GB1568688A

GB1568688A - Preparation of graft copolymers of starch

Info

Publication number: GB1568688A
Application number: GB965076A
Authority: GB
Original assignee: Stadex AB
Current assignee: Stadex AB
Priority date: 1977-03-10
Filing date: 1977-03-10
Publication date: 1980-06-04

Description

(54) PREPARATION OF GRAFT COPOLYMERS OF STARCH (71) We, AKTIEBOLAGET STADEX, a Swedish Company, of Kopparbergsgatan 31, S-214 44 Malmo, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the preparation of graft copolymers of starch. The word 'graft' denotes the modification of a substrate polymer (in this case starch) by chemical attachment or growth of a different polymer at sites on the chains of the substrate polymer.

It is generally known that a graft copolymer retains some of the basic properties of the original polymer (starch) and adds some properties of the polymer grafted-on. In this way, a graft copolymer can have a composite of properties of the two homopolymers. Thus, in this way, copolymers may be 'tailor-made' by selecting a basic polymer with known properties and then grafting onto it a polymer having other selected properties, e.g. particular functional groups.

Methods of grafting vinyl and vinylidene monomers to starch are of three main types: chain transfer, physical activation and chemical activation.

Graft copolymers prepared by chain transfer, i.e. polymerizing the monomer simply in the presence of starch, are always obtained in admixture with homopolymer.

The physical activation includes mechanical degradation or gamma irradiation (60cho) and also gives rise to the formation of mixtures of graft and block copolymers. The chemical processes comprise homolytic oxidation of starch (with persulfates or hydrogen peroxide and ferrous ion) and introduction of hydroperoxide groups (with ozone-oxygen mixtures), both also resulting in a mixture of graft and homopolymer. A more specific activation is obtained by the ceric ion oxidation of starch whereby only graft polymers are formed. Unfortunately tetravalent cerium salts are very expensive and can only be used with a restricted amount of monomers.

According to U.S. Patent No. 3,457,198 acid manganic polyphosphate can be used as grafting initiator for graft copolymerization of acrylic monomers on cellulosic material, provided reactive sites including carboxyl, keto and aldehyde groups are present. These reactive sites are introduced by a mild oxidative treatment, e.g. a mild hypochlorite bleach, prior to grafting.

It has now surprisingly been found that this acid manganic polyphosphate initiator can be used as initiator in graft copolymerization of a wide range of vinyl and vinylidene monomers on starch without any oxidative pretreatment.

The invention therefore provides a process for the synthesis of graft copolymers of starch which comprises grafting at least one ethylenically unsaturated monomer onto starch in the presence of a trivalent manganese complex with a polyphosphate as grafting initiator.

The starch suitable as starting material in the process of the invention can be starch obtained from different sources, such as potato starch, corn starch, wheat starch, rice starch or a waxy starch. The graft polymerization may take place with the starch in granular or in gelatinized form. The starch starting material may also be a chemically or physically modified starch such as a starch degraded by the action of heat, acid or enzymes, a cold water soluble starch, or a starch derivative such as a starch ether or starch ester.

In the process of this invention, the compounds that can be grafted to starch are polymerizable monomers which may have vinyl (CH2=CH-) or vinylidene groups (CH2=C < ). Suitable classes of unsaturated monomer are: (1) polymerizable nitriles such as acrylonitrile and its homologs (2) polymerizable amides such as acrylamide and its homologs (3) acrylic acid and its esters (4) methacrylic acid and its esters (5) vinyl esters, e.g. vinyl acetate (6) monomers with amino substituents such as cationic methacrylate monomers, e.g. dimethyl amino ethyl-methacrylate in acidic medium (7) vinyl ethers (8) vinyl pyridines and 1 - vinyl - 2pyrrolidone (9) maleic and fumaric acids, and (10) styrene and its derivatives These monomers may be used alone or in combination with each other, for example a mixture of acrylonitrile and one or more of acrylamide, vinyl acetate and styrene.

The main features of the process of the invention for producing graft-copolymers of starch using a trivalent manganese polyphosphate complex as initiator are its simplicity, specificity, versatility and low cost.

