Method for the continuous production of anionic polymers using radicals

1. A method for continuously preparing an anionic polymer by radical polymerization, the method comprising:

introducing at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, and a mixture thereof; water; at least one initiator; and optionally at least one chain transfer agent into a tubular reactor, thus forming a fluid stream in the tubular reactor,

carrying out a polymerization reaction in the tubular reactor, optionally with a heating means to initiate the polymerization reaction and with a residence time in the reactor of greater than 1 min,

subjecting the fluid stream during the polymerization reaction to an oscillatory movement,

and

obtaining the anionic polymer in solution in water at an outlet of the tubular reactor,

wherein

the at least one monomer is optionally partially neutralized,

a temperature of the water is greater than 20° C.,

a solids content of the water solution of the anionic polymer is between 20% and 60% by weight, relative to a total weight of the anionic polymer, and a molecular weight Mw of less than 10 000 g/mol,

the tubular reactor has a length L R of at least 5 m and comprises at least one tubular section of length L S and internal diameter D such that L S is at least 20 times larger than D,

each tubular section comprises, over an entire length, a plurality of stationary baffles having holes, opposing the fluid stream, in a form of washers having a diameter identical to the internal diameter D of the tubular section,

each tubular section is connected to a device, which subjects the fluid stream to an oscillatory movement,

an amplitude of the oscillatory movement is from 0.3×d 2 to 4×d 2 , wherein d 2 is the external diameter of the stationary baffles,

a frequency of the oscillatory movement is from 0.1 to 100 Hz,

and

the residence time in the tubular reactor, size of the holes in the baffles, spacing of the baffles, and oscillatory movement of the device provides at any point in the tubular reactor, a homogeneous fluid stream.

2. The method of claim 1 , wherein D is less than 20 cm.

3. The method of claim 1 , wherein the baffles are spaced out, regularly or irregularly, by a distance ranging from 1D to 3D.

4. The method of claim 1 , wherein the baffles comprise concentric annular holes such that a d 2 /d 1 ratio varies from 1.2 to 5, wherein d 2 is an external diameter of the washer and d 1 the internal diameter of the washer.

5. The method of claim 4 , wherein amplitude of the oscillatory movement varies from 0,3×d 2 to 4×d 2 , and frequency of the oscillatory movement varies from 0.1 to 100 Hz.

6. The method of claim 1 , wherein the reactor comprises one or more devices which provide or discharge heat so as to allow different temperatures from one area to the other.

7. The method of claim 1 , wherein the at least one initiator is introduced at an inlet and/or downstream of the tubular reactor, one or more times.

8. The method of claim 1 , wherein the at least one monomer is introduced at an inlet and/or downstream of the tubular reactor, one or more times.

9. The method of claim 1 , wherein a catalyst based on water-soluble metal salts is also introduced, at an inlet and/or downstream of the tubular reactor, one or more times.

10. The method of claim 1 , wherein

at least one other ethylenically unsaturated monomer is provided, and

the at least one other ethylenically unsaturated monomer is selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), maleic acid, fumaric acid, crotonic acid, itaconic acid, an unsaturated acrylic acid telomer, and a monomer of formula (I):

wherein:

R a , R b , and R c independently represent H or CH 3 , and

n is an integer varying between 0 and 2.

11. The method of claim 1 , wherein the at least one monomer is acrylic acid and the anionic polymer obtained is poly(acrylic acid) having a molecular weight Mw of from 1000 to 10000 g/mol and a polydispersity index of from 1.5 to 4.

12. The method of claim 1 wherein a neutralizing agent is injected into the tubular reactor, in an area where a rate of conversion of the monomer is greater than 90%.

13. The method of claim 1 , wherein the tubular reactor is connected, at an outlet, to another tubular reactor optionally equipped with an oscillatory device, to a stirred reactor, and/or to a flash distillation column.