THERMOASSOCIATIVE AND EXCHANGEABLE COPOLYMERS, COMPOSITION COMPRISING SAME

Abstract

Compositions resulting from the mixing of at least one copolymer A1, resulting from the copolymerization of at least one monomer functionalized by diol functions with at least one styrenic monomer, and at least one compound A2 comprising at least two boronic ester functions. They have very varied rheological properties depending on the proportion of compounds A1 and A2 used. Composition resulting from the mixing of at least one lubricating oil with such a composition of associative and exchangeable polymers and use of this composition for lubricating a mechanical part.

Claims

1-22. (canceled)

23. A composition resulting from the mixing of at least one polydiol random copolymer A1 comprising at least from 2 mol % to 50 mol % of at least one monomer M3 of general formula (X): ##STR00044## (X) in which: Z.sub.1, Z.sub.2 and Z.sub.3, which may be identical or different, represent groups chosen from a hydrogen atom, a C.sub.1-C.sub.12 alkyl, and a group OZ or C(O)OZ with Z being a C.sub.1-C.sub.12 alkyl. and a compound A2 comprising at least two boronic ester functions.

24. The composition as claimed in claim 23, in which the polydiol random copolymer A1 is a copolymer resulting from the copolymerization: of at least one first monomer M1 of general formula (I): ##STR00045## in which: R.sub.1 is chosen from the group formed by H, CH.sub.3 and CH.sub.2CH.sub.3; x is an integer ranging from 1 to 18; y is an integer equal to 0 or 1; X.sub.1 and X.sub.2, which may be identical or different, are chosen from the group formed by hydrogen, tetrahydropyranyl, methyloxymethyl, tert-butyl, benzyl, trimethylsilyl and t-butyldimethylsilyl; or X.sub.1 and X.sub.2 form, with the oxygen atoms, a bridge having the following formula ##STR00046## in which: the asterisks (*) symbolize the bonds to oxygen atoms, R.sub.2 and R.sub.2, which may be identical or different, are chosen from the group formed by hydrogen and a C.sub.1-C.sub.11 alkyl; or X.sub.1 and X.sub.2 form, with the oxygen atoms, a boronic ester having the following formula: ##STR00047## in which: the asterisks (*) symbolize the bonds to oxygen atoms, R.sub.2 is chosen from the group formed by a C.sub.6-C.sub.30 aryl, a C.sub.7-C.sub.30 aralkyl and a C.sub.2-C.sub.30 alkyl; with at least one second monomer M2 of general formula (II): ##STR00048## in which: R.sub.2 is chosen from the group formed by H, CH.sub.3 and CH.sub.2CH.sub.3; R.sub.3 is chosen from the group formed by: C(O)OR.sub.3; OR.sub.3; SR.sub.3 and C(O)N(H)R.sub.3 with R.sub.3 being a C.sub.1-C.sub.30 alkyl group, and with at least one third monomer M3 of general formula (X): ##STR00049## in which: Z.sub.1, Z.sub.2 and Z.sub.3, which may be identical or different, represent groups chosen from a hydrogen atom, a C.sub.1-C.sub.12 alkyl, and a group OZ or C(O)OZ with Z being a C.sub.1-C.sub.12 alkyl.

25. The composition as claimed in claim 23, in which the third monomer M3 is styrene.

26. The composition as claimed in claim 24, in which the random copolymer A1 results from the copolymerization of at least one monomer M1 with at least two monomers M2 bearing different groups R.sub.3 and at least one monomer M3.

27. The composition as claimed in claim 26, in which the two monomers M2 of the random copolymer A1 have the general formula (II-B): ##STR00050## in which: R.sub.2 is chosen from the group formed by H, CH.sub.3 and CH.sub.2CH.sub.3; R.sub.3 is a C.sub.9-C.sub.30 alkyl group.

28. The composition as claimed in claim 24, in which the side chains of the random copolymer A1 have a mean length ranging from 8 to 20 carbon atoms.

29. The composition as claimed in claim 24, in which the random copolymer A1 has a molar percentage of monomer M1 of formula (I) in said copolymer ranging from 1% to 30%.

30. The composition as claimed in claim 23, in which the random copolymer A1 has a number-average degree of polymerization ranging from 40 to 2000 and a polydispersity index (Ip) ranging from 1.05 to 4.0.

