GB2150578A - Process for preparing low molecular weight acrylamide polymers - Google Patents

Abstract

Low molecular weight homo- and co-polymers of acrylamide and methacrylamide are prepared by solution or emulsion polymerization of the monomer in the presence of a chain transfer agent selected from erythorbic acid (or an alkali metal salt thereof), ascorbic acid (or an alkali metal salt thereof), vitamins, and butylated hydroxytoluene.

Description

SPECIFICATION Process for preparing low molecular weight acrylamide polymers This invention relates to an improved process for preparing low molecular weight water soluble polymers and copolymers of acrylamide by emulsion or solution polymerization using a chain transfer agent.

Solution or emulsion polymerization of acrylamide is known in the prior art. The prior art teaches the use of aliphatic alcohols, such as isopropyl alcohol, ethanol and methanol, as chain transfer agents for the solution polymerization of acrylamide. Other techniques use various sulfur or organo sulfur compounds as chain transfer agents such as thioglycol, thioglycolic acid and ndodecyl mercaptan (U.S. Patents 4,196,272 and 4,245,072). Another technique, used for solution polymerization of acrylamide, uses redox initiators wherein the bisulfite ion acts as a chain transfer agent.

U.S. Patent 4,307,215 teaches the use of formic acid and alkali metal formates as chain transfer agent using inverse emulsion polymerization conditions.

All of the priot art compounds have problems which make them difficult, at best, to use commercially. For example, they are either not efficient and require high concentrations of the chain transfer agent to obtain desired molecular weight reduction or they have strong pungent odors which are frequently carried over to the final polymer, thus severely limiting the commercial market potential; some are corrosive or toxic and some exhibit a strong retarding effect on acrylamide polymerization thus contributing to loss in productivity.

In accordance with the present invention improved chain transfer agents have been found which are useful in the solution or emulsion polymerization of acrylamide and related monomers. These are: erythorbic acid and its alkali metal salts, ascorbic acid and its alkali metal salts, vitamins, and butylated hydroxytoluene.

The compounds used as chain transfer agents in the practice of this invention are well known compounds, most of which have been used as additives in the food and beverage industry. They are readily available, safe to handle, and do not exhibit toxicity or health hazards associated with prior art compounds. This is particularly important, since the resulting polymers are used in various cosmetic formulations and also as additives in the food industry.

Although solution polymerization can be used, emulsion polymerization, particularly inverse aqueous emulsion polymerization, is preferred since this overcomes the problems of high viscosity and poor heat transfer. As a result, water to monomer ratio can be kept quite low thereby increasing the amount of polymer produced in a given reaction batch.

High polymerization temperature is not required in the practice of this invention. Thus, the extent of polymer branching would be reduced thereby improving product quality. Due to high energy costs the lower polymerization temperature would also improve the product economics.

An emulsion is generally obtained by dispersing one phase in the form of very fine droplets (i.e. dispersed phase) into another phase (i.e. continuous phase) in the presence of a suitable surfactant. Most common form of emulsion is an oil-in-water emulsion where oil is the dispersed phase and water is the continuous phase Another form of emulsion is the water-in-oil emulsion wherein water is the dispersed phase. This is sometimes also referred to as inverse emulsion.

The term emulsion as used in this invention refers to only water-in-oil emulsions, unless specified otherwise.

Surfactants, as is well known, can be classified in terms of the HLB system, i.e. "Hydrophile Lipophile Balance". In general, to prepare water-in-oil emulsions, the surfactants used will be oil soluble and have a HLB range of from 1 to 12 preferably from 4 to 6. The required HLB range can also be obtained by using blends of surfactants, including ionic and non-ionic. Suitable surfactants include sorbitan fatty acid esters, mono and diglycerides of fatty acids, polyoxyethylene sorbitol esters, polyethylene sorbitan fatty acid esters and polyoxyethylene alcohols.

