WO2016020232A1 - Lubrication method with alkoxylated perfluoropolyether - Google Patents

Abstract

A lubrication method comprising applying to a surface to be lubricated a polymer comprising a partially or fully fluorinated, straight or branched, polyoxyalkylene chain (chain R_f) having two chain ends, wherein one or both chain end(s) bear(s) a hydroxy- or alkoxy-terminated polyoxyalkylene chain free from fluorine atoms (chain R_a), said chain comprising from 4 to 50 fluorine-free oxyalkylene units, said units being the same or different from one another and being selected from -CH₂CH₂O- and -CH₂CH(J)O-, wherein J is independently straight or branched alkyl or aryl, with the proviso that, if both chain ends bear a hydroxy-terminated chain Ra comprising only -CH₂CH₂O- units, chain Rf does not consist only of -CF₂CF₂O-units. Lubricant compositions containing the polymer and methods for preparing the polymer are also herein disclosed.

Description

Lubrication method Cross reference to related applications

This application claims priority from European application No. 14179867.8 filed on 5^th August 2014, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

The present invention relates to a lubrication method, to polymers for use in such method and to lubricant compositions containing such polymers.

Background Art

Polyoxyalkylene glycols (in the following referred to as “PAGs”) are used in a variety of applications, such as the lubrication of gears, transmission systems, air conditioning (A/C) systems, metalworking fluids as well as hydraulic fluids. For this purpose, PAGs can be formulated as aqueous or non-aqueous compositions containing specific additive packages to improve their performances. However, it has been observed that, when PAGs are used as base oils in such compositions, they do not provide satisfactory performances for applications requiring harsh conditions, such as high temperatures and extreme friction conditions.

(Per)fluoropolyethers (in the following referred to as “PFPEs”) have been also long since known as base oils or as additives in several lubricant applications. However, despite their outstanding performances under harsh conditions, they have scarce compatibility with mineral and synthetic hydrogenated lubricants, which limits their use in formulations with conventional lubrication systems.

US 7230140 ASAHI GLASS COMPANY, LIMITED 20050908 discloses a PFPE derivative of the formula (I):
HO-(CH₂CH₂O)_r(CH₂CH(OH)CH₂O)_p-CH₂CF₂O(CF₂CF₂O)_mCF₂CH₂O-(CH₂CH(OH)CH₂O)_q(CH₂CH₂O)_s-H (I)
wherein:
m is an integer of from 3 to 200,
each of r and s, which are independent of each other, and is an integer of from 0 to 100, and each of p and q, which are independent of each other and, is an integer of from 0 to 100.

The derivative of formula (I) is said to be useful as lubricating oil or as coating agent and is said to be less likely to undergo decomposition, and to be free from deterioration during its use. It is worth noting that the PFPE backbone of the derivative of formula (I) comprises only -CF₂CF₂O- repeating units; indeed, in this document it is stated that PFPE derivatives comprising also -CF₂O- units further contain -OCF₂O- units that may cause decomposition or deterioration (reference is made to col. 1, lines 21 – 25).

Thus, this prior art document teaches to improve the stability of PFPE lubricants by selecting a PFPE chain without -CF₂O- units. The description discloses in particular a preferred compound of formula:
HO(CH₂CH₂O)_rCH₂CF₂O(CF₂CF₂O)_mCF₂CH₂O(CH₂CH₂O)_sH (Ic)
wherein r and s are simultaneously 1
and a compound of formula:
HO(CH₂CH(OH)CH₂O)_pCH₂CF₂O(CF₂CF₂O)_mCF₂CH₂O(CH₂CH(OH)CH₂O)_qH (Id)
wherein p and q are simultaneously 1. Examples 6-1, 6-2 and 7 illustrate the synthesis of certain compounds complying with formulae (Ic) and (Id) above. Nevertheless, this prior art does not teach to prepare compounds complying formula (I) having r and s higher than 1.

US 2006252910 ASAHI GLASS COMPANY, LIMITED 20061109 relates to fluoropolyether compounds complying with formula (A) below:
(A) (X-)_xY(-Z)_z
wherein:
- X is a group represented by the following formula (X):
HO-(CH₂CH₂O)_a(CH₂CH(OH)CH₂O)_b-(CH₂)_c-CF₂O(CF₂CF₂O)_d- (X)
wherein a is an integer of from 0 to 100, b is an integer of from 0 to 100, c is an integer of from 1 to 100, and d is an integer of from 1 to 200;
- Z is a group represented by the following formula (Z):
R^FO(CF₂CF₂O)_g- (Z)
wherein R^F is a C₁-₂₀ perfluoroalkyl group or a group having an etheric oxygen atom inserted between carbon-carbon atoms of such a perfluoroalkyl group (the group has no -OCF₂O- structure), and g is an integer of from 3 to 200;
- Y is a (x+z) valent perfluorinated saturated hydrocarbon group, or a (x+z) valent perfluorinated saturated hydrocarbon group having an etheric oxygen atom inserted between carbon-carbon atoms, having no -OCF₂O- structure;
- X, Z: x is an integer of at least 2, z is an integer of at least 0, and (x+z) is an integer of from 3 to 20, provided that when x is at least 2, groups represented by the formula (X) may be the same or different, and when z is at least 2, groups represented by the formula (Z) may be the same or different (reference is made in particular to [0015] – [0019]). This document teaches that fluoropolyether compounds (A) are useful, inter alia, as lubricants, especially in the form of solutions with organic solvents (par. [0013] and [0024]), and that they are less subject to deterioration, as they do not contain -OCF₂O- units in their molecular structure. Indeed, in formula (A), group Y is an at least trivalent group which does not contain -OCF₂O- units. Thus, this document teaches away from PFPE lubricants comprising -CF₂O- repeating units. In addition, this prior art does not teach to synthesise fluoropolyether compounds having more than one ethoxylated units in the chain ends.

US 2008132664 ASAHI GLASS COMPANY, LIMITED 20080605 relates to an ether composition comprising a polyether compound (A) and an ether compound (B), said composition being useful, inter alia, as lubricant (see par. [0007] and [0025]). Examples of ether compounds (B) comply with formulae (B-4) and (B-5) below:
(B-4)
HOCH₂CH(OH)CH₂OCH₂CF₂O(CF₂CF₂O)_d7(CF₂O)_g2CF₂CH₂OCH₂CH(OH)CH₂OH
(B-5)
HOCH₂CH₂OCH₂CF₂O(CF₂CF₂O)_d8(CF₂O)_g3CF₂CH₂OCH₂CH₂OH.

In formula (B-4), d7 is a positive number of at least 1, g2 is a positive number of at least 0, and the average molecular weight of the compound represented by formula (B-4) is from 500 to 2,000;

In formula (B-5), d8 is a positive number of at least 1, g3 is a positive number of at least 0, and the average molecular weight of the compound represented by the formula (B-5) is from 500 to 2, 000.