31. The composition as claimed in claim 23, in which compound A2 is a compound of formula (III): ##STR00051## in which: w.sub.1 and w.sub.2, which may be identical or different, are integers chosen between 0 and 1; R.sub.4, R.sub.5, R.sub.6 and R.sub.7, which may be identical or different, represent a group chosen from a hydrogen atom, a hydrocarbon-based group comprising from 1 to 30 carbon atoms, optionally substituted with one or more groups chosen from: a hydroxyl, a group OJ or C(O)O-J with J being a hydrocarbon-based group comprising from 1 to 24 carbon atoms; L is a divalent bonding group chosen from the group formed by a C.sub.6-C.sub.18 aryl, a C.sub.6-C.sub.8 aralkyl and a C.sub.2-C.sub.24 hydrocarbon-based chain.

32. The composition as claimed in claim 23, in which compound A2 is a random copolymer resulting from the copolymerization of at least one monomer M4 of formula (IV): ##STR00052## in which: t is an integer equal to 0 or 1; u is an integer equal to 0 or 1; M and R.sub.8 are identical or different divalent bonding groups, chosen from the group formed by C.sub.6-C.sub.18 aryl, a C.sub.7-C.sub.24 aralkyl and a C.sub.2-C.sub.24 alkyl, X is a function chosen from the group formed by OC(O), C(O)O, C(O)N(H), N(H)C(O), S, N(H), N(R.sub.4) and O with R.sub.4 being a hydrocarbon-based chain comprising from 1 to 15 carbon atoms; R.sub.9 is chosen from the group formed by H, CH.sub.3 and CH.sub.2CH.sub.3; R.sub.10 and R.sub.11, which may be identical or different, represent a group chosen from a hydrogen atom, a hydrocarbon-based group comprising from 1 to 30 carbon atoms, optionally substituted with one or more groups chosen from: a hydroxyl, a group OJ or C(O)O-J with J being a hydrocarbon-based group comprising from 1 to 24 carbon atoms; with at least one second monomer M5 of general formula (V): ##STR00053## in which: R.sub.12 is chosen from the group formed by H, CH.sub.3 and CH.sub.2CH.sub.3; R.sub.13 is chosen from the group formed by a C.sub.6-C.sub.18 aryl, a C.sub.6-C.sub.18 aryl substituted with a group R.sub.13, C(O)OR.sub.13; OR.sub.13, SR.sub.13 and C(O)N(H)R.sub.13 with R.sub.13 being a C.sub.1-C.sub.30 alkyl group.

33. The composition as claimed in claim 32, in which at least one of the following three conditions is met: either, in formula (IV): u=1, R.sub.9 is H and R.sub.8 represents a C.sub.6-C.sub.18 aryl or a C.sub.7-C.sub.24 aralkyl and the double bond of the monomer M4 of formula (IV) is directly connected to the aryl group; or, in formula (V): R.sub.12 represents H and R.sub.13 is chosen from the group formed by a C.sub.6-C.sub.18 aryl and a C.sub.6-C.sub.18 aryl substituted with a group R.sub.13 with R.sub.13 being a C.sub.1-C.sub.25 alkyl group and the double bond of the monomer M5 of formula (V) is directly connected to the aryl group. or, copolymer A2 comprises at least one third monomer M3 of formula (X) ##STR00054## in which: Z.sub.1, Z.sub.2 and Z.sub.3, which may be identical or different, represent groups chosen from a hydrogen atom, a C.sub.1-C.sub.12 alkyl, and a group OZ or C(O)OZ with Z being a C.sub.1-C.sub.12 alkyl.

34. The composition as claimed in claim 32, in which the chain formed by the sequence of groups R.sub.10, M, X and (R.sub.8).sub.u with u equal to 0, in which the chain formed by the sequence of groups R.sub.10, M, X and (R.sub.8).sub.u with u equal to 0 or 1 of the monomer of formula (IV) has a total number of carbon atoms ranging from 8 to 38.

35. The composition as claimed in claim 32, in which the side chains of copolymer A2 have a mean length of greater than or equal to 8 carbon atoms.

36. The composition as claimed in claim 32, in which copolymer A2 has a molar percentage of monomer of formula (IV) in said copolymer ranging from 0.25% to 30%.

37. The composition as claimed in claim 32, in which copolymer A2 has a number-average degree of polymerization ranging from 50 to 1500 and a polydispersity index (Ip) ranging from 1.04 to 3.54.

38. The composition as claimed in claim 23, in which the content of copolymer A1 ranges from 0.1% to 50% by weight relative to the total weight of the composition.

39. The composition as claimed in claim 23, in which the content of compound A2 ranges from 0.1% to 50% by weight relative to total weight of the composition.

40. The composition as claimed in claim 23, in which the mass ratio between copolymer A1 and compound A2 (ratio A1/A2) ranges from 0.005 to 200.

41. The composition as claimed in claim 23, which also comprises at least one exogenous compound A4 chosen from 1,2-diols and 1,3-diols.

