Process for sulphonating aromatic polyether sulphones

The invention relates to a process for the sulfonation of aromatic polyether sulfones by means of a solution of sulfur trioxide in concentrated sulfuric acid and using a solvent for the corresponding polyether sulfone.

Aromatic polyether sulfones are understood to mean polymers whose monomeric, repeating units are at least<ul id="ul0001" list-style="none" compact="compact"><li>a) a sulfone group between two aromatic residues and</li><li>b) an ether bond between two aromatic residues</li></ul> have, wherein the two aromatic radicals, between which the sulfone group is located, are each bound to another aromatic radical via an ether bond. The aromatic radicals present in the monomeric unit can be linked to the corresponding substituents in each case in the o-, m- or p-position. The aromatic radicals optionally carry further substituents, but in each aromatic radical of the monomer unit there is at least one hydrogen atom bonded to the aromatic ring which can be substituted by the sulfonic acid group by means of a sulfonation reaction.

Typical representatives of such polyether sulfones are polymers which have the following, repeating structural unit<chemistry id="chem0001" num="0001"><img file="EP0341473B1_D0001.tif" /></chemistry> Such polymers are commercially available products.

Aromatic polyether sulfones (PÄS) are known polymers which have already been used for various areas of application, making use of the good thermal, mechanical and chemical stability. Examples of application areas are auto parts, circuits and the use for coatings. Polyether sulfones have also already been used for the production of membranes which are used in material separation processes. Here the stability of the material against water, chlorine and a number of organic solvents is used. Membranes made of polyether sulfones are for example in the<u>EP-A-0 121 911</u> described.

Membranes made of such materials, however, have insufficient hydrophilicity for a number of uses in which high hydrophilicity is required, which is manifested, for example, in too slow wetting by liquid aqueous systems. An attempt was therefore made to modify polyether sulfones by introducing ionic or dissociable substituents. One of the options used for this is the introduction of sulfonic acid groups, which can then be converted into salts. The aromatic polyether sulfones are substituted on the aromatic ring system and are often carried out using a sulfonating agent. Chlorosulfonic acid can be used as the sulfonating agent. A corresponding method is for example in the<u>U.S. Patent 4,508,852</u> described. A disadvantage of using chlorosulfonic acid is that side reactions occur. Thus, not only sulfonated but also chlorosulfonated products are obtained in the process of the aforementioned US Pat. In addition, the reaction described there in organic solvents at temperatures of more than 40 ° C. sometimes leads to undesired crosslinked polymers.

Attempts to sulfonate polyether sulfones with concentrated sulfuric acid have led to the result that this process can only be carried out for certain polymers whose monomer units are appropriately substituted. That's how it is<u>EP-A-0 008 894</u> It can be seen that phenylene residues, each of which have an ether bond in the 1.4 position, can be sulfonated by concentrated sulfuric acid, but not phenylene residues which have a sulfone group and an ether bond in the p-position to one another. The reason for this is probably the strong electron-withdrawing effect of the sulfone group. Although EP-A-0 008 894 states that chlorosulfonic acid or oleum act on these inert phenylene groups mentioned, it advises against the use of these sulfonating agents, since only high degrees of sulfonation and / or polymer degradation can be achieved here. A controllable sulfonation reaction with chlorosulfonic acid or oleum is not possible. The same statement is that<u>EP-A-0 112 724</u> refer to. According to the statements of <u>U.S. Patent 3,709,841</u> The sulfonation of polyether sulfones can also be achieved using chlorosulfonic acid or oleum when working in an inert solvent. Chlorinated hydrocarbons are mentioned as suitable solvents. One of the disadvantages of using these solvents is their toxicity. The disposal of used solvents also poses problems. In comparison to the known processes described, it would therefore be desirable to use sulfonation using concentrated sulfuric acid, which contains sulfur trioxide (oleum, fuming sulfuric acid), as a solvent.

The object of the present invention was to develop a process for the sulfonation of polyether sulfones which, without the use of organic solvents, has a controllable sulfonation, inter alia also of inert aromatic residues in the polymers by means of sulfur trioxide in concentrated sulfuric acid and leads to sulfonated polymers without the formation of large amounts of by-products or degradation products, especially in the case that linear polyether sulfones are used which are based on the following monomeric units ,<chemistry id="chem0002" num="0002"><img file="EP0341473B1_D0002.tif" /></chemistry>

The object is achieved by a process according to the preamble of claim 1, which is characterized in that concentrated sulfuric acid is used as the solvent, that the sulfur trioxide content, based on the total amount of pure sulfuric acid present in the reaction mixture, is kept at a value of less than 6 percent by weight during the entire duration of the sulfonation and that the temperature of the reaction mixture is kept below +30 ° C. throughout the entire duration of the reaction.

