Sulphonation process for low crosslinked polystyrene

Crosslinked styrenic polymers may be sulphonated in a controlled manner in an organic solvent at elevated temperature using SO.sub.3 in the presence of a trialkyl phosphate. The process does not introduce further crosslinking into the polymer permitting it to have an extremely high water uptake. The resulting polymer is useful as a hydrogel.

FIELD OF THE INVENTION 
The present invention relates to the sulphonation of polymers. More 
particularly it relates to the sulphonation of crosslinked vinyl polymers. 
BACKGROUND OF THE INVENTION 
Sulphonated polymers have been known for some time. Non-crosslinked 
sulphonated vinyl polymers have been known since at least the early 
nineteen sixties. 
U.S. Pat. No. 3,072,618 issued Jan. 8, 1963 in the name of Albin F. 
Turback, assigned to Esso Research and Engineering, discloses a process 
for the sulphonation of non-crosslinked polystyrene. In fact the 
disclosure of the patent makes it clear that it is important to reduce the 
number of crosslinks to the smallest possible number (Col. 1, line 31). 
The object of U.S. Pat. No. 3,072,618 was to prepare water soluble 
polymers which would be useful as thickeners, impregnants, adhesives, soil 
conditioners and textile sizes. 
Greek Patent 86.0636 in the names of G. Valkanas and P. Rigas issued Mar. 
18, 1986 discloses that polystyrene crosslinked by reaction with a 
di-alkylhalo aromatic compound in the presence of a Lewis Acid is useful 
in agricultural applications after it has been sulphonated. The Greek 
Patent discloses a process for sulphonation in which a gel of the 
crosslinked polystyrene is sulphonated with sulfuric acid. From a 
practical point of view such a process results in a significant amount of 
acid water which must be separated and treated before safe disposal. This 
is a disadvantage which needs to be overcome before the technology 
disclosed in the Greek Patent can be effectively commercialized. 
The polymers disclosed in the Greek Patent are not water soluble as they 
are intended to stay in the ground to hold water in the soil. The polymers 
disclosed in U.S. Pat. No. 3,072,618 are intended to be water soluble. 
Interestingly applicants have discovered that the process designed for use 
with non-crosslinked polymers is also useful with crosslinked starting 
materials in accordance with the present invention. 
U.S. Pat. No. 4,209,592 issued Jun. 24, 1980, assigned to Rohm and Haas 
Company discloses a process to manufacture ion exchange resin. In the 
process SO.sub.3 is used both as a crosslinking agent and as a 
sulphonating agent. Uncrosslinked polystyrene is reacted directly with 
SO.sub.3 in an halogenated solvent. While the resulting polymer is a 
sulphonated crosslinked vinyl aromatic monomer it does not have the same 
water uptake as the polymers of the present invention. Particularly, the 
sulphonated croslinked vinyl aromatic polymers of the present should have 
a water up take of at least 15000, preferably 20000, most preferably 25000 
or greater weight % of water. It is clear from the examples of U.S. Patent 
the water uptake is below about 100 weight %. 
U.S. Pat. No. 4,448,935 issued May 15, 1984, assigned to National Starch 
and Chemical discloses a process for simultaneously crosslinking and 
sulphonating polystyrene. The patent contains a limitation that the 
process is conducted a low temperatures from 0.degree. to 25.degree. C. 
The process of the present invention operates a higher temperatures than 
those disclosed in the reference. The process of the present invention 
provides a controlled sulphonation process which may be carried out at 
elevated temperatures. Most importantly the process does not introduce a 
significant further amount of crosslinking into the crosslinked polymer. 
SUMMARY OF THE INVENTION 
Accordingly the present invention provides a process to produce a 
sulphonated crosslinked vinyl aromatic polymer having a water up take of 
not less than 150 times its weight (e.g. 15000 weight %) comprising: 
(a) swelling a crosslinked vinyl aromatic polymer in an organic solvent at 
temperature and pressure conditions so that said organic solvent remains a 
liquid; 
(b) optionally separating the swollen insoluble crosslinked vinyl aromatic 
polymer from said organic solvent; 
(c) contacting for a period of time from ten minutes to five hours at a 
temperature greater than 30.degree. C. said swollen crosslinked vinyl 
aromatic polymer in said organic solvent with a mixture comprising a 
trialkyl phosphate and sulfur trioxide in a molar ratio from 1:1 to 1:4 
the weight ratio of said sulfur trioxide to said crosslinked polymer being 
from 1:3 to 1: 0.5; and 
(d) separating the resulting sulphonated crosslinked vinyl aromatic polymer 
from the solution of organic solvent and mixture of said trialkyl 
phosphate and sulfur trioxide.

