PREPARATION OF A WATER-SOLUBLE POLYMERIC DISPERSANT HAVING LOW POLYDISPERSITY The present invention relates to the preparation of a low polydispersity index, water-soluble polymeric dispersant. Water soluble polymeric dispersants such as low molecular weight polyacrylates are widely used in detergent formulations as crystal growth inhibitors, in water treatment processes as antiscalants, and as dispersants in clay and mineral processing as well as in coating formulations.

Water-soluble polymeric dispersants that are produced by conventional free radical polymerization methods typically have a high polydispersity index (defined by weight average molecular weight over number average molecular weight or MW/M,,).

Although the theoretical limit for polydispersity (MW/Mn) in radical chain polymerizations is 1. 5 or less, in practice the distribution is greater than 2.

The performance of water-soluble polymers can depend strongly on both their molecular weight and MW/M n. For example, detergent performance is known to depend strongly on polymer molecular weight and structure. In conventional detergents, under low temperature U. S. wash conditions, clay removal is maximized and calcium carbonate fabric encrustation is minimized when detergents are formulated using sodium polyacrylate of approximately 5000 molecular weight.

Higher molecular weight polymers, which are less effective crystal growth inhibitors and clay dispersants, allow calcium carbonate deposition to increase, resulting in reduced clay cleaning performance. Thus, there is an obvious need for controlled molecular weight and low polydispersity for acrylate polymers. (See Witiak in Detergents and Cleaners : A Handbook for Formulators, ed., K. Robert Lange, Hanser Publishers, Munich, Vienna, New York, pp 113-132 (1994) ; and Zini, ("Polymeric Additives for High Performing Detergents" (1995).

In another example of the desirability of low polydisperse polyacrylates, Farrar et al. (U. S. Patent 4, 507, 422) discloses that water-soluble polymeric dispersants have greatly improved pigment dispersing properties if the polymer has a MWtMn of less than 1. 5. It is presumed that molecular weights that are too low or too high interfere with the function of the optimal molecular weight fractions by competitive absorption or dilution effects.

Farrar et al. teaches that water-soluble polyacrylates having a low M. (that is, a weight average molecular weight of about 1000 to 10, 000) and low MW/Mn can be achieved by a variety of techniques including a) using conventional polymerization methods to obtain a polymer having high polydispersity, then fractionally precipitating the polymer to obtain polymers having a polydispersity below 1. 5 ; b) synthesizing the polymer in the presence of isopropanol as a chain regulator under closely monitored and controlled conditions ; and c) preparing a water-insoluble acrylate polymer having the desired molecular weight and MJM,,, then hydrolyze the acrylate to the free acid.

However, all of these approaches are labor intensive and inefficient.

In U. S. Patent 5, 412, 047, Georges et al. discloses the use of the carbonyl- containing nitroxide 4-oxo-TEMPO as a free radical agent suitable for controlling the molecular weight of acrylate polymers. Georges et al. further teaches (column 13, lines 65-68 to column 14, lines 1-4) that nitroxide compounds that do not contain oxo groups, while satisfactory for the purpose of moderating the polymerization of a wide variety of different monomer types,"were completely ineffective when used in homopolymerizations of acrylate monomers (for example, n-butylacrylate." Presumably, the homopolymer (column 14, lines 42-45)"was sufficiently thermally unstable or the stable free radical had a sufficiently strong inhibitory effect under the reaction conditions so as to preclude homoacrylate polymer product formation." The reaction conditions disclosed by Georges et al. are in the range of 100°C and 180°C ; at temperatures below 100°C (column 16, lines 13-16),"the reaction rate is slow and industrially impractical without the aid of an acid or base accelerating additive compound."At these elevated temperatures, the polymerization reactor must be equipped to operate at elevated pressure inasmuch as water is used as the solvent.

In view of the deficiencies in the art, it would be desirable to prepare more efficiently and under milder conditions a polyacid (or acid salt) having controlled molecular weight and low polydispersity.