The trivalent manganese polyphosphate complex may be derived from pyrophosphate, tripolyphosphate, or metaphosphates. The manganic pyrophosphate complex, which is made from readily available chemicals, is preferred. Thus, the manganic pyrophosphate initiator is made by oxidation of manganous (Mn2+) ions, e.g.

from manganous sulfate, by permanganate (Mn7+) ions, e.g. from potassium permanganate, in a solution of pyrophosphate ions which can be obtained from tetrasodium pyrophosphate. Since all these chemicals are available in abundance and at low prices, the initiator for the synthesis of graft copolymers of starch according to the method of this invention i.e. manganic pyrophosphate, is very cheap.

The versatility of the process of the invention lies in the wide range of conditions available for synthesis of the graft copolymers. This leads to a whole series of different types of graft copolymers which can be characterized by grafting parameters as percentage add-on, which is the percent synthetic polymer in the graft copolymer; the grafting efficiency, which is defined as the percentage of the total synthetic polymer formed in grafting reaction that has been grafted to starch; the grafting frequency, which is expressed as the number of anhydroglucose units per grafted chain; and the average molecular weights of the grafted branches.

The graft copolymerization of starch according to the process of the invention is carried out in a slurry or solution of starch, preferably in water, having a concentration varying from 1 to 25% by weight. In order to prevent oxygen from interfering with the copolymerization reaction, the air is preferably displaced by an inert gas such as nitrogen. The pH of the reaction medium is preferably adjusted below pH 7 and more preferably to between I and 2 by addition of a suitable amount of a mineral acid such as sulfuric acid, phosphoric acid, nitric acid, hydrochloric acid. The manganic polyphosphate initiator may. be added to the reaction medium before or after addition of the monomer or at intervals during the graft copolymerization. The initial concentration of the trivalent manganese ions in the reaction medium may vary from 0.5x 10-3 to 30x 10-3 mole/l whereas the free polyphosphate, defined as the difference between total polyphosphate concentration added and thrice the initial trivalent manganese ion concentration, may vary from 0 to lOOx10-3 mole/l. The monomer concentration can cover a wide range dependent on the type of monomer used and the graft copolymer to be obtained. The graft copolymerization of the starch can take place at temperatures from room temperature to 600C and preferably at 25 500 C. The time of the reaction may vary from 30 to 300 minutes depending on the other operating conditions. After grafting the product can be isolated from the medium by the usual methods.

Under a given set of reaction conditions starch may be graft-copolymerized with unsaturated monomer to yield products with either increased hydrophobic or increased hydrophilic properties or having ionic (anionic or cationic) character.

For example, grafting of methyl methacrylate, methyl acrylate or acrylonitrile imparts greater hydrophobicity to starch. The same products can be made anionic in character by alkaline hydrolysis of ester or nitrile groups along the grafted chains. Grafting of acrylamide may lead to greater hydrophilicity. Cationic properties may be imparted to the substrates by grafting monomers with amino substituents, e.g. acidic salts of dimethyl-amino ethylmethacrylate, etc.

Graft copolymers having novel composite properties of the two homopolymers are produced. The following Examples serve to illustrate the essential features of this invention.

EXAMPLE 1 This Example illustrates the grafting of acrylonitrile to native potato starch.

The initiator was prepared as follows. 5 ml of a solution of Mn2+ ions prepared by dissolving 0.876 g MnSO4. H2O in 100 ml of distilled water were added to a solution of 2.89 g of sodium pyrophosphate (Na4P2O7 l0H2O) in 50 ml of distilled water. The pH of the resulting solution was adjusted to 6 by adding conc. H2SO4 using a pH-meter. (The pH-meter was calibrated with standard buffers. A glass electrode was used for all pH measurements with a saturated calomel electrode as reference).

The Mn2+ ions were then oxidized to Mn3+ ions by titrating potentiometrically with Mn7+ ions (obtained by dissolving 0.205 g KMnO4 in 100 ml distilled water) using a platinum electrode as reference. 5 ml of the permanganate solution were required, giving a concentration of Mn3+ ions of 5.4 10-3 M in the initiator prepared.

5 ml of 95-97% H2SO4 were added to 1.05 1 of distilled water. 7.5 g (on a moisture free basis) of native potato starch were slurried together with 100 ml of the acidified distilled water in a reaction vessel which was immersed in a thermostatically controlled water bath, maintained at 300 C. The reaction vessel was equipped with a stirrer, a condenser, a thermometer and a dropping funnel with a nitrogen bubbler. 25 ml of the initiator solution (prepared as described above giving a concentration of 1.08x10-3 M of Mn3+ ions) and 10 ml of purified acrylonitrile (AN) were added to the dropping funnel. A brisk stream of oxygenfree nitrogen was bubbled first through the contents of the dropping funnel and then through the starch slurry in the reaction vessel before the gas was led out of the system. This was continued for 30 min.