DETAILED DESCRIPTION 
Vinyl polymers useful in accordance with the present invention typically 
contain aromatic rings. Preferred polymers are styrenic in nature. The 
polymers may be prepared in accordance with the teachings of the Greek 
Patent 86.0636 by dissolving polystyrene in a polar organic solvent then 
reacting the resulting solution with a dialkylhalo aromatic compound in 
the presence of a Lewis Acid such as SbCl.sub.5, FeCl.sub.3 and 
ZnCl.sub.2, preferably, SbCl.sub.5. The details of the process are 
disclosed in the literature. e.g. "Crosslinking of Polystyrene by Mono- 
and Difunctional Agents", Nikolaos A. Peppas and George N. Valkanas, 
Makromolekulare Chemie 62 (1977) 163; and "Friedal - Crafts Crosslinking 
of Polystyrene" N. Grassie and J. Gilks, Journal of Polymer Science: 
Polymer Chemistry Edition, Vol. 11, 1531-1552. 
Polymers produced by such a process may be characterized as comprising 
polymers of C.sub.8-12 vinyl aromatic monomers and optionally minor 
amounts of non-aromatic copolymerizable vinyl monomers well known to those 
skilled in the art of styrenic copolymer technology which have been 
crosslinked with a dialkylhalo aromatic compound in the presence of a 
lewis acid. Typically the aromatic compound is a benzene compound. The 
alkyl groups are generally C.sub.1-4 alkyl radicals which are halogenated. 
Preferably the halogen atom is a chlorine atom. The aromatic compound may 
be further substituted by up to two C.sub.1-4 alkyl radicals. Useful 
dialkyl halo aromatic compounds include 1, 4-dichloromethyl benzene and 2, 
5-dimethyl-1, 4-dichloromethyl benzene. (also referred to as 2, 5 
bis(chloromethyl)-p-xylene). 
It is also possible to produce crosslinked vinyl aromatic polymers by the 
direct polymerization of a monomer mixture comprising essentially one or 
more mono-vinyl aromatic monomers and one or more poly- or di-vinyl 
aromatic monomers. The system which is most well known is the production 
of crosslinked polystyrene by polymerizing styrene and a small amount of 
divinyl benzene or its analogues such as diisopropenyl benzene etc., or 
other crosslinking agents such as ethylene glycol dimethacrylate (EGDM) or 
PEGDM etc. Typically the ratio of styrene to divinyl benzene will be from 
97:3 to 99.95:0.05 preferably from 98:2 to 99.95:0.05, most preferably 
from 99:1 to 99.95:0.05. 
Such polymers might be characterized as copolymers comprising from 95 to 
99.9 most preferably from 98 to 99 weight % of one or more C.sub.8-12 
vinyl aromatic monomers which are unsubstituted or substituted by a 
C.sub.1-4 alkyl radical or a chlorine atom and from 5, most preferably 
from 2 to 0.05 weight % of a divinyl aromatic monomer, or other known 
crosslinking agents. 
It is pointed out in Greek Patent 86.0636 that other aromatic containing 
polymers may be crosslinked using a Lewis Acid in the presence of a 
dialkylhalo aromatic reactant Suitable other polymers may include: 
(a) copolymers comprising from 10 to 40 weight % of a copolymer comprising 
40 to 60 weight % of one or more C.sub.8-12 vinyl aromatic monomers which 
are unsubstituted or substituted by a C.sub.1-4 alkyl radical or a 
chlorine atom and from 60 to 40 weight % of one or more C.sub.3-8 vinyl 
nitriles which have been grafted onto from 90 to 60 weight % of a 
homopolymer of one or more C.sub.4-6 conjugated diolefins; and 
(b) copolymers comprising from 40 to 60 weight % of one or more C.sub.8-12 
vinyl aromatic monomers which are unsubstituted or substituted by a 
C.sub.1-4 alkyl radical or a chlorine atom and from 60 to 40 weight % of 
one or more C.sub.4-6 conjugated diolefins. 
