) with at least a —CH
2
OH end group.
The organic bases used in the process of the present invention are selected from alcoholates and thioalcoholates of alkaline or alkaline-earth metals, having alkyl, cycloaliphatic, aromatic chain or combinations thereof; or nitrogenous bases which do not give condensation reactions with the fluorinated ester, such as for example amidation reactions. The following bases are preferably used: sodium or potassium alcoholates such as for example potassium terbutylate or sodium ethylate, more preferably sodium ethylate; in the class of the nitrogenous bases, alkyl, cycloalipahtic and aromatic tertiary amines, such as for example triethylamine, diazabicyclo-octane (DABCO), pyridine, can be mentioned. Also a mixture of the above mentioned organic bases can be used. Preferably as organic bases the class of the above mentioned alcoholates is used.
As solvents of the reaction inert solvents, such as for example, tetrahydrofuran, diglyme, dioxane or protic solvents such as for example methanol, ethanol, isopropanol, isobutanol or mixtures thereof, preferably ethanol, etc., can be used. Also mixtures of inert solvents and protic solvents can be used.
As reducing agents metal hydrides, such as for example lithium-aluminum hydride (LiAlH
4
), sodium-boron hydride (NaBH
4
), lithium-boron hydride, etc., dissolved in the above mentioned reaction solvents, can be used. Preferably as reducing agent, sodium-boron hydride is used.
In formula (I) R
f
preferably comprises the following repeating units statistically distributed along the polymer chain:
—(CF
2
CF
2
O)—, —(CFYO)— wherein Y is equal to F or CF
3
,
—(C
3
F
6
O)—,
—(CF
2
(CF
2
)
z
O)— wherein z is an integer equal to 2 or 3,
—CR
4
R
5
CF
2
CF
2
O— wherein R
4
and R
5
, equal to or different from each other are selected from H, Cl or perfluoroalkyl group having

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