It is worth noting that polyether compound (A) contains at least two -CF₂CF₂O- units and does not contain -OCF₂O- units and that this document teaches that high chemical stability can be achieved thanks to the presence of such compound (A). Furthermore, compound (B-4) comprise only one -OCH₂CH(OH)CH₂- unit at each end of the PFPE chain, while compound (B-5) comprises only one –OCH₂CH₂- unit at each chain end of the PFPE chain.

US 3810874 MINESOTA MINING AND MANUFACTURING COMPANY 19740515 discloses linear polyfunctional-terminated poly(perfluoroalkyleneoxide) compounds of formula:
A-[CF₂-O-(CF₂CF₂O)_m-(CF₂O)_n-CF₂]A'
wherein A or A' could be -CH₂OH, or -CH₂OCH₂CH(OH)CH₂OH; and the ratio m/n is 0. 2/1 to 5/1, preferably 0. 5/1 to 2/1.

These compounds are said to be suitable for use as lubricants and as viscosity index additives for perhalogenated lubricants. However, this document does not disclose or suggest PFPE derivatives bearing a plurality of oxyalkylene end units at each end of the PFPE chain.

WO WO 2013/086264 DU PONT 20130613 teaches that Fomblin^® PFPE lubricants to be used in the compositions of the invention are functionalized PFPEs that range in molecular weight from 500 to 4000 atomic mass units and have general formula:
X-CF₂-O(CF₂-CF₂-O)-(CF₂O)_q-CF₂-X,
wherein X may be -CH₂OH, CH₂(O-CH₂-CH₂)_nOH, CH₂OCH₂CH(OH)CH₂OH or -CH₂O-CH₂-piperonyl (page 22, lines 4 – 8).

This document does not mention or suggest Fomblin^® PFPE lubricants terminated with a specific number of ethylene oxide and/or longer oxide units.

WO WO 2014/090649 SOLVAY SPECIALTY POLYMERS ITALY S.P.A. discloses a process for the alkoxylation of (per)fluoropolyether alcohols. More in particular, the exemplified ethoxylated PFPE diols have the following structure:
HO(CH₂CH₂O)_pCH₂CF₂O(CF₂CF₂O)_n(CF₂O)_mCF₂CH₂(OCH₂CH₂)_pOH.

WO WO 93/19142 THE BRITISH PETROLEUM COMPANY PLC discloses a lubricating oil composition comprising (1) a lubricating base oil and (2) a monofunctional PFPE ester derivative, which is obtained by the reaction of a PFPE carboxylic acid with one poly(alkylene oxide) alcohol.

Brief description of drawings

Figure 1 illustrates the results of a thermogravimetric analysis (TGA) carried out on two polymers according to the present invention.

Summary of invention

The Applicant has now surprisingly found out that polymers comprising a (per)fluoropolyether chain having two chain ends, wherein one or both chain ends bear a definite number of oxyalkylene units free from fluorine atoms show higher thermal stability, improved lubrication properties (such as lower wear) and improved performances at extreme pressures with respect to PAGs. It has also been observed that such polymers are endowed with improved compatibility with PAGs. Therefore, they are suitable for use in lubrication methods comprising applying such polymers or compositions containing them to a surface to be lubricated.

The polymers for use in the method of the invention comprise a partially or fully fluorinated, straight or branched, polyoxyalkylene chain (chain R_f) having two chain ends, wherein one or both chain end(s) bear(s) a hydroxy-, alkoxy- or acyloxy- terminated polyoxyalkylene chain free from fluorine atoms (chain R_a), said chain comprising from 4 to 50 fluorine-free oxyalkylene units, said units being the same or different from one another and being selected from -CH₂CH₂O- and -CH₂CH(J)O- , wherein J is independently straight or branched alkyl or aryl, preferably methyl, ethyl or phenyl, with the proviso that, if both chain ends bear a hydroxy-terminated chain R_a comprising only -CH₂CH₂O- units, chain R_f does not consist only of -CF₂CF₂O- units.

The polymers for use in the method of the invention will be herein after also generically referred to as "PFPE-PAGs" and, more specifically, as “mono- or bi-functional "PFPE-PAGs, depending on whether one or both chain ends bear a chain R_a.

The invention further relates to lubricant compositions comprising the PFPE-PAGs and to methods for preparing the PFPE-PAGs.

Definitions

For the purposes of the present description, the term “(per)fluoropolyether” stands for “fully or partially fluorinated polyether".

The acronym PFPE stands for (per)fluoropolyether.

The term "(halo)alkyl” denotes a hydrocarbon group wherein one or more hydrogens can be replaced by one or more halogen atoms, preferably fluorine atoms.

The indefinite article “a” stands for “one or more”, unless indicated otherwise. Substantives in the plural form are to be construed as including also the singular form, i.e. as “one or more”, unless indicated otherwise. For example, the term “compounds” is to be construed as “a compound” or “one or more compounds”, unless indicated otherwise.

When ranges are indicated, range extremes are included.

Typically, the PFPE-PAGs for use in the method of the invention comply with formula (I) below:
A-O-R_f-(CF₂)_x-CFZ-CH₂-O-R_a (I)
wherein:
- R_f is a (per)fluoropolyoxyalkylene chain having an average number molecular weight M_n ranging from 100 to 8,000, preferably from 300 to 6,000, more preferably from 800 to 3,000, and comprising, preferably consisting of, repeating units, which may be equal to or different from one another, selected from:
(i) -CFXO-, wherein X is F or CF₃,
(ii) -CFXCFXO-, wherein X, equal or different at each occurrence, is F or CF₃, with the provision that at least one of X is –F,
(iii) -CF₂CF₂CW₂O-, wherein each of W, equal or different from each other, are F or H,
(iv) -CF₂CF₂CF₂CF₂O-,
(v) –(CF₂)_j-CFZ’-O- wherein j is an integer from 0 to 3 and Z’ is a group of general formula –OR_f’T, wherein R_f’ is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFXO- , -CF₂CFXO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X being independently F or CF₃ and T being a C₁-C₃ perfluoroalkyl group;
- Z is fluorine or CF₃;
- x is 0 or 1, with the proviso that, when, x is 1, Z is F;
- A is -(CF₂)_x-CFZ-CH₂-O-R_a, wherein x and Z are as defined above, or is a straight or branched C₁-C₄ perfluoroalkyl group wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups and
- R_a is a hydroxy-, alkoxy- or acyloxy- terminated polyoxyalkylene chain free from fluorine atoms (chain R_a), said chain comprising from 4 to 50 fluorine-free oxyalkylene units, said units being the same or different from one another and being selected from -CH₂CH₂O- and -CH₂CH(J)O-, wherein J is as defined above
with the proviso that, if A is -(CF₂)_x-CFZ-CH₂-R_a and chain R_a is hydroxy-terminated and comprises only -CH₂CH₂O- units, chain R_f does not consist only of -CF₂CF₂O- repeating units.

