Amphiphilic block polymers soluble in strongly saline medium

1. A process for preparing a block copolymer, the process comprising a step (E) of micellar radical polymerization comprising placing the following in contact, in an aqueous medium (M):

hydrophilic monomers, dissolved or dispersed in said aqueous medium (M);

hydrophobic monomers in the form of a micellar solution containing, in dispersed form in the aqueous medium (M), micelles comprising the hydrophobic monomers, wherein the hydrophobic monomers comprise t-butylstyrene;

at least one radical polymerization initiator,

wherein said micelles comprise at least one surfactant of amphoteric nature.

2. The process as claimed in claim 1 , wherein the amphoteric surfactant is selected from the group consisting of:

betaines;

sulfobetaines;

alkylamphoacetates and alkylamphodiacetates;

alkylamphopropionates or alkylamphodipropionates;

alkyl amphohydroxypropyl sultaines;

iminopropinates;

alkyl amine oxides; and

mixtures of surfactants comprising one or more of the abovementioned compounds.

3. The process as claimed in claim 2 , wherein the amphoteric surfactant is selected from the group consisting of:

cetyl betaine;

sulfobetaines;

alkylamphoacetates and alkylamphodiacetates;

alkylamphopropionates or alkylamphodipropionates;

alkyl amphohydroxypropyl sultaines;

iminopropionates;

lauramine oxide; and

mixtures of surfactants comprising one or more of the abovementioned compounds.

4. The process as claimed in claim 1 , wherein the hydrophobic monomers of step (E) comprise a C 1 -C 30 alkyl α,β-unsaturated ester.

5. The process as claimed in claim 4 , wherein the hydrophobic monomers of step (E) comprise a C 4 -C 22 alkyl α,β-unsaturated ester.

6. The process as claimed in claim 5 , wherein the hydrophobic monomers of step (E) comprise lauryl methacrylate.

7. The process according to claim 1 , wherein the at least one radical polymerization initiator is water-soluble or water-dispersible.

8. The process according to claim 1 , comprising placing the following in contact, in the aqueous medium (M):

hydrophilic monomers, dissolved or dispersed in said aqueous medium (M);

hydrophobic monomers in the form of a micellar solution;

at least one radical polymerization initiator, and

at least one radical polymerization control agent, wherein said micelles comprise at least one surfactant of amphoteric nature.

9. The process as claimed in claim 8 , wherein the radical polymerization control agent is a compound which comprises a thiocarbonylthio —S(C═S)— group.

10. The process according to claim 9 , wherein the thiocarbonylthio —S(C═S)— group is a xanthate.

11. The process as claimed in claim 8 , wherein the radical polymerization control agent is an oligomer which is of water-soluble or water-dispersible nature bearing a thiocarbonylthio —S(C═S)— group and which

is soluble or dispersible in the aqueous medium (M) used in step (E); and/or

is not suitable for penetrating into the micelles of the micellar solution.

12. The process as claimed in claim 11 , wherein the radical polymerization control agent is a prepolymer bearing at least one thiocarbonylthio —S(C═S)— group obtained on conclusion of a step (E 0 ), prior to step (E), said step (E 0 ) comprising placing in contact

hydrophilic monomers;

a radical polymerization initiator; and

a control agent bearing a thiocarbonylthio group —S(C═S)—.

13. The process according to claim 12 , wherein the hydrophilic monomers of step (E 0 ) are identical to those of step (E).

14. A polymer composition obtained according to the process of claim 1 .

15. A method comprising regulating rheology of a liquid medium by dissolving in the liquid medium the polymer composition or the polymer contained in the polymer composition as claimed in claim 14 .

16. The method as claimed in claim 15 , wherein said polymer composition is dissolved in an aqueous medium comprising more than 50 g/l of salt.

17. The method according to claim 15 , wherein the liquid medium is an aqueous medium.

18. The method according to claim 17 , wherein the aqueous medium is for oil or natural gas extraction.