The present invention addresses the deficiencies in the art by providing a method for preparing a dispersion or a solution of a polymer having a backbone that contains pendant acid groups and pendant acid salt groups, comprising the steps of

contacting together under polymerization conditions a solvent, a polymerizable acid monomer, a polymerizable acid salt monomer, a radical initiator, and a stable free radical agent, with the proviso that when a) the polymerization is carried out at a reaction temperature greater than the boiling point of the mixture at ambient pressure ; and b) the stable free radical agent is a carbonyl containing nitroxide ; the mole-to-mole ratio of the acid salt monomer to the acid monomer is not less than 51 : 49 and not greater than 95 : 5.

In another aspect, the present invention is a method for preparing a polymer having a backbone that contains pendant acid groups and pendant acid salt groups comprising the steps of contacting together under polymerization conditions, a polymerizable acid monomer, a polymerizable acid salt monomer, and a radical initiator wherein the mole-to-mole ratio of the acid monomer to the acid salt monomer is from about 5 : 95 to about 49 : 51.

A low polydisperse, controlled molecular weight water-soluble polymer containing pendant acid groups and pendant acid salt groups can be prepared by aqueous polymerization with appropriate pH control or non-aqueous precipitation polymerization in the presence of a stable free radical agent. As used herein, the term"acid salt"refers to a deprotonated acid. Preferably the acid groups are carboxylic acid groups or sulfonic acid groups, more preferably carboxylic acid groups.

The polymers are advantageously prepared from a polymerizable acid monomer and an acid salt monomer in a suitable solvent, which may be water- containing or organic and non-aqueous. Preferred solvents include water, Cl-C, alcools, and toluene, and mixtures thereof. Water is a more preferred solvent.

Preferred polymerizable acid monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, crotonic acid, vinyl acetic acid, acryloxypropionic acid and 2-acrylamido-2-methyl-1-propane sulfonic acid, styrene sulfonic acid, and combinations thereof. More preferred polymerizable acid monomers include acrylic acid, methacrylic acid, maleic anhydride, maleic acid, and

combinations thereof. A preferred copolymer is prepared by the polymerization of about 50 to 80 mole equivalents of a partially neutralized acrylic acid and about 20 to 50 mole equivalents maleic anhydride. (Although maleic anhydride is not strictly speaking an acid, it is often conveniently used instead of maleic acid ; structural units of maleic acid and its monobasic or dibasic salt will form in situ during the course of the polymerization reaction. Thus, for the purposes of this disclosure, maleic anhydride is an acid monomer.) Acrylic acid is the most preferred acid monomer for homopolymerization reactions. Preferred acid salts are alkali metal and ammonium salts of an acid monomer, particularly sodium and potassium salts of acrylic acid or maleic acid.

It may also be desirable in some instances to include a small amount of a polymerizable nonionic compound or a polymerizable cationic or amine salt with the polymerizable acid monomer. Examples of suitable polymerizable nonionic compounds include acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate ; methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate ; acrylamides and substituted acrylamides. An example of a suitable polymerizable cationic salt is the chloride salt of trimethyl aminoethyl methacrylate ; suitable amine salts include aminoalkyl methacrylates such as aminoethylmethacrylate. In general, the amount of these ancillary nonacid monomers does not exceed 10 mole percent.

The mixture containing a polymerizable acid monomer and an acid salt monomer is said to contain a partially neutralized acid monomer. It is to be understood that the term"partially neutralized acid monomer"may include more than one monomer. A polymer having a backbone that contains pendant acid groups and pendant acid salt groups is said to be a partially neutralized polymer. That is to say, at least some of the pendant acid groups exist in the acid salt form, preferably, the alkali metal or ammonium salt form. The term"percent neutralized"refers to the mole percent of pendant groups that are in the acid salt form. Preferably, the polymer is not less than 40 percent neutralized (that is, 40 parts salt and 60 parts acid), more preferably not less than 51 percent neutralized, and most preferably not less than 60 percent neutralized ; and preferably not more than 95 percent neutralized, more preferably not more than 85 percent neutralized, and most preferably not more than

80 percent neutralized. Partial neutralization can be accomplished by addition of an appropriate amount of base to the acid monomer. Suitable bases include, but are not restricted to ammonium, alkali metal, and alkaline earth hydroxides and carbonates such as sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, and calcium carbonate.

