Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1998-06-25
2001-02-13
Henderson, Christopher (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S211000
Reexamination Certificate
active
06187884
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a process for preparing highly concentrated aqueous solutions of low molecular mass homopolymers of N-vinylpyrrolidone.
The preparation of N-vinylpyrrolidone polymers by free-radical polymerization is known. The mechanism of polymerization under various conditions is described, for example, in Polymer Journal, 17 (1985) 143-152. Polymerization in organic solvents, for example in alcoholic solution in accordance with U.S. Pat. No. 4,053,696, leads to polyvinylpyrrolidone of low molecular mass, since the organic solvents may act as chain regulators. To prepare highly concentrated aqueous solutions of the polymer, however, at least the majority of the organic solvents have to be distilled off and then either disposed of or reprocessed.
The polymerization of N-vinylpyrrolidone in aqueous solution has to date usually been carried out in the presence of hydrogen peroxide as initiator, as is described, for example, in U.S. Pat. No. 2,335,454. In this case the molecular weight of the polyvinylpyrrolidone depends on the hydrogen peroxide concentration: low molecular weights result from high hydrogen peroxide concentrations, and vice versa. Highly concentrated aqueous solutions of the polyvinylpyrrolidone, however, cannot be prepared, since high monomer concentrations of >30% by weight make it impossible to control the exothermic reaction, and in the case of feed techniques the highly grafting effect of the hydrogen peroxide leads to an unwanted buildup of molecular weight.
WO 94/26796 discloses the preparation of low molecular mass copolymers of N-vinylimidazole and N-vinylpyrrolidone using an azo initiator in aqueous solution in the presence of a sulfur compound as regulator.
DE 2 218 935 describes the polymerization of N-vinylpyrrolidone, alone or in a mixture with minor amounts of other monoolefinically unsaturated monomers, in an aqueous medium in the presence of water-insoluble polymerization catalysts which form free radicals and which are added in the form of a fine suspension in an aqueous solution of the N-vinylpyrrolidone polymers.
It is an object of the present invention to provide a simple process for preparing highly concentrated aqueous solutions of low molecular mass homopolymers of N-vinylpyrrolidone.
We have found that this object is achieved by carrying out polymerization in aqueous solution in the presence of particular polymerization regulators.
BRIEF SUMMARY OF THE INVENTION
The present invention therefore provides a process for preparing highly concentrated aqueous solutions of low molecular mass homopolymers of N-vinylpyrrolidone by free-radical solution polymerization in an aqueous medium with H
2
O
2
as initiator, which comprises carrying out polymerization in the presence of from 0.1 to 30% by weight, based on the N-vinylpyrrolidone employed, of a polymerization regulator selected from the group consisting of C
1
-C
6
-alkanols, hydroxylamine salts and water-soluble compounds containing sulfur in bonded form.
One advantage of this process is the increased space-time yield of the polymerization plant. In addition, the highly concentrated polymer solutions are also advantageous in terms of transportation and of conversion to a powder product.
DETAILED DESCRIPTION OF THE INVENTION
Suitable initiators for the free-radical solution polymerization of the N-vinylpyrrolidone are those customary for aqueous solution polymerization. Preferred polymerization initiators include water-soluble peroxides and hydroperoxides, such as tert-butyl hydroperoxide, tert-amyl hydroperoxide, cumene hydroperoxide, pinane hydroperoxide, peroxodisulfuric acid and its salts, especially its alkali metal salts or ammonium salts, and also percarbonates, peroxo esters and hydrogen peroxide. It is preferred to use hydrogen peroxide. It is water-soluble, inexpensive, commercially available and does not contaminate the polymer solution with decomposition products. The initiator is employed in amounts of from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, based on the amount of N-vinylpyrrolidone. The commercially available 30 or 50% strength by weight solution if desired may be diluted to concentrations of about 10% by weight before being added to the reaction solution.
H
2
O
2
is advantageously used together with transition metal compounds whose metal in aqueous solution is able to exist in different oxidation states, examples being iron(II) salts or copper(II) salts, i.e. redox initiator systems.
The polymerization medium used is water. Polymerization is preferably conducted at a pH in the range from 6 to 9 in order to avoid hydrolysis of the N-vinylpyrrolidone. Preferably, therefore, before beginning the polymerization the solutions of the individual components are adjusted to a pH within this range using a suitable base, such as aqueous sodium hydroxide or aqueous ammonia solution, or the pH of the reaction medium is kept within the range from 6 to 9 during polymerization by adding an appropriate base.
The polymerization temperatures should be chosen so that the half-lives of initiator decomposition are from 0.5 to 5 hours, preferably from 1 to 3 hours. This is usually the case at from 60 to 85° C. Therefore, polymerization is conducted preferably in the range from 60° C. to 85° C., in particular at from 65to 80° C.
Polymerization is preferably conducted in the presence of polymerization regulators which contain sulfur in bonded form, as specified, for example, in WO 94/26796. Examples of such compounds include inorganic hydrogen sulfites, disulfites and dithionites or organic sulfides, disulfides, polysulfides, sulfoxides, sulfones and mercapto compounds. Specific examples of polymerization regulators are the following: di-n-butyl sulfide, di-n-octyl sulfide, diphenyl sulfide, thiodiglycol, ethylthioethanol, diisopropyl disulfide, di-n-butyl disulfide, di-n-hexyl disulfide, diacetyl disulfide, di-t-butyl trisulfide and dimethyl sulfoxide. Compounds preferably employed as polymerization regulators are mercapto compounds, dialkyl sulfides, dialkyl disulfides and/or diaryl sulfides. Examples of these compounds are ethyl thioglycolate, cysteine, 2-mercaptoethanol, 1,3-mercaptopropanol, 3-mercaptopropane-1,2-diol, 1,4-mercaptobutanol, mercaptoacetic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, thioglycerol, thioacetic acid, thiourea and alkyl mercaptans such as n-butyl mercaptan, n-hexyl mercaptan or n-dodecyl mercaptan. Of these, preference is given to the use of mercapto alcohols and mercapto carboxylic acids.
Other suitable regulators are hydroxylamine salts, such as hydroxylammonium sulfate.
Particularly preferred polymerization regulators are C
1
-C
6
alcohols, of which ethanol, n-propanol and isopropanol are very particularly suitable.
The polymerization regulators are used in amounts of from 0.1 to 30% by weight, preferably from 0.1 to 20% by weight, based on the N-vinylpyrrolidone monomer employed in the polymerization. Mixtures of the polymerization regulators that are to be employed in accordance with the invention can also be used.
The polymerization of N-vinylpyrrolidone in water at a high monomer concentration of >30% by weight can for safety reasons not be carried out by the batch procedure. Consequently, semibatch techniques and feed techniques are preferably employed as polymerization methods. In the case of these process variants at least one of the components, or a mixture of two or more components, is added continuously or in portions within a certain period to a mixture of the remaining components. For instance, a solution of the polymerization regulator and an initiator solution can be added to a mixture of N-vinylpyrrolidone and water at the polymerization temperature. An alternative method is to add hydrogen peroxide to the initial charge, at or below the polymerization temperature, and to add only the regulator, or a solution of the regulator, to the reaction mixture within a predetermined period of time after the polymerization temperature has been reached.
Blankenburg Rainer
Kothrade Stephan
Peters Iris
Sanner Axel
BASF - Aktiengesellschaft
Henderson Christopher
Keil & Weinkauf
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