It has surprisingly been found that if these measures are followed, a controllable sulfonation of aromatic polyether sulfones is possible and polymer degradation can be largely or completely avoided. The degree of sulfonation, ie the quotient of the total number of sulfonic acid groups in the polymer and the total number of repeating monomer units can be conveniently controlled by specifically setting the polymer concentration, the concentration of sulfur trioxide and the reaction time. A very simple way to do this is to choose the appropriate reaction time, because the reaction can be ended at any time by adding water to the reaction mixture or pouring the reaction mixture into water. The process according to the invention is suitable for the sulfonation of polyether sulfones which contain aromatic radicals and correspond to the definition mentioned at the outset. In the context of the present invention, aromatic polyether sulfones are also understood to mean copolymers in which - in addition to other monomer units - the monomer units mentioned at the outset are present. The substitution of hydrogen atoms by sulfonic acid residues takes place on the aromatic residues of the polyether sulfone groups and possibly also on aromatic residues of other monomer units.

It is an essential part of the process according to the invention that care is taken to ensure that the sulfur trioxide content is lower than 6% by weight during the entire duration of the sulfonation, this value being based on the total amount of pure sulfuric acid present in the reaction mixture. The sulfur trioxide content can be adjusted via the amounts of sulfuric acid and sulfur trioxide used, although any water content of the sulfuric acid used must be taken into account, ie the sulfur trioxide content is based on 100% anhydrous sulfuric acid. If the sulfur trioxide in solution (oleum, fuming sulfuric acid) is mixed with concentrated sulfuric acid and the oleum is thereby diluted, the content of sulfur trioxide which must be observed naturally refers to the sum of the amount of 100% sulfuric acid resulting from the Oleum originates and that used to dilute the oleum.

It is possible that the concentrated sulfuric acid used, which has not yet been mixed with oleum and / or the polyether sulfone used contains water. In this case, the sulfur trioxide added, for example in the form of oleum, first reacts with water before the polymer is substituted. In this case, therefore, more oleum can be used, ie the initial sulfur trioxide content can be higher than calculated from the 6% limit. Only when the water has been converted to sulfuric acid with sulfur trioxide does the sulfonation reaction begin, during which the specified limit of 6% by weight must not be reached. It is expedient to work well below this limit, namely with less than 3% by weight and possibly even with not more than 1 to 1.5% by weight of sulfur trioxide during the entire reaction time.

The sulfur trioxide content in the sulfuric acid does not necessarily have to be in the range of a few percent by weight. Depending on the type of polymer used, the reaction time and / or temperature, the sulfonation can be carried out to the desired extent even with minimal amounts of sulfur trioxide.

Compliance with the limit for the sulfur trioxide content can be achieved, for example, by concentrating SO before the reaction₃-free sulfuric acid with the calculated amount of oleum added, the mixture cools and then the polymer in solid form or dissolved in concentrated sulfuric acid. This is a preferred embodiment of the method according to the invention. Another is to first dissolve the polymer in concentrated sulfuric acid and then slowly add oleum. Here too, the sulfur trioxide content in the reaction mixture can be kept below 6% during the reaction. Another, but less preferred, possibility of carrying out the process according to the invention is to add sulfur trioxide in pure solid or gaseous form to a solution of the polymer in concentrated sulfuric acid. This embodiment is less preferred because either locally high concentrations of SO₃ (Addition in solid form) or local overheating (addition as gas) can occur, which promote polymer degradation. In the two preferred embodiments mentioned, in which oleum is used, its sulfur trioxide content is preferably 65% ​​by weight.

Another well-suited way of obtaining sulfuric acid containing sulfur trioxide, which is used as the reaction medium, is, besides the use of oleum, by adding a compound to concentrated sulfuric acid which leads to the formation of sulfur trioxide in concentrated sulfuric acid.

Phosphorus pentoxide, which liberates sulfur trioxide from concentrated sulfuric acid, can preferably be used for this. M type connections are also suitable^{n +}(P₂O₇)^2-_{n / 2}, where M is a metal and n is the valence of this metal.