Meanwhile the temperature in the reaction vessel was raised to 300 C. The content of the dropping funnel was then emptied into the reaction vessel and the reaction carried out under a slow stream of N2 gas for 75 minutes. The polymerization was terminated by adding 0.5 g of hydroquinone dissolved in 25 ml of distilled water. The products were filtered, thoroughly washed with water and dried overnight in a stream of dry air at 600C. The weight of the dry product was 13.6 g. That this product was a graft copolymer was shown by extracting the products with N,N-dimethylformamide which is a good solvent for polyacrylonitrile but a non-solvent for the graft copolymer.

Negligible amounts of the products were soluble in this solvent. This is a proof of the high grafting efficiency with this system. In the absence of any substrate, but under exactly the same conditions, very low yields of the polymer, i.e. polyacrylonitrile, were obtained. This indicates the high specificity of the initiating system. The average molecular weights determined viscometrically in N,N-dimethylformamide at 25"C were about 86,000.

EXAMPLE 2 This Example describes the grafting of acrylamide to native potato starch. The polymerization procedure was the same as in Example 1 except that 10 g of acrylamide monomer (a powder) were added to the reaction vessel containing the starch slurry in acidified water in the beginning while the initiator solution was kept in the dropping funnel during N2 bubbling. After the termination of the graft polymerization reaction, the products were centrifuged and a portion of the clear solution precipitated in methanol to determine the amount of homopolymer formed. 1.28 g of homopolymer were formed. The graft copolymer weighed 9.31 g indicating an add-on of 19.4%. The total conversion of acrylamide to polyacrylamide was 30.9% in the grafting reaction. In the absence of any substrate, the conversion of acrylamide to polyacrylamide, in otherwise exactly similar conditions, was 21.6 /".

EXAMPLE 3 Starch is readily grafted with methacrylic acid esters according to the method of the present invention. When 10 ml of purified methyl methacrylate were used instead of acrylonitrile in the conditions of Example 1, a product yield of 12.4 g was obtained indicating a conversion of monomer to polymer of 52.1%. No polymer formation was observed in the absence of any substrate. The grafting efficiency was nearly 100%.

EXAMPLE 4 This Example is an illustration of the grafting of acrylic acid esters. Thus, under conditions similar to Example 1 with the use of 10 ml of distilled methyl acrylate monomer instead of acrylonitrile, a product yield of 12.41 g was obtained which corresponded to a monomer conversion to poly(methyl acrylate) of 51.5%. In the absence of any substrate the conversion of methyl acrylate to polymer was only 0.6%.

EXAMPLE 5 According to the process of this invention, different derivatives of starch may also be grafted with vinyl monomers.

This example illustrates graft copolymerization to Farinex A 90, a commercial oxidized starch manufactured by AB Stadex, Malt5, Sweden, prepared by oxidation of a slurry of native potato starch by sodium hypochlorite. With 7.5 g (on a dry basis) of Farinex A 90 and acrylonitrile as monomer, grafting under the conditions of Example 1 took place readily giving a product yield of 13.0 g, corresponding to an add-on of 40.9%.

Negligible homopolymerization was observed and thus a grafting efficiency approaching nearly 100% was obtained.

EXAMPLE 6 Posamyl* E, a quaternary cation active derivative of native potato starch prepared by reacting a quaternary reagent to give a degree of substitution corresponding to a nitrogen content of 0.287% on absolutely dry starch, was also readily graft copolymerized with acrylonitrile. Thus, with 7.5 g (on a dry basis) of Posamyl E and under the conditions in Example 1 a product yield of 13.15 g was obtained corresponding to an add-on of 41.6%. Negligible homopolymerization was observed.

EXAMPLE 7 This Example demonstrates grafting of acrylonitrile to gelatinized starch. Thus, 7.5 g (on a dry basis) of native potato starch were gelatinized in 100 ml of distilled water by heating to 750C. The starch dispersion was then cooled to 30"C. 0.5 ml of 95-97% concentrated sulfuric acid was then added to the dispersion. Rest of the procedure was the same as in Example 1. A product yield of 10.4 g was obtained corresponding to an add-on of 27.5%. Negligible homopolymer formation was observed.