42. A lubricant composition resulting from the mixing of at least: one lubricant oil; and one composition as defined in claim 23.

Description

FIGURES

[0578] FIG. 1 schematically represents a random copolymer (P1), a gradient copolymer (P2) and a block copolymer (P3); each circle represents a monomer unit. The difference in chemical structure between the monomers is symbolized by a different color (light gray/black).

[0579] FIG. 2 schematically represents a comb copolymer.

[0580] FIG. 3 schematically illustrates the exchange reactions of boronic ester bonds between two polydiol random polymers (A1-1 and A1-2) and two boronic ester random polymers (A2-1 and A2-2) in the presence of diols.

[0581] FIG. 4 schematically illustrates and represents the crosslinking of the composition according to the invention in tetrahydrofuran (THF).

[0582] FIG. 5 schematically represents the behavior of the composition of the invention as a function of the temperature. A random copolymer (2) bearing diol functions (function A) can thermoreversibly associate with a random copolymer (1) bearing boronic ester functions (function B) via a transesterification reaction. The organic group of the boronic ester functions (function B) which exchanges during the transesterification reaction is a diol symbolized by a black crescent. It forms a chemical bond (3) of boronic ester type with release of a diol compound.

[0583] FIG. 6 represents the change in relative viscosity (unitless, on the y-axis) as a function of the temperature ( C., on the x-axis) of compositions A, B, C and D.

[0584] FIG. 7 represents the change in relative viscosity (unitless, on the y-axis) as a function of the temperature ( C., on the x-axis) of compositions A, E and F.

[0585] FIG. 8 represents the change in relative viscosity (unitless, on the y-axis) as a function of the temperature ( C., on the x-axis) of compositions E and F for three successive cycles (1, 2 and 3) of heating and then cooling.

[0586] FIG. 9 represents the mass (%, on the y-axis) as a function of the temperature ( C., on the x-axis) of the polydiol A-1a (solid line), of the polydiol A-1c (dotted line) and of the polydiol A-1b (dashes) during a temperature ramp from 25 C. to 600 C. under a dinitrogen atmosphere.

[0587] FIG. 10 represents the mass (%, on the y-axis) as a function of time (minutes, on the x-axis) of the polydiol A-1a (solid line), of the polydiol A-1c (dotted line) and of the polydiol A-1b (dashes) during an isotherm at 150 C. under a dinitrogen atmosphere.

[0588] FIG. 11 represents the change in relative viscosity (unitless, on the y-axis) as a function of the temperature ( C., on the x-axis) of composition G for three successive cycles (1, 2 and 3) of heating and then cooling.

EXPERIMENTAL SECTION

[0589] The examples that follow illustrate the invention without limiting it.

1. Synthesis of Random Copolymers A1 Bearing a Diol Function

[0590] 1.1: Starting with a Monomer Bearing a Diol Function

[0591] In one embodiment, the random copolymers A1 of the invention are obtained according to reaction scheme 11 below:

##STR00043##

[0592] The copolymer obtained after removing the RAFT chain end contains, inter alia, styrene as comonomer and the thiocarbonylthio residue was removed, for example by converting it into a thioether.

1.1.1. Synthesis of the Monomer M1 Bearing a Diol Function

[0593] The synthesis of a methacrylate monomer bearing a diol function is performed in three steps (steps 1, 2 and 3 of reaction scheme 11) according to the protocol below:

First Step:

[0594] 42.1 g (314 mmol) of 1,2,6-hexanetriol (1,2,6-HexTri) are placed in a 1 L round-bottomed flask. 5.88 g of molecular sieves (4 ) are added, followed by 570 mL of acetone. 5.01 g (26.3 mmol) of para-toluenesulfonic acid (pTSA) are then slowly added. The reaction medium is stirred for 24 hours at room temperature. 4.48 g (53.3 mmol) of NaHCO.sub.3 are then added. The reaction medium is stirred for 3 hours at room temperature before being filtered. The filtrate is then concentrated under vacuum using a rotary evaporator until a suspension of white crystals is obtained. 500 mL of water are then added to this suspension. The solution thus obtained is extracted with 4300 mL of dichloromethane. The organic phases are combined and dried over MgSO.sub.4. The solvent is then totally evaporated off under vacuum at 25 C. using a rotary evaporator.