Preferred R_f chains in the PFPE-PAGs of formula (I) are those selected from formulae (a) - (c) here below:
(a) -(CF₂O)_n(CF₂CF₂O)_m(CF₂CF₂CF₂O)_p(CF₂CF₂CF₂CF₂O)_q-
wherein m, n, p, q are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement, with the proviso that if, p and q are simultaneously 0, n is not 0; when m is other than 0, the m/n ratio is preferably between 0.1 and 20; when (m+n) is other than 0, (p+q)/(m+n) is preferably between 0 and 0.2;
(b) -(CF₂CF(CF₃)O)_a(CF₂CF₂O)_b(CF₂O)_c(CF(CF₃)O)_d-
wherein a, b, c, d are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; with the proviso that, at least one of a, c and d is not 0; when b is other than 0, a/b is preferably between 0.1 and 10; when (a+b) is different from 0 (c+d)/(a+b) preferably is between 0.01 and 0.5, more preferably between 0.01 and 0.2;
(c) -(CF₂CF(CF₃)O)_e(CF₂O)_f(CF(CF₃)O)_g-
wherein e, f, g are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when e is other than 0, (f+g)/e is preferably between 0.01 and 0.5, more preferably between 0.01 and 0.2.

PFPE-PAGs of formula (I) wherein chain R_f complies with formula (a) as defined above are particularly preferred in the method of the invention.

Typically, in the PFPE-PAGs of formula (I), chain R_a complies with formula (R_a-I) below:
(R_a-I) -(CH₂CH₂O)_r(CH₂CH(CH₃)O)_s(CH₂CH(CH₂CH₃)O)_t(CH₂CH(Ph)O)_uR¹
wherein r, s, t and u are independently selected from 0 and a positive number, with r+s+t+u ranging from 4 to 50, preferably from 4 to 15, more preferably from 4 to 10, and R¹ is selected from hydrogen, C₁-C₄ straight or branched alkyl, preferably methyl, and -C(O)R², wherein R² is C₁-C₄ straight or branched (halo)alkyl, with the proviso that, if chain R_f of compounds (I) comprises only -CF₂CF₂O- units and bears at both ends a chain (R_a-I) wherein s, t and u are 0, R¹ is not hydrogen.

In one preferred embodiment, in chain (R_a-I), r is a positive number ranging from 4 to 15, preferably from 4 to 10, s, t and u are 0 and R¹ is selected from hydrogen or methyl, with the proviso that, if chain R_f of compounds (I) comprises only -CF₂CF₂O- units and bears at both ends a chain (R_a-I), R¹ is not hydrogen.

In another preferred embodiment, r, t and u are 0, s is a positive number ranging from 4 to 15, preferably from 4 to 10, and R¹ is selected from hydrogen or methyl.

In another preferred embodiment, r and s are positive numbers and t and u are 0, r+s ranges from 4 to 15, preferably from 4 to 10, and R¹ is selected from hydrogen or methyl.

When two or more of the -CH₂CH₂O-, -CH₂CH(CH₃)O-, -CH₂CH(CH₂CH₃)O- and -CH₂CH(Ph)O- units are present in chain (R_a-I), they can be arranged in blocks or they can be disposed at random.

According to one preferred embodiment, the PFPE-PAGs are bifunctional PFPE-PAGs complying with formula (I-A) below:
R_a-O-CH₂-CF₂-O-R_f-CF₂-CH₂-O-R_a (I-A)
wherein:
- R_a is as defined above and
- R_f complies with formula (a) as defined above.

According to another preferred embodiment, the PFPE-PAGs are monofunctional PFPE-PAGs complying with formula (I-B) below:
A-O-R_f-CF₂-CH₂-O-R_a (I-B)
wherein:
- A is a straight or branched C₁-C₄ perfluoroalkyl group wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups,
- R_a is as defined above and
- R_f complies with formula (a) as defined above.

In the method of the invention, bifunctional PFPE-PAGs of formula (I-A) are preferred.

A preferred group of PFPE-PAGs of formula (I) complies with formula (I*) below:
A-O-R_f-(CF₂)_x-CFZ-CH₂-O-R_a (I*)
wherein:
- R_f is a (per)fluoropolyoxyalkylene chain having an average number molecular weight M_n ranging from 100 to 8,000, preferably from 300 to 6,000, more preferably from 800 to 3,000, and comprising, preferably consisting of, repeating units, which may be equal to or different from one another, selected from
(i) -CFXO-, wherein X is F or CF₃,
(ii) -CFXCFXO-, wherein X, equal or different at each occurrence, is F or CF₃, with the proviso that at least one of X is –F,
(iii) -CF₂CF₂CW₂O-, wherein each of W, equal or different from each other, are F or H,
(iv) -CF₂CF₂CF₂CF₂O-,
(v) –(CF₂)_j-CFZ’-O- wherein j is an integer from 0 to 3 and Z’ is a group of general formula –OR_f’T, wherein R_f’ is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFXO- , -CF₂CFXO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X being independently F or CF₃, and T being a C₁-C₃ perfluoroalkyl group;
- Z is fluorine or CF₃;
- x is 0 or 1, with the proviso that, when, x is 1, Z is F;
- A is (CF₂)_x-CFZ-CH₂-O-R_a, wherein x and Z are as defined above, or is selected from straight or branched C₁-C₄ perfluoroalkyl groups wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups and
- R_a complies with formula (R_a-I*) below:
-(CH₂CH₂O)_r(CH₂CH(CH₃)O)_s(CH₂CH(CH₂CH₃)O)_t(CH₂CH(Ph)O)_uR¹ (R_a-I*)
wherein:
- r, s, t and u are independently selected from 0 and a positive number and r+s+t+u is a positive number ranging from 4 to 50, preferably from 4 to 15, more preferably from 4 to 10; and
- R¹ is selected from hydrogen, C₁-C₄ straight or branched alkyl, and -C(O)R², wherein R² is C₁-C₄ straight or branched (halo)alkyl, with the proviso that, when R¹ is hydrogen, at least one of s, t and u is not 0.

