The present invention relates to a method of producing water-soluble copolymers of at least one water-soluble N-vinyllactam and at least one hydrophobic comonomer by free-radical polymerization of the monomers in an organic solvent, and to the copolymers obtainable by the method and their use.
The production of copolymers from N-vinyllactams and hydrophobic comonomers by free-radical polymerization is known. The production of such copolymers takes place in an organic solvent, for example an alcohol or in a mixture of water and organic solvent with a high content of solvent. Usually, the polymerization is carried out under reflux of the solvent. The hydrophobic monomers that are more readily volatile compared to the N-vinyllactams pass in this way into the gas phase and into the condensate.
For many application purposes, copolymers are desired which dissolve in water to give clear solutions, i.e. the FNU value of a 5% strength by weight solution should be <20. However, there is the problem that differing reactivities and differing polarity of the monomers can lead to increases in the concentration of the hydrophobic monomers which results in homopolymers which are not water-soluble being able to be formed from the hydrophobic monomers. Even in small amounts in the range from 500 to 1000 ppm, such homopolymers lead to turbidity of an aqueous solution of the copolymers. The increases in concentration of hydrophobic monomers can arise in particular in the gas phase and in the condensate, and also on the reactor wall and the surface of the polymerization medium.
U.S. Pat. No. 5,395,904 describes the polymerization of vinylpyrrolidone and vinylacetate by controlled polymerization according to the feed method. An alcoholic solvent is used which can comprise up to 50% by weight of water.
U.S. Pat. No. 5,319,041 describes the preparation of copolymers of vinylpyrrolidone and vinyl acetate by polymerization according to the feed method with control of the polymerization temperature.
U.S. Pat. No. 5,502,136 describes a method of producing copolymers of vinylpyrrolidone and vinyl acetate according to the feed method, where the feeds are controlled via a scheme defined by specific mathematical formulae.
U.S. Pat. No. 4,520,179 and U.S. Pat. No. 4,554,311 describe the polymerization of vinylpyrrolidone and vinyl acetate with t-butyl peroxypivalate as initiator in water or water/alcohol mixtures. The initiator used therein allows the production of copolymers with a narrow molecular weight distribution, that does not lead to water-soluble products with a FNU value of <20.
EP-A 161 describes a method of producing copolymers of vinylpyrrolidone and vinyl acetate where, after the polymerization, an after polymerization with specific initiators is carried out. However, the polymers have high residual contents of vinyl acetate and are not sufficiently nonturbid.
EP-A 795 567 describes the production of copolymers of vinyllactams and hydrophobic monomers by polymerization in aqueous solution.
EP-A discloses the production of copolymers of vinylpyrrolidone and vinyl esters which dissolve in water to give clear solutions, where, at a certain point during the polymerization, a solvent exchange is carried out in order to remove volatile constituents. This method is relatively complex.
DE-A 22 18 935 describes the copolymerization of N-vinylpyrrolidone with various water-soluble and water-insoluble comonomers. Use is made here of water-insoluble initiators which are used in the form of a finely divided suspension in an aqueous solution of the copolymers. However, in the case of the water-insoluble comonomers, this does not likewise lead to the desired water-soluble copolymers with a FNU value of <20.