The total concentration of surfactants used can vary from 0.5 to 20.0 parts by weight per 100 parts by weight of the oil phase, preferably from 2.0 to 15.0 parts by weight of the oil phase.

Acrylamide is preferably polymerized in an inert atmosphere (e.g. N2 or CO2) at a pH of from 3 to 8. When the pH is higher than about 9, the amide groups may be hydrolyzed and when the pH is less than about 2.5. imidization often occurs resulting in the formation of crosslinked, water insoluble polymer.

The water to oil phase ratio is important in the practice of this invention for stability and efficiency. In general, water to oil phase ratio should be about 1:5 and more preferably about 1:2. However water to oil phase ratio of 1:1 or even 2:1 can be used as long as the resulting emulsion is stable. Since the monomer is dissolved in the water phase, it is best to keep the concentration of oil phase as low as possible so as to increase the yield of polymer in a given batch.

The polymerization temperature will usually be in the range 20"C to 80"C and preferably in the range of 25"C to 60"C.

The free radical initiators used in the practice of this invention are the well known organic peroxides and azo initiators. For purposes of illustration, suitable initiators include: t-Butyl peroctoate, t-amyl peroctoate, t-butyl peroxypivalate, t-amyl peroxidepivalate, a-cumyl peroxypivalate, t-butyl peroxneohexanoate, t-amyl peroxyneohexanoate, t-butyl peroxyneodecanoate, t-amyl peroxyneodecanoate, dibenzoyl peroxide diacetyl peroxide, dilauroyl peroxide, di(2ethylhexyl) peroxydicarbonate, di(phenoxyethyl) peroxydicarbonate, 1 , 1 -d i(t-butylperoxy) cyclohexane, 1 , 1 -di(t-butylperoxy) 3, 3, 5-trimethyl cyclohexane, 2, 5-dimethyl-2, 5-di(2-ethylhexanoylperoxy)hexane, 2,5-dimethyl-2, 5-di(benzoylperoxy)hexane, di-t-butyl peroxide, di-(t-butyldiperoxy)azelate, t-butyl cm my peroxide, azo-bisisobutyronitrile, 2,2 '-azobis(2, 4-dimethylvaleronitrile), 2 t-butylazo-2-cyano-4-methoxy4-methylpentane, 2-t-butylazo-2-cyano-4-methylpentane, 2-t-butylazo-2-cyanopropane, 2-t-butylazo-2-cyanobutane, 1-t-butylazo-1-cyanocyclohexane, t-butyl hydroperoxide, and cumene hydroperoxide.

Water soluble free radical initiators such as potassium persulfate, ammonium persulfate and hydrogen peroxide can also be used.

A mixture of two or more free radical initiators can also be used. The initiators concentration will usually be in the range of 0.005 to 5.0 parts by weight per 100 parts by weight of monomer, preferably 0.05 to 2.5 parts by weight per 100 parts by weight of monomer.

Acrylamide or methacrylamide is polymerized in the practice of this invention to produce low molecular weight nonionic homopolymers. However, it is also possible to copolymerize them with one or more comonomers. Suitable comonomers include, for example, acrylic acid, methyacrylic acid, N,N-dimethylaminoethlmethacrylate, sodium acrylate, isopropyl acrylamide, and acrolein. As is well known to one skilled in the art, the comonomers can be selected to produce either cationic or anionic polyelectrolytes.

During copolymerization, the concentration of acrylamide (or methacrylamide) is 10 to 90% of total monomer by weight and preferably about 10 to 75% by weight.

Chain transfer compounds by definition are used to control the growth of the polymer chain and thus the molecular weight. Under idealized conditions, these compounds should not have an effect on the rate of polymerization. They are usually used in a concentration range of from 0.001 to 5.0 parts by weight per 100 parts by weight of monomer, more preferably from 0.01 to 2.5 parts by weight per 100 parts by weight of monomer.

The following compounds are effective chain transfer agents of this invention.