It is to be understood that the acid salt monomer can be, or include, a salt of a variety of acid monomers. For example, if the acid monomer were acrylic acid, the acid salt is preferably the salt of acrylic acid, but could include, for example, the sodium salt of maleic acid.

The initial concentration of the acid monomer and the acid salt monomer in the solvent, preferably water, based on the weight of the monomers and the solvent, is preferably not less than about 0. 1 weight percent, more preferably not less than about 1 weight percent, preferably not more than about 20 weight percent, more preferably not more than about 10 weight percent. In the case where a stable free radical is not used, the preferred initial concentration of the acid monomer and the acid salt monomer is not greater than about 10, more preferably not greater than 5, and most preferably not greater than 3 weight percent.

The stable free radical agent is any agent that contains or produces free radicals that persist in some amount approximately throughout the course of the reaction in which the radicals are present. The agent may naturally contain stable free radicals or can be made to contain stable free radicals by processes such as thermal or photolytic decomposition. Examples of stable free radical agents include nitroxide free radicals such as 2, 2, 6, 6-tetramethyl-1-piperidinyloxy free radical (TEMPO) ; 4-hydroxy-2, 2, 6, 6-tetramethyl-1-piperidinyloxy free radical ; 2, 2, 5, 5- tetramethyl-1-pyrrolidinyloxy free radical ; 3-carboxy-2, 2, 5, 5-tetramethyl-1- pyrrolidinyloxy free radical ; 4-oxo-2, 2, 6, 6-tetramethyl-1-piperidinyloxy free radical (4- oxo-TEMPO) ; and di-t-butylnitroxide ; stable free radicals generated from dithiocarbamates and other disulfides such as tetraethyl thiuram disulfide S-benzyl dithiocarbamate, cyclic disulfides ; and aminophenyldisulfides, and benzyl N, N- diethyidithiocarbamate, which decompose to form sulfur centered stable free radicals ; stable free radicals generated from azo compounds such as phenyl azo triphenylmethane, which decompose to form carbon centered stable free radicals ;

stable free radicals generated from polyarylalkyl, polyarylalkanol, or diaryl ketone compounds such as triphenylmethane, diphenylmethanol, and benzophenone, which decompose to form carbon centered stable free radicals ; and aryl azo oxy radicals.

Nitroxides are preferred stable free radicals, with TEMPO and 4-oxo-TEMPO being more preferred. The preferred mole-to-mole ratio of the acid monomer to the stable free radical is application dependent, and is preferably not more than 200 : 1, more preferably not more than 100 : 1, most preferably not more than 50 : 1 and not less than 5 : 1, more preferably not less than 10 : 1, and most preferably not less than 20 : 1.

Molecular weight of the polyacid can generally be controlled by adjusting the ratio of the acid monomer to the initiator and stable free radical ; in general, the higher the ratio, the higher the molecular weight.

The polymerization reaction is carried out in the presence of a sufficient amount of a suitable initiator to intiate polymerization. Suitable initiators have a half- life of preferably not more than about 2 minutes, more preferably not more than about 30 seconds, and most preferably not more than about 5 seconds. Examples of suitable initiators include persulfates, peroxides, hydroperoxides, and azo compounds. Since short initiator half-lifes are advantageous, it may be desirable to include a half-life reducing agent such as sodium, potassium, or ammonium bisulfite.

More preferred initiators are sodium persulfate, potassium persulfate, ammonium persulfate, benzoyl peroxide, and a combination of sodium, potassium, or ammonium persulfate and sodium, potassium, or ammonium bisulfite. An example of a preferrred commercially available initiator is Vaso 52 (2, 2'-azobis (2, 4-dimethylpentane nitrile), available from E. I. DuPont de Nemours & Co., Inc.) The preferred weight : weight ratio of the acid monomer and the acid salt monomer (in combination, the partially neutralized acid monomer) to initiator is not less than about 3 : 1, more preferably not less than about 5 : 1 ; and preferably not more than 50 : 1, more preferably not more than 20 : 1.