Preferred R_f chains in the PFPE-PAGs of formula (I*) are those selected from formulae (a*) - (c*) here below:
(a*) -(CF₂O)_n(CF₂CF₂O)_m(CF₂CF₂CF₂O)_p(CF₂CF₂CF₂CF₂O)_q-
wherein m, n, p, q are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when m is other than 0, the m/n ratio is preferably between 0.1 and 20; when (m+n) is other than 0, (p+q)/(m+n) is preferably between 0 and 0.2;
(b*) -(CF₂CF(CF₃)O)_a(CF₂CF₂O)_b(CF₂O)_c(CF(CF₃)O)_d-
wherein a, b, c, d are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; with the proviso that, at least one of a, c and d is not 0; when b is other than 0, a/b is preferably between 0.1 and 10; when (a+b) is different from 0 (c+d)/(a+b) preferably is between 0.01 and 0.5, more preferably between 0.01 and 0.2;
(c*) -(CF₂CF(CF₃)O)_e(CF₂O)_f(CF(CF₃)O)_g-
wherein e, f, g are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when e is other than 0, (f+g)/e is preferably between 0.01 and 0.5, more preferably between 0.01 and 0.2.

A first group of preferred compounds (I*) complies with formula (I*-A) below:
R_a-O-CH₂-CF₂-O-R_f-CF₂-CH₂-O-R_a (I*-A)
in which:
- R_f complies with formulae (a*) – (c*) as defined above; and
- R_a complies with formula (R_a-I*) as defined above.

Preferably, in compounds (I*-A), chain R_f complies with formula (a*).

A second group of compounds (I*) complies with formula (I*-B) below:
A-O-R_f-CF₂-CH₂-O-R_a (I*-B)
in which:
- R_f complies with formulae (a*) – (c*) as defined above; and
- R_a complies with formula (R_a-I*) as defined above, and A is a straight or branched C₁-C₄ perfluoroalkyl group wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups.

Preferably, in compounds (I*-B), chain R_f complies with formula (a*).

Preferred compounds (I*-A) and (I*-B) are those wherein r, t and u are 0 and s is a positive number ranging from 4 to 15, preferably from 4 to 10, and those wherein r+s is a positive number ranging from 4 to 15, preferably from 4 to 10, and t and u are 0.

Preferably, in compounds (I*-A) and (I*-B), R¹ is methyl or C(O)CH₃.

PFPE-PAGs of formula (I*) represent a further aspect of the present invention.

The PFPE-PAGs for use in the method of the invention can be obtained by reaction of a mono- or bi-functional PFPE alcohol with an alkoxylating agent in such an amount as to obtain from 4 to 50, preferably from 4 to 15, more preferably from 4 to 10, oxyalkylene units at one or both chain ends.

For the purposes of the obtainment of the PFPE-PAGs of formula (I), the mono- or bi-functional PFPE alcohol complies with formula (II) below:
Y-O-R_f-(CF₂)_x-CFZ-CH₂-OH (II)
wherein:
- R_f, x and Z are as defined above;
- Y is (CF₂)_x-CFZ-CH₂-OH, wherein x and Z are as defined above, or is selected from straight or branched C₁-C₄ perfluoroalkyl groups wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups and
the alkoxylating agent is selected from ethylene oxide, propylene oxide, 1,2-butylene oxide and styrene oxide and a mixture of two or more thereof.

Specifically, bifunctional PFPE-PAGs (I-A) wherein chain R_a complies with formula (R_a-I) as defined above wherein R¹ is hydrogen can be obtained by reaction of a bifunctional PFPE alcohol of formula (II-A) below:
HO-CH₂-CF₂-O-R_f-CF₂-CH₂-OH (II-A)
wherein R_f complies with formula (a) as defined above
with ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide or with a mixture of two or more thereof.

Monofunctional PFPE-PAGs (I-B) wherein chain R_a complies with formula (R_a-I) as defined above wherein R¹ is hydrogen can instead be obtained by reaction of a monofunctional PFPE alcohol of formula (II-B) below:
A-O-R_f-CF₂-CH₂-OH (II-B)
wherein R_f complies with formula (a) as defined above and A is a straight or branched C₁-C₄ perfluoroalkyl group wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups
with ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide or with a mixture of two or more thereof.

Mono- and bifunctional PFPE-PAGs wherein chain R_a complies with formula (R_a-I) in which R¹ is C₁-C₄-straight or branched alkyl can be obtained according to known methods by alkylation of the corresponding mono- and bifunctional PFPE-PAGs wherein chain R_a complies with formula (R_a-I) in which R¹ is hydrogen.

Mono- and bifunctional PFPE-PAGs wherein chain R_a complies with formula (R_a-I) in which R¹ is -C(O)R² as defined above can be obtained according to known methods by acylation the corresponding mono- and bifunctional PFPE-PAGs wherein chain R_a complies with formula (R_a-I) in which R¹ is hydrogen.

PFPE alcohols of formula (II-A) or (II-B) can be manufactured by chemical reduction of corresponding PFPE carboxylic acids or esters according to several methods known in the art, using reducing agents such as NaBH₄, or by catalytic hydrogenation, as disclosed, for example, in US 6509509 AUSIMONT SPA 20010705 , US 6573411 AUSIMONT SPA 20021121 , WO WO 2008/122639 SOLVAY SOLEXIS SPA 20081016 . Precursors of PFPE carboxylic acids or of PFPE esters can be manufactured according to different methods, e.g. by oxypolymerization of fluoroolefins or by ring opening polymerization of HFPO (hexafluoropropylene oxide), as taught in US 3847978 MONTEDISON SPA 19741112 , US 3766251 MONTEDISON SPA 19731016 , US 3715378 MONTEDISON SPA 19730206 , US 3665041 MONTEDISON SPA 19720523 , US 4647413 MINESOTA MINING & MFG 19870303 , EP 151877 A MINNESOTA MINING & MFG 19850821 , US 3442942 MONTEDISON SPA 19690506 , US 577291 AUSIMONT SPA 19880707 , US 5258110 AUSIMONT SRL 19931102 or US 7132574 SOLVAY SOLEXIS SPA 20060711 .

Preferably, the PFPE-PAGs for use in the method of the invention present invention are synthesised following the process (or “method”) disclosed in international patent application WO WO 2014/090649 SOLVAY SPECIALTY POLYMERS ITALY S.P.A. 20140619 . This method comprises the use of a boron-based catalytic species, wherein sais species is prepared by first providing a mixture of a PFPE alcohol containing a catalytic amount of the corresponding alkoxide and then bringing into contact such mixture with a catalytic amount of a boric acid triester of the same PFPE alcohol.

