Production of polymaleic acid

A polymer or copolymer of maleic acid is produced by neutralizing (I) maleic acid in an aqueous medium with (III) an alkali metal hydroxide or ammonia at a molar ratio of (I) to (III) in the range of 100:0 to 100:90, polymerizing (I) with a polymerization initiator, adding (III) a further amount of the alkali metal hydroxide or ammonia at a molar ratio of (I) to (III) in the range of 100:100 to 100:130 in total through the previous addition and this addition and continuing the polymerization to the complete effect.

The present invention relates to a process for producing polymaleic acid 
and maleic acid copolymers. More particularly, the invention relates to a 
process for producing polymaleic acid and maleic acid copolymers 
effectively in an aqueous medium. The invention process provides a 
copolymer containing the maleic acid component at a high content such as 
50 mole percent or above. 
A polymaleic acid and a copolymer thereof with another monomer have been 
used as a dispersing agent and a scaling inhibitor. Polymaleic acid is 
produced in the prior art by using a peroxide such as benzoyl peroxide as 
a polymerization initiator in an organic solvent such as toluene or 
xylene. However, from an industrial viewpoint, an aqueous medium is far 
more desirable than the organic medium with respect to the ease of 
production, reduction of cost and the prevention of fire. However, studies 
of the polymerization of polymaleic acid salts in an aqueous solvent have 
scarcely been reported. Only copolymers of maleic acid with acrylic acid 
or vinyl acetate have been reported. 
It has been suggested in the literature that when the molar ratio of maleic 
acid to polymaleic acid or maleic acid copolymer used as the scaling 
inhibitor is reduced, the scaling-inhibiting activity is reduced due to a 
reduction of charge density. Thus, it is supposed that the polymaleic acid 
and maleic copolymer having a high maleic acid content have an excellent 
scaling inhibiting effect. Under these circumstances, it has been demanded 
to produce these polymers efficiently at a low cost. 
After intensive investigations of the production of polymaleic acid and 
maleic acid copolymers under these circumstances, the inventors have found 
that the polymaleic acid and maleic acid copolymers can be produced 
efficiently in an aqueous medium under specified conditions, though the 
production of them in an aqueous medium has been considered to be 
difficult. The present invention has been completed on the basis of this 
finding. 
According to the invention, a polymer of maleic acid is produced by the 
steps of: 
neutralizing a monomer of (I) maleic acid or anhydride thereof in an 
aqueous medium with (III) an alkali metal hydroxide or ammonia at a molar 
ratio of (I) to (III) in the range of 100:0 to 100:90, 
polymerizing the monomer with a polymerization initiator, 
adding (III) a further amount of the alkali metal hydroxide or ammonia at a 
molar ratio of (I) to (III) in the range of 100:100 to 100:130 in total 
through the previous addition and this addition and 
continuing the polymerization to the complete effect. 
When a monomer mixture of maleic acid and another monomer is used in the 
process, a copolymer of maleic acid is obtained. A suitable co-monomer 
(II) may be used with the monomer of maleic acid or anhydride thereof to 
obtain a copolymer. It may be added to the polymerization system before 
the first neutralization step, after the first neutralization step or 
after the second neutralization step. Moreover it is also practical that a 
part of the co-monomer is added before the first neutralization step and 
the other is added later. The co-monomer (II) may be used in an amount of 
up to 100 moles, preferably up to 50 moles, per 100 moles of the monomer 
of maleic acid or anhydride thereof (I). 
A co-monomer to be used in the invention process is preferably defined 
below. It includes as preferable embodiments an unsaturated alcohol such 
as (meth)allyl alcohol, (meth)allyl glyceryl ether and polyoxyethylene 
monoallylether, sodium unsaturated sulfonate such as sodium (meth)allyl 
sulfonate and sodium styrene sulfonate, (meth)acrylic amide and a hydroxy 
group-having (meth)acrylic ester such as hydroxyethyl (meth)acrylate. 
In other words, the invention relates to a process for producing polymaleic 
acid and its copolymers by homopolymerizing maleic acid or its anhydride 
(I) or copolymerizing it with another polymerizable monomer (II) in an 
aqueous medium to form maleic acid homopolymer or copolymer, characterized 
by polymerizing maleic acid or its anhydride (I) in the presence of a 
polymerization initiator in an aqueous solution after neutralization with 
an alkali metal hydroxide or ammonia (III) in a molar ratio of (I) to 
(III) of 100:0 to 100:90, then adding the alkali metal hydroxide or 
ammonia (III) to the polymerization system to control the molar ratio of 
(I) to (III) to 100:100 or 100:130 and further continuing the reaction to 
complete the polymerization reaction. 
Particularly preferred copolymerizable monomers (II) include compounds of 
the general formula: 
##STR1## 
wherein R.sub.1 represents hydrogen or a methyl group and A represents 
--CH.sub.2 --OH, 
##STR2## 
--CH.sub.2 --OR.sub.2 .sub.n OH (OR.sub.2 being an oxyalkylene group 
having 2 or 3 carbon atoms and n being an integer of 1 to 50), 
##STR3## 
(M being an alkali metal or ammonium), --CONH.sub.2 or --COOCH.sub.2 
CH.sub.2 OH. They are suitable for use in the process of the present 
invention owing to their solubility in water. They may be used either 
alone or in the form of a mixture of two or more of them. The molar ratio 
of (I) to (II) is preferably in the range of 100:0 to 100:100 from the 
viewpoint of the scaling inhibition capacity. Particularly when A in the 
formula of the copolymerizable monomer (II) is --CH.sub.2 --OR.sub.2 
.sub.n OH, the molar ratio of (I) to (II) in the range of 100:0 to 100:50 
is preferred. 
The process of the present invention will now be described in detail. An 
aqueous solution of an alkali metal hydroxide, such as sodium hydroxide or 
potassium hydroxide, or aqueous ammonia (III) is added to an aqueous 
solution or suspension of maleic acid or maleic anhydride (I) prior to the 
polymerization or the aqueous maleic acid solution is subjected to the 
polymerization directly without the addition of the alkaline solution in 
the first stage. The molar ratio of (I) to (III) is in the range of 100:0 
to 100:90, preferably 100:0 to 100:80. When the molar ratio of (I) to 
(III) in the initial stage is 100:100 to 100:110, the resulting polymer 
will have a molecular weight distribution broader than that of a polymer 
obtained by the process of the present invention as will be shown in the 
following examples and comparative examples. When the former polymer is 
used as, for example, a scaling inhibitor, its effect is inferior to that 
of the polymer obtained by the process of the present invention. 
In the production of the copolymer, the whole or part of the 
copolymerizable monomer (II) may be added to said aqueous solution in the 
initial stage. Thereafter, a polymerization initiator and, if necessary, 
the copolymerizable monomer (II) are added dropwise to the aqueous 
solution so as to carry out the polymerization reaction in the first 
stage. After the polymerization reaction in the first stage, the 
polymerization initiator is added dropwise to the polymerization system 
after the alkali metal hydroxide or ammonia (III) is added at once or 
while the latter is added dropwise thereto so as to complete the 
polymerization. In the course of the supplementation of the alkali metal 
hydroxide or ammonia, the copolymerizable monomer (II) may be added to the 
polymerization system. In the process of the present invention, the final 
molar ratio of (I) to (III) should be regulated in the range of 100:100 to 
100 to 130. Outside this range, no high rate of polymerization can be 
obtained. 
According to the process of the present invention as described above, the 
polymaleic acid and its copolymer having a narrow molecular weight 
distribution can be obtained with a high rate of polymerization. The 
polymaleic acid and its copolymer obtained by the process of the invention 
are useful as scaling inhibitors for boiler or desalination facilities, 
detergent builders, dispersants for inorganic pigments, etc. 
The reaction temperature in the production of the polymer is 60.degree. to 
160.degree. C. From the viewpoint of the reaction efficiency, the reaction 
temperature is preferably 80.degree. to 140.degree. C. The practical 
monomer concentration in the polymerization step is in the range of 30 to 
85 wt. %. The reaction time is 2 to 12 h. 
The polymerization initiators include water-soluble initiators such as 
persulfates, hydrogen peroxide, t-butyl peroxide and water-soluble azobis 
compounds, among which the persulfates, hydrogen peroxide and t-butyl 
peroxide are preferred. They are used in an amount of 0.5 to 200 mol for 
100 mol of the monomer. 
Though the most preferred polymerization solvent is water, it is preferred 
from the viewpoints of the solubility of the monomer and economy to use a 
solvent having a water content of at least 60 wt. %, preferably at least 
90 wt. %. As the solvents other than water, alcohols such as ethanol and 
isopropanol and ketones such as methyl ethyl ketone and methyl isobutyl 
ketone may be used so far as the effects of the present invention are not 
damaged. 
The polymer obtained by the process of the present invention has a 
molecular weight of 200 to 20,000, preferably 300 to 5,000. The polymer 
has a molecular weight distribution narrower than that of polymaleic acid 
or maleic acid copolymer produced in the prior art. Therefore, when the 
polymer is used as the scale inhibitor in boiler or desalination 
facilities, or as a detergent builder or dispersant for inorganic 
pigments, it exhibits a more excellent performance. According to the 
process of the present invention, polymaleic acid and its copolymer may be 
produced efficiently in an aqueous solvent at a low cost.