Second Step:

[0595] 5.01 g (28.8 mmol) of the product thus obtained are placed in a 1 L round-bottomed flask. 4.13 g (31.9 mmol) of DIPEA and 37.9 mg (0.31 mmol) of DMAP are then placed in the flask, followed by 5.34 g (34.6 mmol) of methacrylic anhydride. The flask is then stirred at room temperature for 24 hours. 0.95 g of methanol (29.7 mmol) is then added to the solution and the flask is stirred for a further 1 hour. The product is then dissolved in 40 mL of hexane. The organic phase is then washed successively with 25 mL of water, 325 mL of aqueous 0.5 M hydrochloric acid solution, 325 mL of aqueous 0.5 M NaOH solution and again with 25 mL of water. The organic phase is dried over MgSO.sub.4, filtered and then concentrated under vacuum using a rotary evaporator to give a pale yellow liquid.

Third Step:

[0596] 17.23 g (71.2 mmol) of the product thus obtained are placed in a 1 L round-bottomed flask. 90 mL of water and 90 mL of acetonitrile are then placed in the flask, followed by 59.1 mL (159 mmol) of acetic acid. The flask is then stirred for 24 hours at 30 C. while a gentle stream of nitrogen is bubbled through to force the removal of the acetone. The solution thus obtained is extracted with 630 mL of ethyl acetate. The organic phase is washed successively with 530 mL of aqueous 0.5 M NaOH solution and then 330 mL of water. The organic phase is then dried over MgSO.sub.4, filtered and then concentrated under vacuum using a rotary evaporator to give a pale yellow liquid, the characteristics of which are as follows:

[0597] .sup.1H NMR (400 MHz, CDCl.sub.3) : 6.02 (singlet, 1H), 5.49 (singlet, 1H), 4.08 (triplet, J=6.4 Hz, 1H), 3.65-3.58 (multiplet, 1H), 3.57-3.50 (multiplet, 3H), 3.35 (doublet of doublets, J=7.6 Hz and J=11.2 Hz, 1H), 1.86 (doublet of doublets, J=1.2 Hz and J=1.6 Hz, 3H), 1.69-1.31 (multiplet. 6H).

1.1.2. Synthesis of Methacrylate Copolymers Bearing Diol Functions with Removal of the RAFT Chain End

[0598] The synthesis of methacrylate copolymers bearing diol functions is performed in two steps (steps 4 and 5 of reaction scheme 11): [0599] Copolymerization of two alkyl methacrylate monomers with a methacrylate monomer bearing a diol function and a styrene monomer; [0600] Removal of the RAFT chain end (aminolysis of the thiocarbonylthio residue to a thiol followed by Michael addition of the thiol with an alkyl acrylate).

1.1.2.1 Synthesis of the Copolymer A-1a

[0601] More specifically, the synthesis of the copolymer A-1a is performed according to the following protocol:

First Step:

[0602] 12.56 g (37.1 mmol) of stearyl methacrytate (StMA), 12.59 g (49.5 mmol) of lauryl methacrylate (LMA), 2.57 g (24.7 mmol) of styrene (Sty), 2.54 g (12.4 mmol) of methacrylate bearing a diol function obtained according to the protocol described in section 1.1.1, 82.5 mg (0.30 mmol) of cumyl dithiobenzoate, 15 mg (0.09 mmol) of azobisisobutyronitrile (AIBN) and 30 mL of anisole are placed in a 250 mL Schlenk tube. The reaction medium is stirred and degassed for 30 minutes by bubbling nitrogen through, and is then maintained at 65 C. for a period of 24 hours.

Second Step:

[0603] After 24 hours of polymerization, the Schlenk tube is placed in an ice bath to stop the polymerization, and 30 mL of dimethytformamide (DMF) and 0.4 mL of n-butylamine (4 mmol) are added to the solution without degassing the medium. 15 hours later, 3 mL (21 mmol) of butyl acrylate are added. 16 hours later, the polymer isolated by 3 successive precipitations in methanol, filtering and drying under vacuum at 50 C. overnight. A copolymer is thus obtained with a number-average molar mass (M.sub.n) of 53 000 g/mol, a polydispersity index (Ip) of 1.19 and a number-average degree of polymerization (DP.sub.n) of 253. These values are obtained, respectively, by size exclusion chromatography using tetrahydrofuran as eluent and poly(methyl methacrylate) calibration and by monitoring the monomer conversion during the copolymerization.

[0604] A poly(alkyl methacrylate-co-alkyldiol methacrylate-co-styrene) copolymer A-1a containing about 10 mol % of diol monomer units M1 (obtained according to the protocol described in section 1.1.1) is obtained.