In greater detail and with particular reference to the preparation of PFPE-PAGs according to the invention, this process comprises the following steps:
1) separately providing a mixture [M1], comprising a PFPE alcohol of formula (II) as defined above and a catalytic amount of the corresponding alkoxide (herein after “PFPE-alk”);
2) bringing into contact mixture [M1] with a boric acid triester of the same PFPE alcohol (herein after “PFPE-triBor)” in such an amount that the molar ratio PFPE-alk:PFPE-triBor is at least 1, to obtain a mixture [M2];
3) contacting mixture [M2] with a catalytic amount of an iodine source to obtain a mixture [M3];
4) treating mixture [M3] with ethylene oxide, propylene oxide, 1,2-butykene oxide or styrene oxide or a mixture thereof to provide a mixture [M4] containing a PFPE-PAG (I).

In step 1) of the process, mixture [M1] is typically prepared by adding a base to the PFPE alcohol of formula (II) and by allowing the base to react with the PFPE alcohol and form a catalytic amount of the corresponding PFPE-alk dissolved in the PFPE alcohol. The base can be selected from metal hydrides or hydroxides like NaOH, KOH, Ca(OH)₂ and Mg(OH)₂; according to a preferred embodiment, the base is KOH. Typically, the base is used in such an amount to obtain from 1 to 15%, preferably from 2 to 12% of PFPE-alk with respect to the PFPE alcohol. Accordingly, for the purposes of the present description, the expression “catalytic amount of PFPE-alk” is intended to mean a molar amount ranging from 1 to 15% mol, more preferably from 2 to 12% mol with respect to the PFPE alcohol. When a metal hydroxide is used as base, the reaction is typically promoted by heating and the proceeding of the reaction is checked by monitoring the amount of water evaporated off the reaction mixture. When a metal hydride is used as base, the proceeding of the reaction is checked by monitoring the amount of hydrogen evaporated off the reaction mixture.

Step 2) can be performed in two different ways. In a first preferred embodiment, a mixture containing a PFPE-triBor and the PFPE alcohol (herein after referred to as mixture [M_est]) is prepared and then brought into contact with mixture [M1]. Typically, [M_est] is prepared by adding boric acid or a boric acid ester (including mono-, di- and tri-alkyl esters), and allowing the reagents to react until completion of the reaction, i.e. until obtainment of the PFPE-triBor in admixture with the PFPE alcohol. Typically, the esterification reaction is carried out under vacuum and with heating and the completion is checked by monitoring the amount of water (in case boric acid is used) or alcohol (in case an alkyl ester of boric acid is used) evaporated off the reaction mixture. In a second preferred embodiment, the PFPE-triBor is prepared in situ, i.e. by adding to [M1] a boric acid trialkyl ester as defined above; also in this case the reaction is typically carried out under vacuum and with heating and the completion of the reaction is checked in the same way. The molar ratio between the PFPE-alk and the PFPE-triBor is at least 1; according to a preferred embodiment, the PFPE-alk is used in excess with respect to PFPE-triBor, i.e. the molar ratio is higher than 1; still more preferably, the molar ratio is of at least 2. Indeed, it has been observed that when a molar ratio of at least 2 is used, the reaction proceeds faster and a higher conversion is achieved.

Step 3) of the process is typically carried out by adding a catalytic amount of an iodine source, to reaction mixture [M2]. The iodine source can be selected from one or more alkali- or alkaline-earth metal iodides, such as NaI, KI, CaI₂, ammonium iodides, such as NH₄I, elemental iodine and combinations thereof. According to a preferred embodiment, the iodine source is KI. A catalytic amount of iodine source is typically an equivalent amount ranging from 0.01 to 5% with respect to the fluoroalcohol.

Step 4) of the process is typically carried out by adding to mixture [M3] ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide or a mixture thereof in such a stoichiometric amount with respect to PFPE alcohol (II) as to obtain an alkoxylation degree ranging from 4 to 50, preferably from 4 to 15, more preferably from 4 to 10.

The alkoxylation reaction is typically carried out by adding to mixture [M3] one or more aliquots of ethylene oxide, propylene oxide or a mixture thereof and by monitoring the consumption of the oxide(s) and the formation of the PFPE-PAG. When ethylene oxide or propylene oxide is used, the reaction is monitored by checking the ethylene oxide pressure in the reactor. The reaction is typically carried out under heating at a temperature usually ranging from 90°C to 190°C. When ethylene oxide is used as alkoxylating agent, the reaction is carried out at temperatures usually ranging from 110° to 160°C.

Once the reaction is complete, the resulting PFPE-PAG can be isolated from mixture [M4] by conventional techniques, including extraction and distillation. Usually, mixture [M4] is cooled down to room temperature and then diluted with a fluorinated solvent, then treated with a water solution of an inorganic base, typically a carbonate, and the organic phase is separated and submitted to distillation. Examples of fluorinated solvents include, for example, Galden^® PFPEs, hydrofluoroethers (HFEs) including Novec^® HFEs, hydrofluorocarbons (HFCs), like Vertel^® or Fluorinert^®, and fluoroaromatic solvents like hexafluorobenzene and 1,3-hexafluoroxylene. Typically, the fluorinated solvent is 1,3-hexafluoroxylene.

PFPE-PAGs according to the present invention are endowed with lubricant properties and they are advantageous in that they are stable under harsh conditions and in that they have a lower coefficient of friction, lower wear and improved performances under extreme pressure conditions with respect to PAGs. In particular, experiments carried out by the Applicant have demonstrated that PFPE-PAGs of formula (I), in particular those of formula (I-A), are endowed with lower coefficient of friction, lower wear and higher extreme pressure loads with respect to both PAGs having the same molecular weight and PAGs having the same molecular weight as chain R_a in the PFPE-PAG. Experiments carried out by the Applicant further demonstrated that the PFPE-PAGs of formula (I), in particular those of formula (I-A), are endowed with higher thermal stability than PAGs having the same molecular weight.

The PFPE-PAGs of the invention can be used as such of in admixture with further ingredients or additives typically used in the manufacture of lubricant compositions. Accordingly, the present invention further relates to a lubricant composition comprising a PFPE-PAG as defined above, in particular a PFPE-PAG of formula (I), in admixture with further lubricants and/or additives selected from those commonly used in lubricant compositions.

In particular, compositions according to the present comprise:
(a) a PFPE-PAG of formula (I) as defined above; in admixture of one or more of:
(b) a lubricant base oil;
(c) a thickening agent;
(d) an additive; and optionally
(e) a solvent.

Non-limiting examples of lubricant base oils comprise PFPEs, polyalphaolefins (PAO), PAGs, mineral oils, silicon oils, polyphenyethers, etc.

Non-limiting examples of additives comprise antirust agents, antioxidants, thermal stabilizers, pour-point depressants, antiwear agents, including those for high pressures, dispersants, tracers, dyestuffs, talc and inorganic fillers. Examples of dispersants are, for example, surfactants, preferably non-ionic surfactants, more preferably (per)fluoropolyether surfactants and (per)fluoroalkyl surfactants.