Erythorbic acid and its alkali metal salts such as sodium erythorbate, ascorbic acid and its alkali metal salts, vitamins such as Niacin, Vitamin E and Riboflavin, and butylated hydroxytoluene (BHT).

Based on availability and cost, the preferred chain transfer agents in the practice of this invention are sodium erythorbate, erythorbic acid and butylated hydroxytoluene.

These compounds have been classified as antioxidants suitable for use in food, as per FDA regulations (see Kirk-Othmer Encyclopedia of Chemical Technology, Volume 3, Third Edition, page 140, Table 2). Moreover, they do not have a specific tolerance level indicating that they can be safely used at any level as additives in food. Therefore, the preferred chain transfer compounds of this invention can be used alone or in combination with each other to produce low molecular weight acrylamide polymer, which can be safely used for food contact applications.

Examples Definitions of materials used in the Examples OMS-Odorless mineral spirits, marketed under the trademark Shell Sol-71 by Shell Oil Co.

SpanS 80-Sorbitan monooleate from ICI Americas, Inc.

EA-Erythorbic acid from Pfizer Inc.

NaEA-Sodium erythorbate from Pfizer Inc.

Acrylamide monomer, "Fisher Reagent" grade from Fisher Scientific.

50% aqueous acrylamide monomer solution inhibited with about 25ppm of copper (Cu+ +) from Dow Chemicals.

Triply distilled water from Queen City Pure Water (Buffalo, N.Y.). Before use, this water was boiled and allowed to cool while being sparged with N2. Until ready for use, it was stored under a blanket of N2.

LupersolS223M75-di-(2-ethylhexyl) peroxydicarbonate, marketed by the Lucidol Div. of Pennwalt Corp.

VersenexS 80-diethylenetriamine pentaacetic acid, a chelating agent from Dow Chemicals.

Vazo 64-Azo-bis-isobutyronitrile from DuPont.

Vazo 5 2-2, 2'-Azo-bis(2,4-dimethylvaleronitrile) from Du Pont.

BHT-Butylated hydroxytoluene; 2, 6-di-tert-butyl-p-cresol; 2, 6-di-tert-butyl-4-methyl phenol from Aldrich Chemical Co., Inc.

Riboflavin-Vitamin B2 from Roche Vitamin E--Alpha Tocopheryl Acetate from Vornado, Inc.

Niacin-Nicotinic Acid from Lonza Formic Acid-Certified A.C.S., from Fisher Scientic Company.

Acrylamide monomer, ''99 + % Electrophoresis Grade Gold Label" from Aldrich Chemical Company, Inc.

K2S2O8-Potassium Persulfate "Baker Analyzed Reagent" from J.T. Baker Chemical Co.

Lupersol 256-2, 5-dimethyl-2, 5-di-2-ethylhexanoylpernxyJhexane, marketed by the Lucidol Div. of Pennwalt Corporation.

LupersolS 331-80B-1,1-di-(t-butylpernxideicydohexane, marketed by the Lucidol Div. of Pennwalt Corp.

DS-606 4-(t-butylperoxycarbonyl)-3-heXyl-6-(7-(t-butylperoxywarbonyl)heptyl)cyclohexene, marketed by the Lucidol Div. of Pennwalt Corp.

Lupersolo TA-54M75-t-amyl peroxypivalate, marketed by the Lucidol Div. of Pennwalt Corporation.

STANDARD PROCEDURE Formulation Parts by weight Acrylamide 50 Triply distilled water 50 OMS 200 Spano 80 20 Initiator 0.05 or as required Chain transfer agent Procedure Using a triple beam balance (Ohaus), 50 g of solid acrylamide monomer was weighed and then dissolved with stirring (at room temperature) in 50 g of triply distilled water. In a like manner, 20 g of Spans 80 was weighed and dissolved in 200 g of OMS. These two solutions were then mixed in a Waring stainless steel blender at high speed for about 3 minutes to form a water-in-oil emulsion. This emulsion was then transferred to a 1 liter (jacketed) glass reactor equipped with a stirrer, condensor, thermometer and nitrogen sparging line.The reactor bowl itself was immersed in a thermostatically controlled (+ 0.1 do) water bath and preset at the desired reaction temperature (e.g. 35"C). The stirring emulsion was purged with nitrogen for about 1/2 hour before adding the initiator.