1.1.2.2 Synthesis of the Copolymer A-1c

[0605] The synthesis of the copolymer A-1c is performed according to the following protocol:

12.51 g (36.9 mmol) of stearyl methacrylate (StMA), 12.65 g (49.7 mmol) of lauryl methacrylate (LMA), 2.58 g (24.7 mmol) of styrene, 2.51 g (12.4 mmol) of methacrylate bearing a diol function obtained according to the protocol described in section 1.1.1, 15.3 mg (0.06 mmol) of cumyl dithiobenzoate, 4.6 mg (0.03 mmol) of AIBN and 3.2 mL of anisole are placed in a 100 mL Schlenk tube. The reaction medium is stirred and degassed for 30 minutes by bubbling nitrogen through, and is then maintained at 65 C. for a period of 24 hours.

[0606] After 24 hours of polymerization, the Schlenk tube is placed in an ice bath to stop the polymerization, and 20 mL of DMF, 30 mL of tetrahydrofuran (THF) and 0.27 mL of n-butylamine (2.7 mmol) are added to the solution. 16 hours later, 4 mL (28 mmol) of butyl acrylate are added. 24 hours later, the polymer is isolated by 3 successive precipitations in methanol and drying under vacuum at 50 C. overnight. A copolymer is thus obtained with a number-average molar mass (M.sub.n) of 154 000 g/mol, a polydispersity index (Ip) of 1.23 and a number-average degree of polymerization (DP.sub.n) of 893. These values are obtained, respectively, by size exclusion chromatography using THF as eluent and poly(methyl methacrylate) calibration and by monitoring the monomer conversion during the copolymerization.

[0607] A poly(alkyl methacrylate-co-alkyldiol methacrylate-co-styrene) copolymer A-1c containing about 9 mol % of diol monomer units is obtained.

1.1.3. Synthesis of Methacrylate Copolymers Bearing Diol Functions without RAFT Chain Removal

[0608] The synthesis of the methacrylate copolymers bearing diol functions and not having improved properties when compared with the copolymers of the prior art is performed in a single step which consists of the copolymerization of two alkyl methacrylate monomers with a methacrylate monomer bearing a diol function. The term methacrylate copolymers not having improved properties when compared with the copolymers of the prior art means methacrylate copolymers bearing diol functions not containing any styrene monomer and always bearing the thiocarbonylthio residue at the chain end. This copolymer is representative of methacrylate copolymers bearing diol functions obtained by following the protocol described in patent application WO 2015/110642 (experimental section 1.).

[0609] More specifically, the synthesis of the copolymer A-1b is performed according to the following protocol:

First Step:

[0610] 13.38 g (39.5 mmol) of stearyl methacrylate (StMA), 12.58 g (49.4 mmol) of lauryl methacrylate (LMA), 2.01 g (9.9 mmol) of methacrylate bearing a diol function obtained according to the protocol described in section 1.1.1, 93.7 mg (0.34 mmol) of cumyl dithiobenzoate, 12.4 mg (0.08 mmol) of azobisisobutyronitrile (AIBN) and 28 mL of anisole are placed in a 250 mL Schlenk tube. The reaction medium is stirred and degassed for 30 minutes by bubbling nitrogen through, and is then maintained at 65 C. for a period of 18 hours 30 minutes. The polymer is then isolated by 3 successive precipitations in methanol, filtering and drying under vacuum at 50 C. overnight. A copolymer is thus obtained with a number-average molar mass (M.sub.n) of 56 700 g/mol, a polydispersity index (Ip) of 1.21 and a number-average degree of polymerization (DP) of 253. These values are obtained, respectively, by size exclusion chromatography using tetrahydrofuran as eluent and poly(methyl methacrylate) calibration and by monitoring the monomer conversion during the copolymerization. A poly(alkyl methacrylate-co-alkyldiol methacrylate) copolymer A-1b containing about 10 mol % of diol monomer units M1 is obtained.

2. Synthesis of the Poly(Alkyl Methacrylate-Co-Boronic Ester Monomer) Copolymer

[0611] This synthesis is performed according to the protocol described in patent application WO 2016/113229 (experimental section 2.).

3. Rheological Studies

[0612] 3.1 Ingredients for the Formulation of Compositions A to F Lubricant Base Oil

[0613] The lubricant base oil used in the test compositions is an oil from group III of the API classification, sold by SK under the name Yubase 4. It has the following characteristics: [0614] Its kinematic viscosity at 40 C. measured according to the standard ASTM D445 is 19.57 cSt; [0615] Its kinematic viscosity measured at 100 C. according to the standard ASTM D445 is 4.23 cSt; [0616] Its viscosity index measured according to the standard ASTM D2270 is 122; [0617] Its Noack volatility, as a weight percentage, measured according to the standard DIN 51581 is 15; [0618] Its flash point in degrees Celsius measured according to the standard ASTM D92 is 230 C.; [0619] Its pour point in degrees Celsius measured according to the standard ASTM D97 is 15 C. [0620] Polydiol Random Copolymer A-1a (According to 61.1.2)

[0621] This copolymer comprises 10 mol % of monomer bearing a diol function and 24 mol % of styrene monomer. The mean side chain length is 13.5 carbon atoms. Its number-average molar mass is 53 000 g/mol. Its polydispersity index is 1.19. Its number-average degree of polymerization (DP.sub.n) is 253. The number-average molar mass and the polydispersity index are measured by size exclusion chromatography measurement using poly(methyl methacrylate) calibration. This copolymer is obtained by performing the protocol described in section 1.1.2.1 above.