Non-limiting examples of PFPE lubricant base oils such as those disclosed in identified as compounds (1) – (8) EP 2100909 A SOLVAY SOLEXIS SPA  20090916 .

Examples of thickening agents are talc, silica, boron nitride, polyureas, alkali or alkali-earth metals terephthalates, calcium and lithium soaps and complexes thereof and PTFE (polytetrafluoroethylene); among them, PTFE is preferred.

Examples of solvents are fluorinated or partially fluorinated solvents, such as Galden^® PFPEs, Novec® HFEs and other organic solvents like methyl-ethyl-ketone, isopropyl alcohol, butylacetate, etc.

Compositions comprising PFPE-PAGs of formula (I) and a PFPE oil may advantageously be in the form of greases (compositions C-1), i.e. the compositions consist of a PFPE oil, a thickening agent and a PFPE-PAG of formula (I) in an amount ranging from 0.1% to 30% wt, preferably from 3% to 10% wt with respect to the weight of the composition. Experiments carried out by the Applicant have indeed demonstrated that such compositions show significantly lower wear values than greases that do not contain PFPE-PAGs. Thus, the PFPE-PAGs of the invention can act as anti-wear additives for lubricant compositions in the form of greases.

A preferred example of lubricant base oil to be mixed with the PFPE-PAGs of the invention is represented by PAGs. In particular, experiments carried out by the Applicant showed that the PFPE-PAGs of formula (I) according to the present invention, in particular PFPE-PAGs of formula (I-A), are able to dissolve PAGs. Accordingly, the present invention further relates to a lubrication method comprising applying a PFPE-PAG of formula (I) as defined above in admixture with a PAG, and to lubricant compositions (compositions C-2) comprising, preferably consisting of, a PFPE-PAG of formula (I) as defined above in admixture with a PAG and, optionally, one or more additives. Preferred compositions C-2 comprise, preferably consist of, a PFPE-PAG of formula (I) as defined above and a PAG in a weight amount ranging from 5 to 10% with respect to the weight of the composition and, optionally, one or more additives. Accordingly, the present invention further relates to compositions C-2 consisting of:
(a1) a PFPE-PAG of formula (I) as defined above and
(a2) a PAG, preferably a PEG and, optionally
(a3) one or more additives.

Lubricant compositions C-1 and C-2 can be manufactured according to methods known in the art.

Should the disclosure of ant patents, patent applications and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The invention will be disclosed in greater detail in the following Experimental section by means of non-limiting examples.

Experimental section

Materials and methods

The PFPE-PAGs of referred to in the Examples were synthesised following the process in international patent application filed on December 4^th, 2013 with number EP2013/075476 by Solvay Specialty Polymers Italy S.p.A.

PEG 2000 and PEG 400 were purchased from Aldrich^® and were used as such.

Kinematic viscosities measurements were carried out according to the ASTM D445 “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)” at different temperatures.

The viscosity Index, VI, was calculated according to the ASTM D2270 “Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C”.

Kinematic viscosities at different temperatures, not experimentally measured, were calculated according to the Ubbelohde-Walther equation in a temperature range from 20° to 100°C.

Friction and wear properties were determined according to the ASTM D6425 “Standard Test Method for Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV Test Machine” using a ball-on-disk configuration with a pre-load at 50N for 30’’, a load of 300 N for 2 hours, with a frequency of 50 Hz and a stroke of 1 mm.

The pour point of the analyzed samples was determined according to the ASTM D97 “Standard Test Method for Pour Point of Petroleum Products”.

The wear on grease samples has been evaluated according to the ASTM D2266 “Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method)”, at 75°C, with a load of 40Kgf (1200 rpm, 1 h).

Extreme pressure properties of lubricating oils for hydraulics, gears and engines under high-frequency linear oscillation motion were dermined according to the ASTM D7421 “Standard Test Method for Determining Extreme Pressure Properties of Lubricating Oils Using High-Frequency, Linear-Oscillation (SRV) Test Machine 1”.

All the tests performed to evaluate friction and wear properties were carried out at isoviscous conditions in order to compare the same lubricity film layer.

Example 1

A PFPE-PAG of formula:
H(OCH₂CH₂)O_rCH₂CF₂O(CF₂CF₂O)_m(CF₂O)_n-CF₂CH₂O(CH₂CH₂O)_rH
with r=5, p/q~1 and an average molecular weight M_n=2050
was manufactured as indicated above. The product is a transparent liquid at room temperature and the pour point is -18°C.

The calculated kinematic viscosity values are as follows:
at 20°C = 395 cSt;
at 40°C = 137 cSt and
at 100°C = 17.5 cSt
with a corresponding viscosity index equal to 141.

A SRV test was carried out at 70°C, in double, with a kinematic viscosity of the product corresponding to 41 cSt. After the test the average coefficient of friction was 0.14 ± 0.01 and the measured wear-on-the-ball was 1.0 ± 0.1 mm.

Example 2 (Comparative example)

Polyethylenglycol PEG 2000, which is a white solid at room temperature having an average molecule weight of 2000, was heated above its melting point (~50°C) in order to measure the kinematic viscosity. At 100°C the kinematic viscosity was equal to 43.04 cSt.

A SRV test was carried out at 100°C, in double, with a kinematic viscosity of the product corresponding to 43.04 cSt, under the same isoviscous conditions as Example 1. After the test, the average coefficient of friction was 0.20 ± 0.02 and the measure wear-on-the-ball was 1.5 ± 0.1 mm.

Example 3 (Comparative example)

Polyethylenglycol PEG 400 (average molecule weight = 400) was subjected to the SRV test. PEG 400 is a transparent liquid at room temperature having a kinematic viscosity at 20°C = 106.6 cSt, at 40°C = 40.2 cSt and at 100°C=7.03 cSt with a corresponding viscosity index equal to 136.

The SRV test was carried out at 40°C, in double, with a kinematic viscosity of the product corresponding to 40.2 cSt, under the same isoviscous conditions as in Example 1 and in comparative Example 2. After the test, the average coefficient of friction was 0.20 ± 0.02 and the measured wear-on-the-ball was 1.4 ± 0.06 mm.

Examples 1, 2 and 3 above show that the PFPE-PAG is endowed with lower coefficient of friction, lower wear and good viscosity index with respect to:
- both a PEG having the same molecular weight as the PFPE-PAG of Example 1 (Comparative example 2) and
- a PEG having the same molecular weight as the PAG moieties in the PFPE-PAG of Example 1 (Comparative example 3).

Example 4

A PFPE-PAG of formula:

HO(CH₂(CH₃)CHO)_s(CH₂CH₂O)_rCH₂CF₂O(CF₂CF₂O)_m(CF₂O)_n-CF₂CH₂(OCH₂CH₂)_r(OCH(CH₃)CH₂)_sOH

with r=2.5, s=4.5, m/n~1 and an average molecular weight M_n=2050 was manufactured as indicated above.