Polymerization can be carried out using any suitable process. A preferred process includes gradual and continuous addition of initiator ; a more preferred process uses rapid or"pulsed"initiator addition. For example, a partially neutralized acid monomer, water, initiator, optionally co-initiator, and stable free radical can be charged into a reactor under an inert atmosphere at a temperature sufficient to promote polymerization. When the initiator is used with the co-initiator, it is preferred

that the two be added substantialiy simuitaneously, as opposed to one after the other. Although temperatures above the boiling point of the mixture at ambient pressures may be used, it has been surprisingly discovered that a low polydisperse, controlled molecular weight polymer can be prepared at temperatures of less than 100°C, thus eliminating the need to carry out the reaction in a pressurized vessel.

Preferably the reaction is carried out at a temperature not greater than 100°C and not less than 90°C.

M and Mw can be determined by any suitable method, but it is preferred that gel permeation chromatography using well-characterized acrylic acid homopolymers be used in the molecular weight determinations. It has surprisingly been discovered that homo-and copolymers having polydispersities of less than 2, more preferably less than 1. 8, and most preferably less than 1. 5 can readily be prepared under mild conditions and without the need for post-reaction preparation.

The controlled molecular weight, low polydisperse partially neutralized acid polymer of the present invention is useful as an additive for laundry and dishwashing detergents, hard surface cleaners, water treatment and oil field anti-scale agents, thickening agents, and further useful as a dispersant for coatings and a deflocculant for polymers.

The following examples are for illustrative purposes only and are not intended to limit the scope of this invention.

Example 1-Preparation of a Partially Neutralized Acrylic Acid Polymer with TEMPO (In this example, deuterium oxide is used so that percent conversion of the polymer could be determined by proton NMR spectroscopy. Normally, water would be used.) To a 250-mL, 3-necked round-bottom flask equipped with a heating mantle, a reflux condenser, a nitrogen purge, a stir bar, and a temperature probe was added a solution of sodium carbonate in D2O (3. 39 g in 100 mL) and TEMPO (0. 16 g). The solution was heated, and when the temperature reached a range of about 90°C to about 95°C, acrylic acid (6. 28 g) was added by syringe through a septum fitted to one of the necks of the flask. Neutralization was calculated to be 74% and the initial concentration of acrylic acid and the acid salt was 5. 7 %. The temperature was allowed to stabilize to the aforementioned range, whereupon a solution of

ammonium persulfate (0. 26 g in 2 mL of D2O) and sodium bisulfite (0. 26 g in 2 mL of D2O) were added separately and concurrently by syringes through a septum in the flask over a period of about 3 seconds. Samples (2-3 mL) were withdrawn periodically and quenched with a hydroquinone solution (0. 25 mL of 4% hydroquinone in D2O). Percent conversion was determined by proton NMR spectroscopy using 10% acrylic acid in D2O as a standard. Table I summarizes the Mn, the Mw, the polydispersity, and the conversion.

TABLE I Rxn time Mn Mw MW/Mn Conversion (min) (%) 0. 25 1641 2740 1. 67 47. 4 0. 5 1686 2628 1. 56 44. 0 1 1628 2616 1. 61 46. 0 2 1604 2567 1. 60 43. 0 3 1606 2483 1.54 47.2 4 1650 2580 1.56 43.3 5 1618 2589 1. 6 42. 1 15 1640 2572 1. 57 47. 9 30 1653 2724 1. 65 43. 4 60 1699 2644 1. 56 46. 4 90 1623 2692 1. 66 46. 4