EXAMPLE 8 As stated earlier in this specification, starch isolated from different sources such as potato starch, corn starch, wheat starch, rice starch, tapioca starch or a waxy maize starch can also be grafted by vinyl monomers in accordance with the method of this invention.

This Example illustrates grafting of corn starch with acrylonitrile.

1.623 g MnSO4 H2O were dissolved in 100 ml distilled water. 25 ml of this solution of Mn2+ ions were added to 250 ml of a solution of sodium pyrophosphate containing 6.691 g of Na4P2O, 10 H2O. The Mn'+ ions were subsequently oxidized to Mn3e ions using potentiometric titration by nearly 25 ml of a solution of Mn7+ ions obtained by dissolving 0.379 g of KMnO4 in 100 ml of distilled water. The titration procedure was the same as in Example 1.

20 g of corn starch on a dry basis were slurried together with 100 ml of acidified water made by dissolving 5 ml 96% H2SO4 in 900 ml distilled water. 25 ml of initiator solution as synthesized above and 25 ml of acrylonitrile were placed in the dropping funnel. Nitrogen gas was bubbled through the initiator and monomer as well as the corn starch slurry to displace oxygen from the system. After 30 minutes of N2 bubbling, initiator and monomer were added to the reaction vessel and the reaction allowed to proceed for 3 hours in an atmosphere of nitrogen. The temperature in the reaction vessel was maintained between 30 and 33"C by cooling. The product was filtered, washed and dried and weighed. The weight of the product was 37.2 g indicating a conversion of monomer to polymer of 86.0% and an add-on of 46.3%. In the absence of any substrate but in otherwise similar conditions, the conversion of acrylonitrile to polyacrylonitrile was negligible, i.e. 3.5%.

EXAMPLE 9 Grafting of acrylonitrile takes place readily to wheat starch. Thus in conditions exactly similar to Example 8 but with 20 g of dry wheat starch instead of corn starch the weight of the product formed was 37.1 g indicating a conversion of monomer to polymer of 85.5% and an add-on of 46.1%.

As stated in Example 8 of this invention negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 10 This Example illustrates grafting of rice starch with acrylonitrile. In conditions exactly similar to Example 8 but with 20 g of dry rice starch instead of corn starch the weight of the product formed was 36.4 g indicating a conversion of monomer to polymer of 82% and an add-on of 45.05%.

As stated in Example 8 of this invention, negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 11 The grafting of acrylonitrile to tapioca starch is illustrated in this Example. When grafting was carried out as in Example 8 but onto 20 g of dry tapioca starch instead of corn starch the weight of the product formed was 37.55 g indicating a conversion of monomer to polymer of 87.75% and an add-on of 46.7%. As stated in Example 8 of this invention, negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 12 This Example illustrates grafting of acrylonitrile to waxy maize starch. Thus, in conditions similar to those of Example 8 but using 20 g of dry waxy maize starch instead of corn starch, the weight of the product formed was 36.32 g indicating a conversion of monomer to polymer of 81.6% and an add-on of 44.9%. As stated in Example 8 of this invention, negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

* "Posamyl" is a registered trade mark.

EXAMPLE 13 This Example illustrates grafting of acrylonitrile to native potato starch which was first oxidized in alkaline hypochlorite solution containing 2.3% active chlorine and the resulting product then slightly crosslinked with epichlorhydrin. Thus when 20 g of the dry oxidized and crosslinked native potato starch were grafted as in Example 8 the weight of the product formed was 38.0 g indicating a conversion of monomer to polymer of 90% and an add-on of 47.4%. As stated in Example 8 of this invention, negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 14 This Example illustrates grafting of acrylonitrile to native potato starch which was first oxidized in alkali by a hypochlorite solution containing 0.55% active chlorine and the resulting product was then etherified with propylene oxide to a degree of substitution of 0.06. Thus when 20 g of the dried hydroxypropyl ether of oxidized native potato starch were used as the substrate for grafting with acrylonitrile under the conditions of Example 8 the weight of the product formed was 37.6 g indicating a conversion of monomer to polymer of 88% and an add-on of 46.8%. As stated in Example 8 of this invention negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 15 This Example illustrates grafting of acrylonitrile to a tertiary cation active derivative of native potato starch with a degree of substitution of 0.03. Thus when 20 g of the dry cation active derivative were used instead of corn starch but in conditions of Example 8 the weight of the product formed was 36.9 g indicating a conversion of monomer to polymer of 84.5% and an addon of 45.8%. As stated in Example 8 of this invention, negligible conversion of monomer to homopolymer was observed in the absence of any substrate.