[0622] Polydiol Random Copolymer A-1c (According to 61.1.2)

[0623] This copolymer comprises 9 mol % of monomer bearing a diol function and 26 mol % of styrene monomer. The mean side chain length is 13.5 carbon atoms. Its number-average molar mass is 154 000 g/mol. Its polydispersity index is 1.23. Its number-average degree of polymerization (DP.sub.n) is 893. The number-average molar mass and the polydispersity index are measured by size exclusion chromatography measurement using poly(methyl methacrylate) calibration. This copolymer is obtained by performing the protocol described in section 1.1.2.2 above.

[0624] Boronic Ester Random Copolymer A-2:

[0625] This copolymer comprises 5 mol % of monomers bearing boronic ester functions. The mean side chain length is 12 carbon atoms. Its number-average molar mass is 39 000 g/mol. Its polydispersity index is 1.41. Its number-average degree of polymerization (DP.sub.n) is 192. Its number-average molar mass and the polydispersity index are measured by size exclusion chromatography measurement using poly(methyl methacrylate) calibration. This copolymer is obtained by performing the protocol described in section 2 above. [0626] Polydiol Random Copolymer A-1b (According to 1.1.2)

[0627] This copolymer comprises 10 mol % of monomer bearing a diol (and does not contain any styrene). The mean side chain length is 13.8 carbon atoms. Its number-average molar mass is 56 700 g/mol. Its polydispersity index is 1.21. Its number-average degree of polymerization (DP.sub.n) is 253. The number-average molar mass and the polydispersity index are measured by size exclusion chromatography measurement using poly(methyl methacrylate) calibration. This copolymer is obtained by performing the protocol described in section 1.1.3 above.

3.2 Formulation of Compositions for the Viscosity Study

Composition A (Comparative) is Obtained in the Following Manner:

[0628] It contains a solution containing 4.20% by mass of a polymethacrylate polymer in a lubricant base oil from group III of the API classification. The polymer has a number-average molar mass (M.sub.n) equal to 106 000 g/mol, a polydispersity index (Ip) equal to 3.06, a number-average degree of polymerization of 466 and the mean side chain length is 14 carbon atoms.

[0629] This polymethacrylate is used as viscosity-index-enhancing additive. 4.95 g of a formulation with a mass concentration of 42% of this polymethacrylate in a group III base oil and 44.6 g of group III base oil are placed in a flask. The solution thus obtained is stirred at 90 C. until the polymethacrylate has fully dissolved.

[0630] A solution containing 4.20% by mass of this polymethacrylate is obtained. This composition is used as reference for the viscosity study. It represents the rheological behavior of commercial lubricant compositions.

Composition B (Comparative) is Obtained in the Following Manner:

[0631] 6.52 g of polydiol copolymer A-1a (according to 1.1.1) and 58.68 g of a group III base oil are placed in a flask. The solution thus obtained is stirred at room temperature until the polydiol A-1a has fully dissolved. A solution containing 10% by mass of polydiol copolymer A-1a is obtained.

[0632] 4.20 g of this solution of polydiol A-1a at 10% by mass in the group III base oil are mixed with 2.80 g of this same base oil. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 6% by mass of polydiol copolymer A-1a is obtained.

Composition C (Comparative) is Obtained in the Following Manner:

[0633] 7.33 g of poly(boronic ester) copolymer A-2 and 65.97 g of a group III base oil are placed in a flask. The solution thus obtained is stirred at room temperature until the poly(boronic ester) A-2 has fully dissolved. A solution containing 10% by mass of poly(boronic ester) copolymer A-2 is obtained.

[0634] 4.20 g of this solution of poly(boronic ester) A-2 at 10% by mass in the group III base oil are mixed with 2.80 g of this same base oil. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 6% by mass of poly(boronic ester) copolymer A-2 is obtained.

Composition D (Comparative) is Obtained in the Following Manner.

