Method of preparation of cationic polymers with reduced halides content

A method for preparing an aqueous solution of water-soluble cationic polymers P1 wherein the halides content is lower than 10% by weight of the polymer, the viscosity at 25° C. is lower than 200 cps, said viscosity being determined for an aqueous solution of polymers P1 concentrated at 50% by weight, and the cationic charge density is greater than or equal to 4 meq·g−1, characterized in that it comprises the following successive steps:

• a) adding, at a temperature comprised between 0° C. and 120° C., of at least one compound of formula (I) to an aqueous solution of at least one water soluble cationic polymer P2 wherein the halides content is greater than 10% by weight of the polymer, the viscosity at 25° C. is lower than 200 centipoise cps, said viscosity being determined for an aqueous solution of polymers P2 concentrated to 50% by weight, and the cationic charge density being greater than or equal to 4 meq·g−1, the compound of formula (I) being defined by the formula:
• R1—COO−Y1+, wherein:
• R1 represents a hydrogen atom or a saturated alkyl chain, either linear or branched, comprising from 1 to 8 carbon atoms, that may contain at least one nitrogen atom and/or oxygen atom, said chain may be substituted by 1 to 4 carboxylate functional groups of formula —COR;
• Y1+ represents an alkali metal cation, an ammonium ion of formula R2—NH3+, or a quaternary ammonium of formula R3—N+(R4)(R5)(R6);
• R represents an OH group or a group O−Y2+;
• Y2+ represents an alkali metal cation or an ammonium ion of formula R2—NH3+;
• R2 represents a hydrogen atom or a saturated alkyl chain, either linear or branched, comprising from 1 to 4 carbon atoms;
• and
• R3, R4, R5 and R6 represent, independently of each other, a saturated alkyl chain, either linear or branched, comprising from 1 to 4 carbon atoms;
• in order to obtain a mixture;
• b) agitation of the mixture obtained in step a) for at least 5 minutes in order to obtain a stirred mixture;
• c) decreasing of the temperature of the stirred mixture obtained at the end of step b) at a temperature comprised between −10° C. and 50° C. in order to obtain a cooled mixture;
• and
• d) liquid/solid separation of the cooled mixture obtained at the end of step c) in order to obtain an aqueous solution of cationic polymers P1.