The required amount of initiator was weighed in tared glass "petticups" using a Mettler analytical balance ( + 0.0001 g). The "petticup" was then placed in a 10 ml glass beaker to which about 5 ml of acetone was added. This solution was then added to the stirring emulsion.

The beaker with the "petticup" was rinsed again with a few ml (2-5 ml) of acetone and the contents were again added to the reactor. At this point, the electric timer was started to indicate the beginning of polymerization.

When a chain transfer agent was used, it was weighed and transferred to the reactor in the same way. Except the choice of solvent varies depending upon which transfer agent is used, e.g., water for EA, OMS for BHT, water-methanol for Riboflavin. However, the initiator would always be the last component added to the reactor.

At predetermined time intervals, emulsion samples were withdrawn from the reactor using a hypodermic syringe and needle. After weighing the emulsion was precipitated in acetone or methanol with stirring. The syringe and needle were re-weighed to obtain the weight of emulsion sample.

The precipitated polymer was filtered on preweighed filter paper and vacuum dried at about 50"C to constant weight (usually about 1 2 hours). The solids content is obtained from which conversion percentages can be calculated.

For polymerization of aqueous acrylamide solution (from Dow Chemicals), the monomer is first treated with 0.2 parts by weight of VersenexX 80 (chelating agent) per 100 parts by weight of monomer, to remove the inhibitor (copper). Subsequent procedure is the same as for solid acrylamide monomer, described above.

Polymer Solution Viscosity Measurement: The dry polymer was first milled in a Micro-Mill (Model #502 from Technilab Instruments) for about 3 mins. Then 4.00 g of polymer was mixed with 24 ml of methanol with stirring in a 250 ml beaker. After complete polymer wet-out, 1 98 ml of triply distilled water was quickly added and stirring continued for another 5 minutes to obtain 1.8% aqueous polymer solution, which was used for viscosity measurement.

A Brookfield viscometer, Model RVF with a #4 spindle at a speed setting of 4 (rpm) was used for solution viscosity measurements at 25'C.

The STANDARD PROCEDURE was used in all the examples except where indicated otherwise.

Example 1 This example illustrates the use of EA and NaEA as chain transfer agents with different types of free radical initiators, as shown by the results in Table I.

EA was used at a level of 0.1 9 per 100 9 of acrylamide monomer and NaEA at a level of 0.2 per 100 g of monomer.

The initiators were used at a level of 0.1 per 100 9 of monomer, after correcting for any assay differences.

Table I

Brookfield Vis Chain cosity of 1.8% Transfer Polymerization % Con- Polymer Solu Initiator Agent Time, Yins. ::ersion tion at 250C in Centipoise I 55 1 - 1 400 | 74 1 14,000 (14 Pa.s) Luazo 55 | EA | 360 l 75 1 2,500 (2.5 Pa.s) VazoW 52 1 | 245 ! lOo ! 26,000 (26 Pas.) I EA ! 260 1 100 1 7,900 (7.9 Pas.) Lupersol R 256 - | 300 l 100 24,500 (24.5 Pa.s) I EA 300 1 100 I 4,500 (4.5 Pa.s) Lupersole 256 - | 300 1 lOo I 24,500 (24.5 Pa.s) INaEA I 240 1 100 1 4,500 (4.5 Pa.s) Polymerization Temperature: 350C Example 2 The vitamins listed in Table II were used successfully as chain chain transfer agents with various free radical initiators.

The initiators were used at a level of 0.1 g per 100 g of acrylamide monomer, after correcting for any assay differences.

The levels of vitamins used, in gram per 100 g acrylamide monomer, are listed in Table II.