The product is a transparent liquid at room temperature having a pour point of -38°C.

The kinematic viscosity values are as follows:
at 20°C = 274 cSt,
at 40°C = 98 cSt
and at 100°C = 13.7 cSt
with a corresponding viscosity index equal to 142.

A SRV test was carried out at 50°C, in double, with a kinematic viscosity of the product corresponding to 63 cSt. After the test the average coefficient of friction was 0.15 ± 0.004 and the measured wear-on-the-ball was 1.1 ± 0.1 mm.

Example 5

10 g PEG 400 was added to 200 g of the PFPE-PAG of Example 4 in a glass vessel and kept under stirring at room temperature. After 15’ stirring was stopped and a transparent solution was obtained.

Example 6

20 g PEG 400 was added to 200 g of the product of the Example 1 in a glass vessel and kept under stirring at room temperature. After 15’ stirring was stopped and a transparent solution was obtained.

Examples 5-6 show that the PFPE-PAGs of the invention are able to solubilize PAGs.

Example 7

A thermal stability test on the PFPE-PAG of Example 1 was performed by means TGA in air (10°C/min). At 250°C a weight loss of 6% was observed, while at 300°C the corresponding value was 19% (see Figure 1).

Example 8

A thermal stability test on the PEG of Example 2 was performed by means of TGA in air (10°C/min). At 250°C a weight loss of 9.5% was observed, while at 300°C the corresponding value was 34% (see Figure 1).

Examples 7 and Example 8 show that the PFPE-PAGs of the invention have improved thermal stability than PAGs having the same molecular weight.

Example 9 (Comparative example)

A perfluorinated grease was prepared by adding a commercial PFPE, Fomblin^® M30, to the powder of PTFE, in order to reach a cone penetration value of 277 mm/10’.

This grease was subjected to the 4-ball wear test, following the procedure described above. The wear observed on the ball after the test was 1.46 mm + 0.04 mm.

Example 10

The grease prepared in the Example 9 was added with 5% by weight of the PFPE-PAG of Example 1 and was subjected to the 4-ball wear test following the procedure described above.

The wear observed on the ball after the test was 0.80 mm + 0.05 mm.

Example 9 and 10 show that the PFPE-PAGs of the invention are able to act as antiwear additive for perfluorinated greases.

Example 11

The PFPE-PAG of Example 4 was analysed under extreme pressure conditions according to ASTM D7421, under the following operative conditions:
- Temperature: 60°C (kinematic viscosity of the sample oil at this temperature was 43 cSt);
- Stroke: 2 mm
- Frequency: 50 Hz
- Pre-load: 50 N for 30’’
- Load: 100 N for 15’ and increase of 100 N each 2’ up to 2000 N.

The results showed that no seizure occurred at the final load of 2000 N.

Example 12 (Comparative example)

A commercial base oil (without additives) consisting of polypropyleneglycol monobutylether, having an average molecular weight M_n of 1,400 g/mol was analysed. The kinematic viscosity values were as follows:
at 20°C = 316 cSt;
at 40°C = 115 cSt and
at 100°C=20.8 cSt
with a corresponding viscosity index equal to 207.

A SRV test was carried out, in double, at 70°C, with a kinematic viscosity of the product corresponding to 44 cSt. After the test, the average coefficient of friction was 0.22 ± 0.001 and the measured wear on the ball was 1.6 ± 0.1 mm.

The base oil was analysed under extreme pressure conditions according to the ASTM D7421, under the same operative conditions as in Example 11: The results showed that seizure occurred at a load of 1000 N.

The comparison between Example 11 and Example 12 shows that the presence of PFPE sequences in the PFPE-GAG imparts lubrication properties to the PAG base oil, low coefficient of friction, low wear and significantly improved extreme pressure properties.

Claims (13)

1. A lubrication method comprising applying to a surface to be lubricated a polymer complying with formula (I) below:

   A-O-R_f-(CF₂)_x-CFZ-CH₂-O-R_a (I)

   wherein:

   - R_f is a (per)fluoropolyoxyalkylene chain having an average number molecular weight M_n ranging from 100 to 8,000, preferably from 300 to 6,000, more preferably from 800 to 3,000, and comprising, preferably consisting of, repeating units, which may be equal to or different from one another, selected from:

   (i) -CFXO-, wherein X is F or CF₃,

   (ii) -CFXCFXO-, wherein X, equal or different at each occurrence, is F or CF₃, with the proviso that at least one of X is –F,

   (iii) -CF₂CF₂CW₂O-, wherein each of W, equal or different from each other, are F or H,

   (iv )-CF₂CF₂CF₂CF₂O-,

   (v) –(CF₂)_j-CFZ’-O- wherein j is an integer from 0 to 3 and Z’ is a group of general formula –OR_f’T, wherein R_f’ is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFXO- , -CF₂CFXO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X being independently F or CF₃ and T being a C₁-C₃ perfluoroalkyl group;

   - Z is fluorine or CF₃;

   - x is 0 or 1, with the proviso that, when, x is 1, Z is F;

   - A is -(CF₂)_x-CFZ-CH₂-O-R_a, wherein x and Z are as defined above, or is a straight or branched C₁-C₄ perfluoroalkyl group wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups and

   - R_a is a hydroxy-, alkoxy- or acyloxy- terminated polyoxyalkylene chain free from fluorine atoms (chain R_a), said chain comprising from 4 to 50 fluorine-free oxyalkylene units, said units being the same or different from one another and being selected from -CH₂CH₂O-, -CH₂CH(CH₃)O-, -CH₂CH(CH₂CH₃)O- and -CH₂CH(Ph)O-,

   with the proviso that, if A is -(CF₂)_x-CFZ-CH₂-R_a and chain R_a is hydroxy-terminated and comprises only -CH₂CH₂O- units, chain R_f does not consist only of -CF₂CF₂O- repeating units.
2. The method according to claim 1, wherein chain R_f in polymer (I) is selected from formulae (a) - (c) below:

   those selected from formulae (a) - (c) here below:

   (a) -(CF₂O)_n(CF₂CF₂O)_m(CF₂CF₂CF₂O)_p(CF₂CF₂CF₂CF₂O)_q-

   wherein m, n, p, q are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement, with the proviso that if, p and q are simultaneously 0, n is not 0; when m is other than 0, the m/n ratio is preferably between 0.1 and 20; when (m+n) is other than 0, (p+q)/(m+n) is preferably between 0 and 0.2;