EXAMPLE 16 This Example deinonstrates grafting of acrylonitrile to acetyl ester of native potato starch containing 2.6% acetyl groups on a dry basis. Thus, when 20 g of the dry acetyl ester were used as substrate for grafting instead of corn starch under the conditions of Example 8 the weight of the product formed was 37.3 g indicating a conversion of monomer to polymer of 86.5% and an addon of 46.4%. As stated in Example 8 of this invention, negligible conversion of monomer -to homopolymer was observed in the absence of any substrate.

EXAMPLE 17 This Example illustrates grafting of methyl acrylate to corn starch. 0.811 g of MnSO4 H2O was dissolved in 100 ml of distilled water. 25 ml of this solution of Mn2+ ions were added to 6.691 g of Na4P2O7 10 H2O dissolved in 250 ml distilled water. The Mn2+ ions were subsequently oxidized to Mn3+ ions using potentiometric titration by 25 ml of a solution of Mn7+ ions obtained by dissolving 0.189 g of KMnO4 in 100 ml distilled water.

The titration procedure was the same as in Example 1. 7.5 g (on dry basis) of native corn starch were slurried together with 100 ml of acidified distilled water of Example 8.

10 ml of methyl acrylate and 25 ml of initiator solution prepared as above were placed in the dropping funnel. After 15 minutes of bubbling N2 gas through the corn starch slurry, initiator and monomer, the content of the dropping funnel, was added to the reaction vessel. After a reaction time of 2 hours at 300 C, the products were filtered, washed and dried. The product weighed 10.5 g corresponding to an add-on of 28.6%. In the absence of any substrate, negligible conversion of monomer to polymethylacrylate was observed.

EXAMPLE 18 This Example illustrates grafting of methyl methacrylate to corn starch. Thus, in conditions exactly similar to Example 17 but using methyl methacrylate as monomer instead of methyl acrylate, a product yield of 13.8 g was obtained corresponding to an add-on of polymethylmethacrylate of 45.6%. In the absence of any substrate but otherwise exactly similar conditions, no polymer formation was observed.

EXAMPLE 19 According to the process of this invention, vinyl monomers can be graft copolymerized even onto swollen starch (commonly referred to as gelatinized starch). This Example illustrates grafting of acrylonitrile monomer onto gelatinized native potato starch.

Acidified water was prepared by adding 5 ml of 96 /n H2SO4 to 900 ml distilled water.

The initiator solution was prepared as follows. 2.434 g of MnSO4 H2O were dissolved in 100 ml distilled water. 25 ml of this solution of Mn2+ ions were added to 250 ml of a solution of sodium pyrophosphate containing 6.691 g of Na4P201. 10 101120. The Mn2+ ions were subsequently oxidized to Mn3+ ions using potentiometric titration by 25 ml of a solution of Mn7+ ions obtained by dissolving 0.569 g of KMnO4 in 100 ml distilled water, the titration procedure being the same as in Example 1. Then 5 g (on a dry basis) of native potato starch were placed in the reaction vessel together with 75 ml distilled water. 25 ml of initiator solution prepared as above were placed in one chamber of the dropping funnel together with 10 ml of purified acrylonitrile monomer. 25 ml of the acidified water were placed in the second chamber of the dropping funnel. After bubbling purified N2 gas for 30 minutes through the acidified water, initiator, monomer and starch slurry, the temperature of the water bath was raised to 85"C to gelatinize the starch granules (all operations in N2 atmosphere).

The gelatinized starch was cooled to 300C and the content of the dropping funnel (i.e.

initiator solution, monomer and acidified water) was added to the reaction vessel. The reaction was allowed to proceed for 3 hours in an atmosphere of N2.

After the reaction, the products were filtered, washed with methanol and water and dried to constant weight at 600 C. The weight of the dried products were 9.0 g indicating an add-on of 44.4%. In the absence of starch substrate but exactly similar conditions, only a negligible conversion of acrylonitrile to polyacrylonitrile (i.e. about 3%) was obtained.