Table II

Brookfield . - Chain Transfer Polymeri- cosity of 1.

Chain Agent zation Percent Polymer Solu- Transfer Concentration Time, Conver- tion at 250C Initiator Agent in g/100 g Monomer Minutes sion in Centipoise Lupersol R - I 300 1 100 1 14,800 * 256 I I I I| IVitamin B2 10.2 g/100 g Monomer 1 300 1 98 l 1,000 * | (Riboflavin) | I 1 | I LupersolS1 - 1 - I 360 I 85 1 19,500* |Vitamin E 10.66 g/100 g Monomerl 360 I | 57 1 3,000 * 331-80BINiacin 10.1 g/100 g Monomer 1 360 1 53 A 4,800 r tNiacin 10.05 R/lo0 g Monomerl 360 1 56 1 4,000 * * Viscosity equivalents 14,800 cps 14.8 Pa.s l,000 cps 1.0 Pa.s 19,500 cps 19.5 Pa.s 3,000 cps 3.0 Pa.s 4,800 cps 4.8 Pa.s 4,000 cps 4.0 Pa.s Polymerization Temperature: 350C Example 3 The STANDARD PROCEDURES was used herein except that a reaction temperature profile was used. DS-606 is a sequential initiator, wherein the peroxide possesses two initiator groups with different half-life activity. The profile was 35"C for 4.5 hours then raised to 45"C and held at that temperature for 1.5 hours.

EA was used at a level of 0.1 g per 100 g of acrylamide monomer.

DS-606 was used at a level of ""0.1 g per 100 g of acrylamide monomer, after correcting for any assay differences. Sequential initiators are known to produce high molecular weight polymer, and this example shows (in Table Ill) that EA has utility as a chain transfer agent, with this type of free radical initiator Table III

Brookfield Vis cosity of 1.8% Chain Polymer Solu Transfer Polymerization % Con- tion at 250C Initiator Agent Time, Mins. version in Centipoise I - I 360 l 100 1 38,000 (38 Pa.s) DS-606 ≈ EA 1 300 1 100 1 2,800 (2.8 Pa.s) Example 4 BHT is a food grade antioxidant. Certain vitamins, e.g., Ascorbic acid (Vitamin C) and a Tocopheryl Acetate (Vitamin E), are antioxidants as well. This example illustrates the use of BHT as a chain transfer agent as shown in Table IV.

BHT was used at a level of 0.1 g per 100 g of acrylamide monomer.

Lupersols 331-80B, the initiator, was used at a level of 0.1 g per 100 g of acrylamide monomer, correcting for any assay differences.

Table IV

Brookfield Vis Chain cosity of 1.8% Transfer Polymerization % Con- Polymer Solution Initiate; Agent Time, Mins. version at 250C in CPS.

Lupersol - 0 360 1 85 l 19,500 (19.5 Pa.s) 331-80B,, j BET I 360 . 74 1 5,500 (5.5?a.s) Lupersols 300 100 27,500 (27.5 Pa.s) 223M75 | BET 360 360 80 9 500 . a.s) Polymerization TemPerAtUre: 350C Example 5 The experimental procedure used for this Example was as follows: Solid acrylamide was dissolved in triply distilled water until a clear solution was obtained. This solution was poured into a glass reactor, preheated either by a heating jacket or water bath to the reaction temperature. The reactor was equipped with a stirrer, N2 sparge, thermometer, and condensor. The stirrer and N2 sparge were turned on. After 1/2 hour the initiator dissolved in ""5 ml acetone was added to the reactor and the timer was started. The small beaker used to dissolve the initiator was given a rinse with 3-5 mls acetone, and this was added to the reactor.

Below is the formulation used: 25 g acrylamide monomer 500 g triply distilled water 1.0 phm initiator 1.0 phm chain transfer agent.

When the chain transfer agent was used, it was added to the aqueous monomer solution before the initiator was added.