An example which is very suitable for the process according to the invention is potassium disulfate, K₂S₂O₇. Also suitable are further compounds which contain sulfur trioxide contain or covalently bonded manner and release it under the sulfonation conditions. The amount of one or more of the aforementioned compounds to be added is of course limited by the maximum content of sulfur trioxide which, as described above, may be present during the sulfonation reaction, namely less than 6% by weight.

The sulfur trioxide-providing compound can also, as described above for the case of oleum, be added to the sulfuric acid before or after the addition of the polymer.

The use of phosphorus pentoxide also offers the advantage in some cases that the sulfonation can be carried out at lower temperatures than when using oleum, ie at temperatures at which oleum has already solidified.

The advantage of using K₂S₂O₇ is that this salt can be used in the form of an easy-to-handle powder, which makes targeted SO₃Release is possible and local over-concentration of sulfur trioxide can be avoided.

Another important measure for the process according to the invention is temperature control during the sulfonation reaction. In order to keep polymer degradation and side reactions as low as possible, the temperature of the reaction mixture must be lower than 30 ° C. during the entire reaction period. This is done by cooling. It is advisable to ensure that there is no short-term local overheating. If one chooses the embodiment in which oleum is added dropwise to a solution of the polymer in sulfuric acid, this should therefore be done slowly and with stirring of the reaction mixture. Slow dropwise addition means that additional oleum is only added dropwise when the previous amount has been distributed in the mixture by stirring.

To dissolve the polymer, however, the sulfuric acid can be heated to a slightly higher temperature than + 30 ° C. before adding oleum, but expediently not to more than 40 ° C. It is then necessary to cool to the reaction temperature before adding oleum.

The lower limit for the temperature of the reaction mixture during the reaction is chosen so that a homogeneous liquid is still present, ie that no component of the mixture is in the solid state. It is usually advisable not to drop below a temperature of around 5 ° C. The concentrated SO used as solvent₃Free sulfuric acid, as mentioned, can contain water. However, the water content must not be so high that the polymer is no longer soluble in sulfuric acid at the reaction temperature. The water content of the sulfuric acid should not exceed 20% by weight so that the polymer can be dissolved. Normally, a water content of 5% by weight or less is suitable, on the one hand because polyether sulfones dissolve better in higher concentrated sulfuric acid, and secondly so that not too much sulfur trioxide is used for the reaction with water, which proceeds faster than the sulfonation of the polymer. In order to achieve higher degrees of sulfonation, it is advisable not to stop the reaction immediately after the total amount of the reactants has been added to the solvent, but to let the reaction continue for some time, for example 1 to 5 hours, with stirring and cooling. The reaction is advantageously terminated by precipitation of the polymer using water. This can be done by slowly pouring the reaction mixture into cold water. The polymer is then isolated, washed and dried. It should be noted, however, that shorter-chain and / or highly sulfonated polymer units have a higher or complete solubility in water. These can then no longer be precipitated by water. To isolate these products, other ways must therefore be followed. In normal cases, however, no value is placed on these water-soluble by-products, for example When using the sulfonated polyether sulfones for membrane production, chemical resistance, ie insolubility of the (membrane) material with respect to water is required.

The degree of sulfonation of the polymer after the reaction is the quotient of the total number of sulfonic acid groups in the polymer and the total number of repeating monomer units. A degree of sulfonation of 0.2 therefore means that on average one sulfonic acid group is present at every fifth monomer unit. The degree of sulfonation is decisive for the hydrophilicity or the ion exchange capacity of the sulfonated polymer. It can be conveniently determined by titrating the sulfonic acid groups on the one hand and determining the number of monomer units on the other. The latter can be determined arithmetically by dividing the molecular weight determined by customary methods by the calculated molecular weight of the monomer unit.

The ion exchange capacity (IEC) mentioned, which is a measure of the hydrophilicity, can be given in meq / g. 1 meq / g means that 1 mmol protons per gram of polymer can be exchanged for 1 / n mmol of an n-valent cation.

The IEC can in turn be determined by titration.

For some uses, the sulfonated polymer in the form of the free sulfonic acid group (s) is less suitable than in the form of its salt (s), for example metal or ammonium salts. The conversion into these salts can be carried out by neutralization with the corresponding bases in a solvent.