In addition to the polymers disclosed in the Greek Patent the following 
polymers may also be used in accordance with the present invention: 
(a) copolymers comprising from 90 to 60 weight % of one or more C.sub.8-12 
vinyl aromatic monomers which are unsubstituted or substituted by a 
C.sub.1-4 alkyl radical or a chlorine atom, from 10 to 40 weight % of one 
or more C.sub.1-4 alkyl acrylates or methacrylates, and from 0 to 10 
weight % of one or more C.sub.3-6 ethylenically unsaturated carboxylic 
acids and anhydrides of C.sub.4-8 ethylenically unsaturated carboxylic 
acids; and 
(b) graft copolymers comprising from 90 to 60 weight % of a copolymers 
comprising from 90 to 60 weight % of one or more C.sub.8-12 vinyl aromatic 
monomers which are unsubstituted or substituted by a C.sub.1-4 alkyl 
radical or a chlorine atom, from 10 to 40 weight % of one or more 
C.sub.1-4 alkyl acrylates or methacrylates, and from 0 to 10 weight % of 
one or more C.sub.3-6 ethylenically unsaturated carboxylic acids and 
anhydrides of C.sub.4-8 ethylenically unsaturated carboxylic acids grafted 
onto from 10 to 40 weight % of one or more polymers selected from the 
group consisting of homopolymers of C.sub.4-6 conjugated diolefins and 
homo- and co-polymers of C.sub.4-8 alkyl and hydroxy alkyl esters of 
C.sub.3-6 ethylenically unsaturated carboxylic acids which esters form 
homopolymers having a tg of less than -20.degree. C. 
Although the discussion of aromatic monomers has centered on styrene which 
is preferred, suitable vinyl aromatic monomers which are useful in 
accordance with the present invention include C.sub.8-12 vinyl aromatic 
monomers which are unsubstituted or substituted by a C.sub.1-4 alkyl 
radical or a chlorine atom. Such monomers include styrene, .alpha.-methyl 
styrene and chlorostyrene. 
Suitable C.sub.3-8 alkenyl nitriles include acrylonitrile and 
methacrylonitrile. 
Suitable C.sub.1-4 alkyl acrylates or methacrylates include methyl 
acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl 
methacrylate and butyl methacrylate. 
Suitable polymers of C.sub.4-6 conjugated diolefins include homopolymers of 
butadiene or copolymers of butadiene and isoprene. A particularly useful 
polymer of butadiene is the stereospecific polymer having at least 55 wt. 
% preferably more than 90 wt. % most preferably at least 95 wt. % of the 
polymer in the cis configuration. Thus a suitable polymer for use in 
accordance with the present invention is ABS. 
The polymers useful in accordance with the present invention may be a 
copolymer of a vinyl aromatic monomer as discussed above with a conjugated 
diolefin as discussed above. One such polymer is styrene butadiene rubber. 
The crosslinked polymer is one in which the crosslink density is such that 
the molecular weight of the polymer chain between crosslinks is from 
10,000 to 50,0000,preferably from 15,000 to 45,000. The method for 
determining the molecular weight of the polymer chain between crosslinks 
is discussed in the paper Physical Characterization of Suspension 
Crosslinked Polystyrene Particles and Their Sulphonated Products: 1 
Nonionic Networks, F. P. Regas and G. N. Valkanas; POLYMER, 1984, Vol 25, 
February 245, the text of which is hereby incorporated by reference. 
In the process according to the present invention the polymer is first 
swollen in a organic solvent at temperature and pressure conditions such 
that the organic solvent remains a liquid at the temperature of treatment. 
Typically the temperature is from room temperature up to about 70.degree. 
C. Preferred solvents are halogenated C.sub.1-8 alkanes. Particularly 
useful solvents include dichloroethane, dichloromethane and 
1,1,1-trichloroethane 
After the crosslinked polymer is swollen in the solvent the resulting 
swollen crosslinked vinyl aromatic polymer may optionally be separated 
from excess solvent. This treatment may be a simple procedure such as 
decanting the excess solvent or filtering the swollen crosslinked vinyl 
aromatic polymer from the solvent. However, other more sophisticated 
methods may be used to separate the swollen crosslinked vinyl aromatic 
polymer from the solvent such as centrifuging. 
The resulting swollen crosslinked vinyl aromatic polymer is then treated 
with a mixture comprising a trialkyl phosphate and sulfur trioxide in a 
molar ratio from 1:1 to 1:4. Preferably the molar ratio of trialkyl 
phosphate to sulfur trioxide is from 1:2 to 1:3. The weight ratio of 
sulphur trioxide to crosslinked vinyl aromatic polymer to the mix is from 
1:5 to 1:0.5 preferably about 1:1. 