Molecular weight was determine by dilution viscometry, using a Cannon Ubbelhode Dilution Viscommeter (#75), immersed in a thermostatically controlled water bath (i 0.1 C).

The value Mv was determined by using the Mark-Houwink Eqn. n] = KMaV where: K = 6.80 X 10-4 dl/g, a = 0.66, and [nJ is intrinsic viscosity, for polyacrylamide in water at 30 C.

This example illustrates as shown in Table V that EA can successfully lower the molecular weight of polyacrylamide made by solution polymerization, without a large retardation of the polymerization rate.

Table V

Chain Reaction viscosity Average Mole- Transfer Time in % Con cular Weight Mv & ommat; OOC in Initiator Agent minutes version Triply Distilled pater t-amyl- I I peroxy- I - | 60 1 82 | ?66,100 pivalate I I I I Lupersol R I EA i 80 1 89 1 ,5,/00 TA-54M75 I I I Polymerization Temperature: 350C Example 6 The previous examples used various organic peroxides which are oil soluble. This example illustrates the utility of EA as chain transfer agent with a water soluble initiator as shown in Table VI. The initiator was used at a level of 0.1 g per 100 g of monomer after correcting for assay.

Table VI

Chain Transfer Brookfield @@ Agent Polymer SG1 Chain Concentration Polymeri- % ion at Transfer in g/100 g zation time Conver- in Centi Initiator Agent Monomer Minus. sion noise Potassium r I - 1 60 l 100 10,S00* persulfate I (KH2 2 6 l EA I 0.1 1 60 l 100 i 400 * Viscosity equivalents: 10.8 Pa.s and 0.4 Pa.s respectively.

Polymerization Temperature: 350C Example 7 In an attempt to understand the effect of EA upon the reaction, the initiator concentration was held constant at 0.1 g per 100 g of monomer (0.1 phm) and the EA concentration was varied from 0.1 g to 4.0 g per 100 g of monomer, initially.

Afterwards, two additional polymerizations were conducted where EA was kept at 0.1 g per 100 g of monomer (0.1 phm) and the initiator was varied from 0.2 to 0.3 phm.

The results in Table VII show that as the initiator concentration is raised, the polymerization rate increases; however, the polymer molecular weight increases due to the reduced concentration of EA with respect to the initiator concentration. Therefore, while EA is an efficient chain transfer agent, the ratio of EA to initiator may be critical.

Table VII INVERSE E"b5SION POLYs.ERIZATION OF ACRYL.t'!IDE AT 350C Initiator: LUPERSOL 256; Additive: Erythorbic acid Initiator Additive Time Viscosity at 250C phm phm Mins. t Conversion cps 0.1 0.1 360 79 125 (0.125 Pa.s) 0.1 0.25 360 73 125 (0.125 Pa.s) 0.1 1.0 360 59 125 (0.125 Pa.s) 0.1 4.0 360 41 < 100 ( < 0.1 Pa.s) 0.2 0.1 240 100 1,250 (1.25 Pa.s) 0.3 0.1 120 100 5,750 (5.75 Pa.s) Viscosity: Brookfield viscosity of 1.8% solution using spindle #4 at speed #4.

Example 8 The STANDARD PROCEDURE was used herein except the polymerization temperature was 45"C and instead of weighing out 50.0 9 of solid acrylamide monomer and 50.0 g of triply distilled water to make the aqueous acrylamide solution, 100 grams of commercial 50% acrylamide solution was weighed out into a beaker. This solution as received was inhibited with 25 ppm CuSO4 and normally one would add a chelator prior to forming an emulsion. However, no Versenexs 80 chelator was used to remove the Cu+2 inhibitor for the reactions listed in Table VIII.

The initiator was added in the amount of 0.1 9/100 g monomer correcting for assay. The chain transfer agent EA was added prior to the initiator in the amount of 0.1 9/100 g monomer.

Note that EA efficiently lowers the polymer molecular weight while only slightly reducing the polmerization rate.