As mentioned, there are several possibilities for carrying out the process according to the invention, including those in which oleum is used. Of these, two are particularly preferred. The choice of one or the other alternative affects the degrees of sulfonation and yields of sulfonated polyether sulfones. One of the preferred embodiments (Alternative I) is to first polymerize in SO₃to dissolve free concentrated sulfuric acid, if necessary at a slightly elevated temperature, to cool the solution and then slowly add oleum with stirring, for example by dropping. In the second embodiment (alternative II), oleum is first added to concentrated sulfuric acid, the solution is cooled and then the polymer is added in solid form or dissolved in concentrated sulfuric acid. In both cases, the amount of oleum is to be measured so that after the reaction of the sulfur trioxide with water, which may be present in the sulfuric acid and / or in the polymer, the content of sulfur trioxide, based on the total amount of pure sulfuric acid, is less than 6% by weight. % lies. Before the solution preparation begins, the water content of the concentrated sulfuric acid and that of the polymer must be determined.

Alternative I normally leads to a lower yield of sulfonated polymer, which can be obtained by precipitation with water. It is believed that this approach creates a greater proportion of water-soluble compounds than in the case of Alternative II. The yields of water-insoluble polymer in the case of alternative I are about 70, in alternative II they are normally 95-100%, based in each case on the theoretical amount of sulfonated product to be expected according to the amount of polymer used. Alternative II normally leads to higher degrees of sulfonation than alternative I with the same starting quantities.

In a preferred embodiment, the temperature of the reaction mixture during the sulfonation reaction is lower than 12 ° C. This further limits the risk of local overheating and polymer degradation.

The maximum concentration of the polymer in the starting mixture is limited on the one hand by its solubility properties and on the other hand by the processability. Polymer concentrations in the range between 5 and 30% by weight, based on the total amount of pure, anhydrous sulfuric acid, are preferably used.

Polyether sulfones which are particularly suitable for the process according to the invention are polymers which are composed of the following, repeating structural units:<chemistry id="chem0003" num="0003"><img file="EP0341473B1_D0003.tif" /></chemistry>

The molecular weight of the polyether sulfone used can be varied within wide limits. However, care must be taken to ensure that low molecular weight portions of the end product, especially at higher degrees of sulfonation, are water-soluble and are therefore unsuitable for some uses. Polymers are preferably used for the process according to the invention, the weight average molecular weight of which is between 30,000 and 60,000 daltons.

The sulfonated polyether sulfones obtained by the process according to the invention can advantageously be used for the production of membranes. Because of their hydrophilic material, the membranes thus obtained are particularly suitable for areas of application in which rapid wetting by liquid aqueous systems is desired, for example in biotechnology, pharmacy, food technology and wastewater treatment or other processes in which substances are separated from aqueous liquid systems.

Membranes made from these sulfonated polyether sulfones can be made by well known methods, for example by casting a film from a solution of the polymer, evaporating some of the solvent and treating it with a precipitant for the polymer. The membranes can also be produced in the form of hollow threads (capillary membranes), for example by extruding a solution of the polymer through a hollow thread nozzle into a coagulation bath. Here, the nozzle can be immersed directly in the coagulation bath, but there can also be a so-called "air gap" between the two of, for example, a few cm in length, so that the shaped solution below the nozzle is first passed through a short distance in air before it enters the coagulation bath is immersed. Suitable solvents for the processes mentioned are, for example, N, N-dimethylacetamide, N-methyl-pyrrolidone and dimethyl sulfoxide. The polymer solution can also contain other additives to achieve special properties, for example viscosity-increasing additives such as polyvinylpyrrolidone or polyethylene glycol or non-solvents for the sulfonated polyether sulfone such as ethylene glycol, N-methylformamide, glycerol or water. Paraffin oil or a mixture of N, N-dimethylacetamide, glycerol and water can be used as the liquid for filling the lumen in hollow fiber extrusion, for example, water or a mixture of water and the solvent in which the polymer was dissolved before extrusion as a coagulation bath .

Sulfonated polyether sulfones with a degree of sulfonation between 0.001 and 0.6 are preferably used for membrane production. A degree of sulfonation of 0.01 to 0.6 is particularly favorable.

The membranes are particularly suitable for a number of applications if they are in the form of hollow fibers or tubes or as a flat membrane. They are preferably designed as membranes for the microfiltration range, ie with average pore sizes in the range from 0.05 to 5 μm and a porosity between 50 and 85%. The porosity is the quotient of the volume occupied by membrane pores and the total volume of the membrane. It can be determined by measuring the specific weight of the membrane and the specific weight of the non-porous membrane material.