EXAMPLE 20 This Example illustrates grafting of methyl acrylate onto gelatinized starch.

Thus, under the same conditions as in Example 19 except that 10 ml of methyl acrylate were used instead of acrylonitrile, the weight of the washed and dried product was 10.7 g, indicating an add-on of 53.1%. In the absence of starch substrate, only a negligible conversion (about 8%) of methyl acrylate to polymethylacrylate was obtained.

EXAMPLE 21 This Example illustrates grafting of methyl methacrylate onto gelatinized native potato starch.

Thus under the same conditions as in Example 19 except that 10 ml of methyl methacrylate were used instead of acrylonitrile, the weight of the washed and dried product was 9.5 g, indicating an addon of 47%. In the absence of starch substrate, only a negligible conversion (about 1.0%) of methyl methacrylate to polymethylmethacrylate was obtained.

EXAMPLE 22 According to the process of this invention, two vinyl monomers can be simultaneously grafted to yield grafted chains which are copolymers.

This Example illustrates simultaneous grafting of acrylonitrile and acrylamide onto native potato starch.

Acidified water was prepared by adding 5 ml 96% H2SO4 to 900 ml of distilled water.

20 g on a dry basis of native potato starch were placed in the reaction vessel together with 100 ml of acidified water. 25 ml of initiator solution prepared as described in Example 19 of this invention and 25 ml of purified acrylonitrile monomer were placed in one chamber of the dropping funnel. 10 g acrylamide monomer dissolved in 25 ml distilled water were placed in the second chamber of the dropping funnel. After bubbling N2 gas for 30 minutes through acrylonitrile, initiator solution, acrylamide solution and the starch slurry, acrylonitrile monomer and initiator solution were added to the starch slurry followed after 5 minutes by acrylamide solution. The reaction was allowed to proceed for 3 hours in an atmosphere of N2. The product was filtered, washed and dried. The weight of the dried product was 43.6 g. C, N, H and 0 analysis of the product showed that a copolymer having an average of 39.2% by weight of acrylonitrile and 8.9% by weight of acrylamide in total reaction product was grafted onto the native potato starch substrate.

EXAMPLE 23 This Example illustrates simultaneous grafting of acrylonitrile and vinyl acetate onto native potato starch.

Acidified water was prepared by adding 5 ml of 96% H2SO4 to 900 ml distilled water.

20 g on a dry basis of native potato starch were placed in the reaction vessel together with 100 ml of acidified water. 25 ml of initiator solution prepared as described in Example 19 of this invention, 25 ml of purified acrylonitrile monomer and 10 ml distilled vinyl acetate monomer were placed in the dropping funnel. After bubbling N2 gas for 30 minutes through acrylonitrile, vinyl acetate, initiator solution and starch slurry, the content of the dropping funnel was added to the reaction vessel. The reaction was allowed to proceed for 3 hours in an atmosphere of N2. The products were filtered, dried and weighed. The weight of the dried product was 34.5 g. C, H and 0 analysis of the product showed that a copolymer having an average of 38.6% by weight of acrylonitrile and 4.3% by weight of vinyl acetate in total reaction product was grafted onto the native potato starch substrate.

EXAMPLE 24 This Example illustrates simultaneous grafting of methyl acrylate and vinyl acetate onto native potato starch.

100 ml of acidified water (as prepared in Example 22 of this invention) and 7.5 g of native potato starch were placed in the reaction vessel. 25 ml of initiator prepared as described in Example 17 of this invention and 10 ml each of purified methyl acrylate and vinyl acetate monomers were placed in the dropping funnel. After bubbling N2 gas for 30 minutes through the monomers, initiator solution and starch slurry in the reaction vessel the content of the dropping funnel was emptied into the reaction vessel.

The reaction was allowed to proceed in an atmosphere of N2 for 3 hours. After completion of the reaction the products were filtered, washed and dried. The weight of the dry product was 15.0 g corresponding to an add-on of 50% of copolymer of methyl acrylate and vinyl acetate.