[0635] 4.10 g of polydiol copolymer A-1b (according to 1.1.2) and 36.90 g of a group III base oil are placed in a flask. The solution thus obtained is stirred at room temperature until the polydiol A-1b has fully dissolved. A solution containing 10% by mass of polydiol copolymer A-1b is obtained.

[0636] 4.20 g of this solution of polydiol A-1b at 10% by mass in the group III base oil are mixed with 2.80 g of this same base oil. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 6% by mass of polydiol copolymer A-1b is obtained.

Composition E (According to the Invention) is Obtained in the Following Manner:

[0637] 2.80 g of the solution containing 10% by mass of polydiol A-1a prepared previously and 1.4 g of group III base oil are placed in a flask. 2.80 g of the solution containing 10% by mass of poly(boronic ester) A-2 prepared previously are added to this solution. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 4% by mass of polydiol copolymer A-1a and 4% by mass of poly(boronic ester) copolymer A-2 is obtained.

Composition F (Comparative) is Obtained in the Following Manner

[0638] 2.80 g of the solution containing 10% by mass of polydiol A-1b prepared previously and 1.40 g of group III base oil are placed in a flask. 2.80 g of the solution containing 10% by mass of poly(boronic ester) A-2 prepared previously are added to this solution. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 4% by mass of polydiol copolymer A-1b and 4% by mass of poly(boronic ester) copolymer A-2 is obtained.

Composition G (According to the Invention) is Obtained in the Following Manner.

[0639] 1.05 g of the solution containing 10% by mass of polydiol A-1c prepared previously and 5.25 g of group III base oil are placed in a flask. 0.70 g of the solution containing 10% by mass of poly(boronic ester) A-2 prepared previously are added to this solution. The solution thus obtained is stirred at room temperature for 5 minutes. A solution containing 1.5% by mass of polydiol copolymer A-1c and 1% by mass of poly(boronic ester) copolymer A-2 is obtained.

[0640] 3.3 Apparatus and Protocol for Measuring the Viscosity

[0641] The rheological studies were performed using a Couette MCR 501 controlled stress rheometer from the company Anton Paar.

[0642] In the case of the polymer formulations which do not form gels in a group III base oil over the temperature range of the study (compositions A to F), the rheology measurements were performed using a cylindrical geometry of reference DG 26.7. The viscosity was measured as a function of the shear rate for a temperature range extending from 10 C. to 150C. For each temperature, the viscosity of the system was measured as a function of the shear rate from 1 to 100 s.sup.1. The measurements of the viscosity as a function of the shear rate at T=10 C., 50 C., 70 C., 110 C., 130 C. and 150 C. were performed (going from 10 C. to 150 C.). A mean viscosity was then calculated for each temperature using the measurement points located on the same plateau.

[0643] The relative viscosity calculated according to the following formula

[00001]$\left({}_{\mathrm{relative}}=\frac{{}_{\mathrm{solution}}}{{}_{\mathrm{base}.Math..Math.\mathrm{oil}}}\right)$

was chosen to represent the change in viscosity of the system as a function of the temperature, since this magnitude directly reflects the compensation for the natural viscosity loss of a group III base oil of the polymer systems studied.

[0644] 3.4 Rheological Results Obtained

[0645] The relative viscosity of compositions A, E and F was studied for a temperature range extending from 10 to 150 C. whereas that of compositions B, C and D was studied between 10 C. and 110 C. In the cases where the viscosity was not perfectly constant with the shear rate, the viscosity of the solution was calculated by taking the mean of the viscosities obtained on all the shear rates. The relative viscosity of these compositions is illustrated in FIGS. 6, 7 and 8. Copolymer A-1a alone in composition B does not allow a significant compensation for the natural viscosity loss of the group III base oil (FIG. 6). This is likewise the case for the poly(boronic ester) copolymer A-2 when it is used alone in composition C or else for the polydiol copolymer A-1b when it is used alone in composition D (FIG. 6).

[0646] When the polydiol random copolymer A-1a and the poly(boronic ester) copolymer A-2 are present together in the same lubricant composition (composition E), compensation for the natural viscosity loss of the group III base oil which is greater than that which results from the addition of the polymer methacrylate polymer to the group III base oil (composition A) at 150 C. is observed (FIG. 7). At the same time, composition E shows a lower relative viscosity than composition A (reference polymethacrylate) at 10 C. (FIG. 7). The relative viscosity values are also represented for three successive cycles of heating-cooling between 10C to 150 C. (E-1, E-2 and E-3). These values change slightly in the course of the 3 cycles, but still give an increase in the relative viscosity of about 1 between 10 C. and 150 C., reflecting the great compensation for the natural viscosity loss of the group III base oil over this temperature range (FIG. 8).