   (b) -(CF₂CF(CF₃)O)_a(CF₂CF₂O)_b(CF₂O)_c(CF(CF₃)O)_d-

   wherein a, b, c, d are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; with the proviso that, at least one of a, c and d is not 0; when b is other than 0, a/b is preferably between 0.1 and 10; when (a+b) is different from 0 (c+d)/(a+b) preferably is between 0.01 and 0.5, more preferably between 0.01 and 0.2;

   (c) -(CF₂CF(CF₃)O)_e(CF₂O)_f(CF(CF₃)O)_g-

   wherein e, f, g are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when e is other than 0, (f+g)/e is preferably between 0.01 and 0.5, more preferably between 0.01 and 0.2.
3. The method according to claim 1 or 2 wherein chain R_a in polymer (I) complies with formula (R_a-I) below:

   (R_a-I) -(CH₂CH₂O)_r(CH₂CH(CH₃)O)_s(CH₂CH(CH₂CH₃)O)_t(CH₂CH(Ph)O)_uR¹

   wherein r, s, t and u are independently selected from 0 and a positive number, with r+s+t+u ranging from 4 to 50, and R¹ is selected from hydrogen, C₁-C₄ straight or branched alkyl, and -C(O)R², wherein R² is C₁-C₄ straight or branched (halo)alkyl, with the proviso that , if chain R_f of compounds (I) comprises only -CF₂CF₂O- units and bears at both ends a chain (R_a-I) wherein s, t and u are 0, R¹ is not hydrogen.
4. The method according to claim 3 wherein in chain (R_a-I), r is a positive number ranging from 4 to 15, s, t and u are 0 and R¹ is selected from hydrogen or methyl, with the proviso that, if chain R_f of compounds (I) comprises only -CF₂CF₂O- units and bears at both ends a chain (R_a-I), R¹ is not hydrogen.
5. The method according to claim 3 wherein, in chain (R_a-I), r, t and u are 0, s is a positive number ranging from 4 to 15 and R¹ is selected from hydrogen or methyl.
6. The method according to claim 3 wherein in chain (R_a-I), t and u are 0, r+s ranges from 4 to 15 and R¹ is selected from hydrogen or methyl.
7. The method according to any one of claims 1 to 6 wherein polymer (I) complies with formula (I-A) below:

   R_a-O-CH₂-CF₂-O-R_f-CF₂-CH₂-O-R_a (I-A)

   wherein R_a is as defined in any one of claims 1, 3, 4, or 6 and R_f complies with formula (a) as defined in claim 2.
8. A polymer complying with formula (I*)

   A-O-R_f-(CF₂)_x-CFZ-CH₂-O-R_a (I*)

   wherein:

   - R_f is a (per)fluoropolyoxyalkylene chain having an average number molecular weight M_n ranging from 100 to 8,000, preferably from 300 to 6,000, more preferably from 800 to 3,000, and comprising, preferably consisting of, repeating units, which may be equal to or different from one another, selected from

   (i) -CFXO-, wherein X is F or CF₃,

   (ii) -CFXCFXO-, wherein X, equal or different at each occurrence, is F or CF₃, with the proviso that at least one of X is –F,

   (iii) -CF₂CF₂CW₂O-, wherein each of W, equal or different from each other, are F or H,

   (iv) -CF₂CF₂CF₂CF₂O-,

   (v) -(CF₂)_j-CFZ’-O- wherein j is an integer from 0 to 3 and Z’ is a group of general formula –OR_f’T, wherein R_f’ is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFXO- , -CF₂CFXO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X being independently F or CF₃, and T being a C₁-C₃ perfluoroalkyl group;

   - Z is fluorine or CF₃;

   - x is 0 or 1, with the proviso that, when, x is 1, Z is F;

   - A is (CF₂)_x-CFZ-CH₂-O-R_a, wherein x and Z are as defined above, or is selected from straight or branched C₁-C₄ perfluoroalkyl groups wherein one fluorine atom can be substituted by one chlorine atom or one hydrogen atom, with the proviso that, if chlorine is present in group A, it is in a molar amount lower than 2% with respect to the overall amount of end groups and

   - R_a complies with formula (R_a-I*) below:

   -(CH₂CH₂O)_r(CH₂CH(CH₃)O)_s(CH₂CH(CH₂CH₃)O)_t(CH₂CH(Ph)O)_uR¹ (R_a-I*)

   wherein:

   - r, s, t and u are independently selected from 0 and a positive number and r+s+t+u is a positive number ranging from 4 to 50;

   and R¹ is selected from hydrogen, C₁-C₄ straight or branched alkyl, and -C(O)R², wherein R² is C₁-C₄ straight or branched (halo)alkyl, with the proviso that, when R¹ is hydrogen, at least one of s, t and u is not 0.
9. The polymer according to claim 8 wherein chain R_f is selected from formulae (a*) - (c*) here below:

   (a*) -(CF₂O)_n(CF₂CF₂O)_m(CF₂CF₂CF₂O)_p(CF₂CF₂CF₂CF₂O)_q-

   wherein m, n, p, q are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when m is other than 0, the m/n ratio is preferably between 0.1 and 20; when (m+n) is other than 0, (p+q)/(m+n) is preferably between 0 and 0.2;

   (b*) -(CF₂CF(CF₃)O)_a(CF₂CF₂O)_b(CF₂O)_c(CF(CF₃)O)_d-

   wherein a, b, c, d are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; with the proviso that, at least one of a, c and d is not 0; when b is other than 0, a/b is preferably between 0.1 and 10; when (a+b) is different from 0 (c+d)/(a+b) preferably is between 0.01 and 0.5, more preferably between 0.01 and 0.2;

   (c*) -(CF₂CF(CF₃)O)_e(CF₂O)_f(CF(CF₃)O)_g-

   wherein e, f, g are 0 or integers selected in such a way as chain R_f meets the above number average molecular weight requirement; when e is other than 0, (f+g)/e is preferably between 0.01 and 0.5, more preferably between 0.01 and 0.2.
10. The polymer according to claim 9 of formula (I*-A) below

    R_a-O-CH₂-CF₂-O-R_f-CF₂-CH₂-O-R_a (I*-A)

    in which:

    - R_a complies with formula (R_a-I*) as defined in claim 9 and

    - R_f complies with formula (a*) as defined in claim 10.
11. The polymer according to claim 10 wherein r, t and u in formula (R_a-I*) are 0 and s is a positive number ranging from 4 to 15.
12. The polymer according to claim 10 wherein in formula (R_a-I*) r+s is a positive number ranging from 4 to 15 and t and u are 0.
13. A composition comprising:

    (a) a polymer as defined in any one of claims 8 to 12; in admixture of one or more of

    (b) a lubricant base oil;

    (c) a thickening agent;

    (d) an additive; and optionally

    (e) a solvent.

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