Molecular Weight and Polydispersity Measurements The following procedure was used to determine MW, Mn, and polydispersity for all of the polymers described in the examples. A Hewlett Packard HP1 090A gel permeation chromatograph with refractive index detector Model 1047A and autosampler was used to calculate Mw, and Mn. The columns were supplied by Polymer Laboratories. A guard column (PL Aquagel-OH Guard 8 micron) was connected in series with two PL Aquagel-OH 30 8-micron GPC columns, operating range (PEO/PEG MW) of 100-30, 000. Separation was carried out at 35°C, using an injection volume of 100 microliters, and a flow rate of 1. 00 mUmin. The solvent was a

phosphate buffer pH=7, prepared by adding sodium hydrogen phosphate (1. 204 g) and sodium nitrate (21. 247 g) into a 1-L volumetric flask, then diluting the solids with 1 L of HPLC-grade water. The solution was stirred and 5 N NaOH was added dropwise until a pH of 6. 98 to 7. 05 was reached.

Data collection was done using a Model 750-P100, IBM computer ; instrument software was Chemstation for LC, Hewlett-Packard revision A. 05. 01 ; GPC Software was a PL Caliber for HP Chemstation from Polymer Laboratories, version 4. 01, serial 1007. Calibration was done using polyacrylic acid-sodium Salt standards purchased from Polymer Laboratories. The columns were calibrated using the following standards : Mp=1, 250 Batch No. 21423-1 Mp=2, 925 Batch No. 21426-1 Mp=7, 500 Batch No. 21428-1 Mp=16, 000 Batch No. 21430-1 Mp=28, 000 Batch No. 21432-1 where Mp is the peak molecular weight of the standard.

A portion of each standard (0. 25% (w/v)) was dissolved into 5 mL of the phosphate buffer, and shaken for 30 minutes. The dissolved standards were then filtered, placed in autosample vials, and run through the GPC columns using the program described hereinabove. The Polymer Laboratories software was programmed to recognize narrow standards, the flow rate marker, and any other peak needed for a particular calibration curve. The curve was automatically calculated when the peaks, which corresponded to the Mp of the standard, were chosen.

After the polymerization, an aliquot (about 5 mL) of the reaction mixture was removed, and placed in an uncapped glass vial. The vial was placed into a vacuum oven, heated to 45°C, and maintained at a pressure of 20 mm Hg for about 24 hours to dry the sample. A portion of the dried polymer (12. 5 mg) was placed in a vial along with 5 mL of the buffer, and the vial was shaken for 30 minutes. Then, the sample was filtered using Gelman Acrodisc 0. 2 micron PTFE filters on a 3-mL disposable syringe and Mw, Mn, and polydispersity of the sample was determined.

Example 2-Preparation of a Partially Neutralized Acrylic Acid Polymer without TEMPO To a 1000-mL, 5-necked flask equipped with a mechanical stirrer, reflux condenser, and addition ports was added water (233 g), copper sulfate solution (1 g, 0. 15% in water), acrylic acid (12. 5 g) and sodium hydroxide solution (10. 41 g, 50% NaOH in water). The contents were agitated and heated to 95°C 3 C°, whereupon a sodium persulfate solution (3. 6 mL of 23. 8 % sodium persulfate in water) was added via syringe. The reaction was maintained at 95°C 3 C° for 20 minutes, then allowed to cool to room temperature. The percent solids in the resultant polymer solution was measured by proton NMR spectroscopy using 10% acrylic acid in water, which indicated 88% conversion. The Mw was measured to be 5, 488 and the M was 2, 843, giving a polydispersity of 1. 93.

Example 3-Preparation of a Partially Neutralized Acrylic Acid/Maleic Acid Copolymer without TEMPO The procedure of Example 2 was repeated except that maleic anhydride and acrylic acid (6. 25 g each) were used instead of just acrylic acid. Mw and M were 3, 131 and 1, 758 respectively, resulting in a polydispersity of 1. 78.

Example 4-Preparation of a Partially Neutralized Acrylic Acid/Maleic Acid Copolymer with TEMPO The procedure of Example 3 was repeated except that TEMPO (0. 125 g) was added to the reactor prior to heating. Mw and M were 2, 402 and 1, 732 respectively, resulting in a polydispersity of 1. 39.