The polymers obtained by the process according to the invention can advantageously also be processed into membranes in a mixture with another polymer. This procedure has the advantage that membranes can be obtained from the same sulfonated polyether sulfone by mixing with a further polymer selected specifically according to type and amount, the hydrophilicity properties of which can be specifically graded. By mixing with another less hydrophilic polymer, e.g. a polymer that contains no ionic or dissociable groups, a membrane of a certain hydrophilicity can also often be produced more cheaply than by using only a polyether sulfone with a lower degree of sulfonation.

Membranes made from mixtures which have a sulfonate content of between 0.05 and 18% by weight, preferably from 0.1 to 18%, are particularly suitable for a number of uses. This sulfonate content can be determined by titration. It is defined as the proportion by weight of SO₃H groups, based on the total weight of the polymer mixture.

The invention is illustrated by the following exemplary embodiments:

example 1

(Variant I)

200 g (0.86 mol monomer units) dried (in a high vacuum at room temperature to constant weight) polyether sulfone (Victrex® 5200P from ICI) are dissolved in 600 ml (approx. 1104 g) concentrated sulfuric acid (approx. 97%; contains 1. 7 mol H₂O) solved. With constant cooling to 10 ° C and with stirring, 125 ml of 65% oleum (2 mol SO₃) slowly added dropwise (1.5 hours). When the addition is complete, the mixture is stirred at 10 ° C. for a further three hours, then the viscous solution is run into 10-15 l of water and the precipitated polymer is filtered off: the mixture is washed with water until the wash water no longer exhibits an acidic reaction and dried Room temperature. The yield is 140 g of sulfonated polyether sulfone (SPES). The product shows a reduced viscosity of<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>η</mtext></mrow><mrow><mtext>red</mtext></mrow></msub><msup><mrow><mtext> = 0,53·10</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><mtext>l / g</mtext></mrow></math><img file="EP0341473B1_D0004.tif" /></maths> and a degree of sulfonation (measured by titration) of DS = 0.16.

The reduced viscosities in these examples were obtained from solutions of 0.4 g of the polymer in 100 ml of concentrated H₂SO₄ determined at room temperature.

Example 2

(Variant II)

600 ml (approx. 1104 g) concentrated sulfuric acid (approx. 97%; contains approx. 1.7 mol H)₂O) are presented and with 125 ml of 65% oleum (2 mol SO₃) stirred (heating to 60 ° C). The mixture is cooled to about 10 ° C. and 200 g (0.86 mol monomer units) of polyether sulfone (Victrex 5200P; ICI) are added in solid form in one portion with vigorous stirring. The mixture is stirred for a further three hours at 10 ° C. and worked up as in Example 1. The sulfonated material is obtained in a yield of 217 g SPES, which corresponds to the theoretical yield. The product shows a reduced viscosity of<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>η</mtext></mrow><mrow><mtext>red</mtext></mrow></msub><msup><mrow><mtext> = 0,42·10</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><mtext>l / g</mtext></mrow></math><img file="EP0341473B1_D0005.tif" /></maths> and a degree of sulfonation (measured by titration) of DS = 0.22.

The Victrex 5200P used here is a polyether sulfone with the structure specified in claim 12.

Example 3

This example shows the increase in the degree of sulfonation over the reaction time and at elevated temperature. The degree of substitution increases with longer reaction times. The reaction was carried out according to variant II, the polymer being dried before the addition. (Example 2). The reaction temperature was 25 ° C. Depending on the reaction time, the following degrees of sulfonation (DS) and viscosity values ​​resulted:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Time (h)</entry><entry namest="col2" nameend="col2" align="center">DS</entry><entry namest="col3" nameend="col3" align="center">η_red</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">1</entry><entry namest="col2" nameend="col2" align="char" char=",">0,315</entry><entry namest="col3" nameend="col3" align="char" char=",">0,373</entry></row><row><entry namest="col1" nameend="col1" align="right">3</entry><entry namest="col2" nameend="col2" align="char" char=",">0,336</entry><entry namest="col3" nameend="col3" align="char" char=",">0,352</entry></row><row><entry namest="col1" nameend="col1" align="right">5</entry><entry namest="col2" nameend="col2" align="char" char=",">0,347</entry><entry namest="col3" nameend="col3" align="char" char=",">0,350</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">22</entry><entry namest="col2" nameend="col2" align="char" char=",">0,384</entry><entry namest="col3" nameend="col3" align="char" char=",">0,325</entry></row></tbody></tgroup></table></tables>

The 97.00% H₂SO₄ theoretically maximum achievable degree of sulfonation is DS 0.39.