Table VIII

Brookfield Viscosity Chain Polymeri- of a 1.8% Polymer Transfer zation time Percent Solution at 250C in Initiator Agent Minus. Conversion ~ Centipoise Luazo 70 - I 300 1 97 1 16,300 (16.3 Pa.s) I 70 I EA I 360 1 99 1 7,300 (7.3 Pa.s) For the sake of comparison, formic acid was evaluated as a chain transfer agent using the STANDARD PROCEDURE in this experiment.

The initiator was added in an amount of 0.1 9/100 g monomer. Formic acid was also added in the amount of 0.1 9/100 g monomer which is the amount typically employed when adding erythorbic acid or BHT, to reduce polymer molecular weight.

U.S. Patent No. 4,307,215 teaches the use of formic acid and alkali metal formate salts as very efficient chain transfer agents for acrylamide. When evaluating formic acid as a chain transfer agent in the present invention it greatly retarded the rate of polymerization as can be seen in Table A, unlike the chain transfer agents, i.e., EA, BHT etc. of this invention. This rate of retardation is undesirable from an industrial process point of view. In addition, formic acid is corrosive and, hence, it requires special care in handling, in regard to equipment longevity and worker safety.

No viscosity value for the polymer made using formic acid is listed in Table A since the final percent conversion was very low, very little polymer could be recovered.

Table A

Brookfield Vis Chain Polvmerl- cosity of 1N8 ó Transfer zation ;0 Con- Polymer Solution Initiator Agent Time. Yins. version at 250C in Cent.poise Lupersol I Formic Acid 360 256 | ~ |300| 100%| 24,500 ( 24.5 Pa.s) Polymerization Temperature 35"C In a related application (No. ) filed simultaneously herewith a process is disclosed for the preparation of acrylamide polymers in which acrylamide is polymerized in the presence of from 50-250 parts by weight of erythorbic acid or an alkali metal salt thereof per million parts of monomer, i.e. from 0.005 to 0.025% by weight. Accordingly no claim is made herein to such a process.

Claims (11)

1. A process for the preparation of low molecular weight acrylamide polymers which comprises solution or emulsion polymerizing acrylamide and/or methacrylamide or a monomer mixture comprising 10-90% by weight acrylamide and/or methacrylamide and 90-10% by weight of one or more comonomers selected from the following: acrylic acid, methacrylic acid, N,N-Dimethylaminoethylmethacrylate, sodium acrylate, isopropyl acrylamide and acrolein, in the presence of a free radical initiator and a chain transfer agent, wherein said chain transfer agent is erythorbic acid or an alkali metal salt thereof, ascorbic acid or an alkali metal salt thereof, a vitamin, or butylated hydroxytoluene, or a mixture of two or more thereof.

2. A process according to claim 1, wherein the chain transfer agent is used in an amount of from 0.001 to 5.0 parts by weight per 100 parts by weight of monomer.

3. A process according to claim 2, wherein said amount is from 0.05 to 2.5 parts by weight per 100 parts of monomer.

4. A process according to claim 1, 2 pr 3, wherein the chain transfer agent is erythorbic acid or sodium erythorbate.

5. A process according to any one of claims 1-4, which is an emulsion polymerization process.

6. A process according to claim 5, which is an inverse aqueous emulsion polymerization process.

7. A process according to claim 6, wherein the polymerization is effected in the presence of an oil soluble surfactant having an HLB range of from 1 to 12.

8. A process according to claim 7, wherein the sirfactant is present in an amount of from 0.5 to 20.0 parts by weight per 100 parts by weight of the oil phase.

9. A process according to claim 6, 7 or 8, wherein the water to oil phase ratio of said inverse emulsion is in the range 1:1 to 1:5.

10. A process according to any one of claims 1-9, wherein the polymerization temperature is in the range 20 to 80"C.

11. A process according to any one of claims 1-10, wherein the free radical initiator is used in an amount of from 0.005 to 5.0 parts by weight per 100 parts by weight of monomer.