Suitable trialkyl phosphates include tri-C.sub.1-4 alkyl phosphates. A 
particularly useful trialkyl phosphate is triethyl phosphate. 
Sulfur trioxide is a liquid at temperatures up to about 45.degree. C. and 
has a density of about 1.9. The sulfur trioxide may be used in the form of 
a solution dissolved in the organic solvent or an organic solvent soluble 
in or miscible with the organic solvent. The SO.sub.3 may be used as 100% 
(pure SO.sub.3) or obtained from an SO.sub.3 generator or donor such as 
oleum. If oleum is used as a generator or donor it is mixed with the 
solvent and the excess sulfuric acid is separated from the oleum solvent 
mixture prior to the formation of the SO.sub.3 -trialkyl phosphate mixture 
or complex (e.g. the trialkyl phosphate is added to the solution after 
separation of the H.sub.2 SO.sub.4). It is believed that the SO.sub.3 and 
trialkyl phosphate form a complex. 
The swollen crosslinked vinyl aromatic polymer is treated with the mixture 
or complex of trialkyl phosphate and sulfur trioxide for a period of time 
from ten minutes to about five hours, preferably from about thirty minutes 
to about two hours at a temperature from 30.degree. C. to 65.degree. C. 
preferably from about 30.degree. C. up to 55.degree. C. most preferably 
from 30.degree. C. to 45.degree. C. Typically the swollen crosslinked 
vinyl aromatic polymer is added to the trialkyl phosphate and SO.sub.3 or 
a solution of the trialkyl phosphate and SO.sub.3. As the polymer is 
sulphonated it precipitates from the reaction mixture. 
After the sulphonation the crosslinked polymer may be separated from the 
solution of organic solvent and trialkyl phosphate/sulfur trioxide mixture 
by relatively simple means such as filtering. From a commercial point of 
view the solvent is recycled to minimize operating costs. 
Normally the resulting sulphonated polymer is neutralized with a fixed 
alkali such as a hydroxide of a Group I or II metal, or an organic base 
such as ammonium hydroxide, ethanolamine, or other suitable organic bases 
in an alcololic medium generally comprising a C.sub.1-4 alcohol. The salts 
resulting from the neutralization of the sulphonated crosslinked vinyl 
aromatic polymer may be removed from the polymer with the following 
treatment with a C.sub.1-4 alkanol/water mixture. The process may comprise 
suspending and/or washing the neutralized sulphonated crosslinked polymer 
with a C.sub.1-4 alkanol. The C.sub.1-4 alkanol should be at least 50, 
preferably greater than 70 weight % as if too much water is present in the 
alkanol it will swell the polymer and cause problems with drying the 
sulphonated crosslinked vinyl aromatic polymer. Preferred alkanols include 
methanol, ethanol and propanol. 
The resulting sulphonated crosslinked vinyl aromatic polymer may then be 
dried using conventional means and conditions. The dried polymer may be in 
granular form or as a fine powder. 
In some specific applications the sulphonated crosslinked polymer may be 
washed and dried without prior neutralization as described above. 
The present invention will be illustrated by the following nonlimiting 
examples in which unless otherwise specified parts are parts by weight. 
EXAMPLE 1 
Crosslinked polystyrene was prepared by dissolving 24 g of polystyrene in 
125 g of dichloroethane containing 0.2 g of 1, 4-dimethyl-2, 
5-dichloromethyl benzene. This solution was added to a mixture of 180 cc 
of silicone oil and 120cc of dichloroethane at 70.degree. C. and 0.3 cc of 
antimony pentachloride catalyst was added to the resulting solution. The 
reactants were stirred and heated at 70.degree. C. for 5 hours. After that 
time the reaction mass was cooled and a gel of polystyrene was separated 
from the silicone oil and dichloroethane. This resulted in approximately 
125 g of gel. 
125 g of gel produced as described above was swollen in 300 cc of 
dichloroethane. After an hour of swelling a solution consisting of 39 g of 
triethyl phosphate and 25 cc of sulfur trioxide (specific gravity about 
1.97; molar ratio of 1:3) in 300 cc of dichloroethane was added to the 
swollen gel. The solutions were permitted to react at room temperature for 
one hour. At the end of this time the sulphonated polymer had precipitated 
from the dichloroethane in a non swollen state. The sulphonated polymer 
was suspended in methanol then neutralized with 50% aqueous KOH. 