Example 4

The procedure described in Example 1 is repeated with the polymer not previously dried. The sulfonated product shows a reduced viscosity of η_red = 0.58·10^-1l / g with a degree of sulfonation of DS = 0.09.

Example 5

Example 2 was repeated, the starting material having previously been dried in a high vacuum at room temperature. The sulfonated product had a degree of sulfonation of DS = 0.37 with a reduced viscosity of η_red = 0,42·10^-1l / g.

Example 6

Sulfonation of Victrex by SO

3

-Development via P

2

O

5

In 350 ml of 97% H₂SO₄ 70 g of P₂O₅- Powder stirred in quickly. After dissolving the P₂O₅-Powder was down to -13^OC cooled. 100 g of dried Victrex were added to the solution and at about -7^OC stirred for about 5 hours. The reaction was then stopped by carefully adding water and the product was worked up as in Example 1. The Victrex used was the same as in Example 1 Yield: 115 g sulfonated PES<maths id="math0003" num=""><math display="block"><mrow><msub><mrow><mtext>DS = 0.35 η</mtext></mrow><mrow><mtext>red</mtext></mrow></msub><msup><mrow><mtext> = 0,423 . 10</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><mtext> l / g</mtext></mrow></math><img file="EP0341473B1_D0006.tif" /></maths>

Example 7

Sulfonation of Victrex by SO

3

-Development from K

2

S

2

0

7

30 g of K were added to 350 ml of 100% sulfuric acid₂S₂O₇ dissolved at room temperature, then 100 g of dried Victrex were added. Victrex used and refurbishment as example 1<tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">reaction time</entry><entry namest="col2" nameend="col2" align="center">DS</entry><entry namest="col3" nameend="col3" align="center">η_red (10^-1l / g)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">6 H</entry><entry namest="col2" nameend="col2" align="char" char=",">0,190</entry><entry namest="col3" nameend="col3" align="char" char=",">0,361</entry></row><row><entry namest="col1" nameend="col1" align="right">24th H</entry><entry namest="col2" nameend="col2" align="char" char=",">0,272</entry><entry namest="col3" nameend="col3" align="char" char=",">0,287</entry></row><row><entry namest="col1" nameend="col1" align="right">6 d</entry><entry namest="col2" nameend="col2" align="char" char=",">0,310</entry><entry namest="col3" nameend="col3" align="char" char=",">0,229</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">20 d</entry><entry namest="col2" nameend="col2" align="char" char=",">0,398</entry><entry namest="col3" nameend="col3" align="char" char=",">0,266</entry></row></tbody></tgroup></table></tables>

The example described was repeated, the polymer being first dissolved in 100% sulfuric acid. Then K₂S₂O₇ added. The reaction temperature was + 10 ° C.<tables id="tabl0003" num="0003"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="center">reaction time</entry><entry namest="col2" nameend="col2" align="center">DS</entry><entry namest="col3" nameend="col3" align="center">η_red (10^-1l / g)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">6 H</entry><entry namest="col2" nameend="col2" align="char" char=",">0,080</entry><entry namest="col3" nameend="col3" align="char" char=",">-</entry></row><row><entry namest="col1" nameend="col1" align="right">22 H</entry><entry namest="col2" nameend="col2" align="char" char=",">0,127</entry><entry namest="col3" nameend="col3" align="char" char=",">-</entry></row><row><entry namest="col1" nameend="col1" align="right">2nd d</entry><entry namest="col2" nameend="col2" align="char" char=",">0,136</entry><entry namest="col3" nameend="col3" align="char" char=",">0,41</entry></row><row><entry namest="col1" nameend="col1" align="right">4th d</entry><entry namest="col2" nameend="col2" align="char" char=",">0,150</entry><entry namest="col3" nameend="col3" align="char" char=",">0,38</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">7 d</entry><entry namest="col2" nameend="col2" align="char" char=",">0,200</entry><entry namest="col3" nameend="col3" align="char" char=",">0,37</entry></row></tbody></tgroup></table></tables>

Example 8

(Comparative example not according to the invention)

Example 5 was repeated with the reaction temperature set at 40 ° C. After a reaction time of three hours and stirring the solution in water, a colloidal solution of the strongly degraded polymer was obtained, from which no usable product could be filtered off.

Example 9 (comparative example not according to the invention)

Example 6 was repeated, allowing to react at 40 ° C for 24 hours. After pouring into water, only water-soluble reaction product was obtained.