EXAMPLE

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reaction vessel. 25 ml of initiator prepared as described in Example 17 of this invention and 10 ml each of purified methyl acrylate and vinyl acetate monomers were placed in the dropping funnel. After bubbling N2 gas for 30 minutes through the monomers, initiator solution and starch slurry in the reaction vessel the content of the dropping funnel was emptied into the reaction vessel. The reaction was allowed to proceed in an atmosphere of N2 for 3 hours. After completion of the reaction the products were filtered, washed and dried. The weight of the dry product was 15.0 g corresponding to an add-on of 50% of copolymer of methyl acrylate and vinyl acetate. EXAMPLE 25 This Example illustrates simultaneous grafting of acrylonitrile and styrene onto native corn starch. Acidified water was prepared by adding 5 ml 96 /n H2SO4 to 900 ml of distilled water. The initiator solution was prepared as follows. 25 ml of a solution of Mn2+ ions prepared by dissolving 1.6226 g MuSO4. H2O in 100 ml distilled water were added to a solution of 6.6910 g of sodium pyrophosphate (Na4P2O7. 10 H2O) in 250 ml of distilled water. The Mn2 ions were subsequently oxidized to Mn3+ ions using potentiometric titration by 25 ml of a solution of Mn7+ ions obtained by dissolving 0.3794 g KMnO4 in 100 ml distilled water. The titration procedure was the same as in Example 1. 20 g (on a dry basis) of native corn starch were slurried together with 100 ml of the acidified distilled water in a reaction vessel which was immersed in a thermostatically controlled water bath maintained at 300 C. The reaction vessel was equipped with a stirrer, a condenser, a thermometer and a dropping funnel with a nitrogen bubbler. 25 ml of the initiator solution (pH 6), 25 ml of distilled acrylonitrile and 10 ml of distilled styrene were added to the dropping funnel. A brisk stream of oxygen-free nitrogen was bubbled first through the content of the dropping funnel and then through the starch slurry in the reaction vessel before the gas was led out of the system. This was continued for 30 minutes while the temperature in the reaction vessel was raised to 300 C. The content of the dropping funnel was then emptied into the reaction vessel and the reaction carried out in an atmosphere of N2 for 3 hours. After the reaction, the products were filtered, washed with water and ethanol and dried overnight in a stream of dry air at 600C and finally weighed. The product yield was 42.6 g and analysis showed that a copolymer having an average of 30.6 X" of acrylonitrile and 22.1 /" of styrene in total reaction product was grafted onto the native corn starch substrate. EXAMPLE 26 This Example illustrates simultaneous grafting of acrylonitrile and styrene onto an oxidized and crosslinked native potato starch. Thus, under the same conditions as in Example 25 but with a substrate of an oxidized and crosslinked native potato starch instead of corn starch the product yield was 45.3 g. Analysis showed that a copolymer having an average of 35.3% of acrylonitrile and 19.8% of styrene in total reaction product was grafted onto the potato starch derivative. WHAT WE CLAIM IS:

1. A process for the preparation of a graft copolymer of starch which comprises graft polymerising ethylenically unsaturated monomer onto a starch substrate in the presence as an initiator of a complex of trivalent manganese with a polyphosphate.

2. A process as claimed in Claim 1, in which an acid manganic pyrophosphate complex is used as initiator.

3. A process as claimed in Claim 1 or 2, in which said monomer is a vinyl and/or a vinylidene monomer.

4. A process as claimed in Claim 3, in which the vinyl monomer is acrylonitrile.

5. A process as claimed in Claim 3 or 4, in which the vinyl monomer is methyl acrylate.

6. A process as claimed in Claim 3, 4 or 5, in which the vinylidene monomer is methyl methacrylate.

7. A process as claimed in Claim 4, in which said monomer is a mixture of acrylonitrile and one or more of acrylamide, vinyl acetate and styrene.

8. A process as claimed in Claim 1 or 2, in which said monomer is selected from one or more of the classes (1) to (10) of unsaturated monomer as set forth herein.

9. A process as claimed in any preceding claim, in which the starch substrate is potato starch, corn starch, wheat starch, rice starch or a waxy starch.

10. A process as claimed in any preceding claim, in which the starch substrate is in its original granular form.

11. A process as claimed in any one of

Claims 1 to 9, in which the starch substrate is gelatinized starch.

12. A process as claimed in Claim 1 substantially as herein described.

13. A process for the preparation of a graft copolymer of starch substantially as herein described with reference to any one of the foregoing Examples 1 to 26.

14. A graft copolymer of starch when prepared by a process as claimed in any preceding claim.