The resulting polymer was a sulphonated polystyrene hydrogel having a high 
water uptake. (300 g distilled water per g of polymer or 30000 weight %). 
EXAMPLE 2 
A 165 g of a copolymer of 99.8 wt. % of styrene and 0.2 wt. % of 
divinylbenzene was prepared by suspension polymerization using four 
incremental charges of styrene (35.2 g) and divinylbenzene (0.9 g) at 
twenty minute intervals from the exotherm of the initial charge of 
polymer. The reaction was initiated using a mixed initiator system 
comprising 4.2g of 70% perbenzoyl peroxide and 1.4 g of t-butyl 
perbenzoate. The polymerization took 5 hours and produced 165 g of beads 
of crosslinked polystyrene at 98% conversion. 
The beads produced above were swollen in 2800 g of dichloroethane for two 
hours at 30.degree. C. To the swollen beads was added a solution 
comprising 205 g of triethylphosphate and 132 ml of sulfur trioxide (molar 
ratio of 1:3) in about 2400 g of dichloroethane. The reaction mixture was 
stirred for one hour at room temperature. The precipitate resulting from 
the reaction was filtered and neutralized in 1.3 l of methanol and 200 cc 
of 50% KOH. The resulting salt is filtered and dried to yield 390 g of 
product. The salt may be further washed in a 70/30 methanol water mixture. 
The washed product has a water absorption of 350-400 g per g of polymer. 
EXAMPLE 3 
Crosslinked polystyrene was prepared by dissolving 46 g of polystyrene in 
125 g of dichloroethane containing 0.12 g of 1, 4-dimethyl, 2, 
5-dichloromethyl benzene. This solution was added to a mixture 180 cc of 
silicone oil and 120 cc dichloroethane at 70.degree. C. and 1.8 cc 1 M 
antimony pentachloride in dichloromethane was added. The reactants were 
stirred and heated at 70.degree. C. for 5 hours. After that time the 
reaction mass was cooled and a gel of polystyrene was separated from the 
silicone oil and dichloroethane. Approximately 190 g of gel was obtained. 
85 g of gel produced as described above was swollen in 450 cc of 
dichloroethane. After an hour of selling a solution consisting of 24.6 g 
of triethyl phosphate and 16.5 cc of sulfur trioxide (SG 1.97; molar ratio 
1:3) in 150 cc of dichloroethane was added to the swollen gel. The 
solutions were permitted to react for 10 minutes at room temperature. The 
temperature was raised to 35.degree. C. and reaction allowed to proceed 
for 2 hours. At the end of this time the sulphonated polymer had 
precipitated from the dichloroethane in a non-swollen state. The 
sulphonated polymer was suspended in methanol then neutralized with 50% 
aqueous KOH. The resulting polymer was sulphonated polystyrene having a 
high water uptake. Completely purified polymer absorbs 800 g distilled 
water per g of polymer (e.g. 80000 weight % water uptake). 
EXAMPLE 4 
A styrene divinyl benzene copolymer containing 0.25% divinyl benzene was 
prepared according to the following recipe: 
To an aqueous phase that consisted of a solution of 3 g vinyl 523 
(polyvinyl alcohol) and 25 g NaCl in 2000 cc of H.sub.2 O was added with 
agitation (300 RPM) 950 g styrene containing 0.5 g divinyl benzene (55%); 
4 g 70% benzoyl peroxide and 2 g t-butylperbenzoate. The temperature was 
raised to 85.degree. C. and after 30 minutes at this temperature 5 cc 
additions from a feed consisting of 50 g styrene and 4 g divinylbenzene 
(55%) were made every 30 minutes for the first 5 hours. Then every 45 
minutes after 6 hours the temperature was raised to 115.degree. C. and 
held there for 1 hour. A total of 14 additions were made. The reaction 
mixture was cooled. The beads separated and dried. 
19.9 g of the above beads were allowed to swell for 2 hours in a solution 
of 13.2 g triethyl phosphate in 240 cc dichloromethane at 25.degree. C. 
After 2 hours, a solution of 8.7 cc sulfur trioxide in 120 cc 
dichloromethane was added with rapid agitation over several minutes. The 
reaction was stirred at 25.degree. C. for 3.5 hours. The sulphonated beads 
were separated from the solvent and suspended in 400 cc methanol and 
neutralized with 50% aqueous KOH. The salt was filtered and dried. A 
sample of the beads which were washed with water and dried had a water 
absorption in distilled water of 325 g per g polymer (e.g. 32500 weight % 
water uptake).