[0647] When the polydiol random copolymer A-1b and the poly(boronic ester) copolymer A-2 are present together in the same lubricant composition (composition F), a slight compensation for the natural viscosity loss of the group III base oil is observed (FIG. 7). This compensation is lower than in the case of composition E. The relative viscosity values are also represented for the first three successive cycles from 10 C. to 150 C. (F-1, F-2 and F-3). During the first cycle, composition F gives relative viscosity values that are virtually identical to those of composition E from 10 C. to 110 C. On the other hand, when the temperature reaches 130 C. and then 150 C. during the first heating cycle, the relative viscosity drops substantially (FIG. 8). The next two cycles (F-2, F-3) give comparable relative viscosities and an increase in the relative viscosity when the temperature increases (FIG. 8). For these two cycles an increase in relative viscosity of less than 0.5 is reached between 10 C. and 150 C. This result shows that composition F appears to be degraded after its first passage beyond 110 C. The effect of this degradation is to reduce the compensation for the natural viscosity loss of the oil obtained with this composition (up to 110 C. in the first cycle). The change in composition of the polydiol (addition of styrene and removal of the RAFT chain end) thus made it possible to maintain the rheological properties of composition E for several cycles above 110 C.

[0648] The relative viscosity values for formulation G are represented for three successive cycles of heating-cooling between 10 C. to 150 C. (G-1, G-2 and G-3) in FIG. 11. These values change slightly in the course of the three cycles, but still give an increase in the relative viscosity of about 0.55 between 10 C. and 150 C., reflecting the great compensation for the natural viscosity loss of the group III base oil over this temperature range. Furthermore, irrespective of the cycle, the composition gives a relative viscosity ranging from about 1.3 at 10 C. to about 1.85 at 150 C. Composition G thus appears to be more stable than composition F for this study.

4. Thermogravimetric Studies

[0649] 4.1 Apparatus and Protocols for Thermogravimetric Analysis (TGA)

[0650] The thermogravimetric studies were performed using a TG 209 F1 thermogravimetric analyzer from the company Netzsch. The experiments were performed under a stream of 20 mL/minute of dinitrogen. 15 to 30 mg of polymers are placed in an aluminum crucible before each analysis.

[0651] The isotherms were applied for 20 hours at 150 C., whereas the ramps were applied from 25 C. to 600 C. at a heating rate of 10 C./minute.

[0652] 4.2 TGA Results

[0653] The thermal stability of the polydiol A-1a, of the polydiol A-1c and of the polydiol A-1b was studied under a dinitrogen atmosphere via two different protocols. Firstly, the polydiols were subjected to a temperature ramp from 25 C. to 600 C. so as to observe the change in mass of the samples as a function of the temperature (FIG. 9). Secondly, the polydiols were subjected to an isotherm at 150 C. for 20 hours so as to the change in mass as a function of time under these conditions (FIG. 10).

[0654] During the temperature ramp from 25 C. to 600 C., the polydiol A-1a loses 1% of its mass at 290 C. and 5% of its mass at 335 C. and the polydiol A-1c loses 1% of its mass at 240 C. and 5% of its mass at 310 C., whereas the polydiol A-1b loses 1% of its mass at 220 C. and 5% of its mass at 290 C. (FIG. 9). Above 320 C., the three polydiols show a very rapid loss of mass leading to total degradation of these polymers at 450 C. Initiation of the loss of mass thus takes place at lower temperatures for the polydiol A-1b than for the polydiol A-1a and the polydiol A-1c.

[0655] Even during the isotherm of 20 hours at 150 C., the polydiol A-1a loses 0.8% of its mass after 2 hours and 1.1% of its mass after 20 hours and the polydiol A-1c loses 0.4% of its mass after 2 hours and 0.4% of its mass after 20 hours, whereas the polydiol A-1b loses 1% of its mass after 2 hours and 3.3% of its mass after 20 hours (FIG. 10). It is probable that the loss of mass which takes place at the start of the isotherm is attributable to the loss of water which has been adsorbed onto the polymers. This result shows that the polydiol A-1b has a greater loss of mass than the polydiol A-1a and the polydiol A-1c during this isotherm. Furthermore, a constant rate of loss of mass appears to become established after 4 hours of isotherm for the polydiol A-1a and after 7 hours of isotherm for the polydiol A-1b. The polydiol A-1a has a loss of mass of 0.009%/hour, whereas the polydiol A-1b reaches a rate of 0.037%/hour (FIG. 10). The polydiol A-1c does not show any significant loss of mass after 3 hours of isotherm at 150 C. This measurement indicates that the polydiol A-1 b degrades more rapidly than the polydiol A-1a and the polydiol A-1c under these conditions.