The following examples will further illustrate the present invention, which 
by no means limit the invention. 
EXAMPLE 1 
196 g (2.0 mol) of maleic anhydride and 75 g of water were placed in a 
flask fitted with a thermometer, stirrer, dropping funnel, nitrogen inlet 
tube and reflux condenser, and heated to 110.degree. C. under stirring. 
The heating was stopped and then 68 g of a 60% aqueous hydrogen peroxide 
solution was added dropwise thereto over 2 h. In the course of the 
addition, the temperature of the reaction system was elevated to 
128.degree. C. After completion of the addition, the mixture was aged 
under reflux for 30 min. 167 g (2 mol) of 48% sodium hydroxide was added 
dropwise thereto over 2 h and simultaneously 35 g of a 30% aqueous sodium 
persulfate solution and 50 g of a 60% aqueous hydrogen peroxide solution 
were added dropwise thereto over 3 h. After completion of the addition, 
the mixture was aged for 1 h and then cooled to a temperature of below 
40.degree. C. to obtain an aqueous polymaleic acid solution having a solid 
content of 48% and pH of 4.91 (Experiment No. 1). For comparison, the same 
experiment as above was repeated except that no sodium hydroxide was added 
to the polymerization system (Experiment No. 2). Further, 167 g of 48% 
sodium hydroxide was added to a mixture of 196 g of maleic anhydride and 
100 g of ion-exchanged water and then 118 g of a 60% aqueous hydrogen 
peroxide solution and 35 g of 30% sodium persulfate were added dropwise 
thereto under reflux over 6 h to carry out the polymerization (Experiment 
No. 3). The rates of polymerization and average molecular weights of the 
polymerization products were measured to obtain the results shown in Table 
1. The rate of polymerization shown in Table 1 was determined from the 
remaining monomer determined by the bromine/bromide method. The average 
molecular weight of the copolymer was determined according to gel 
permeation chromatography. More particularly, the sodium salt of the 
polymer was diluted to 500 mg/100 ml (solid) with deionized water, 100 
.mu.l of the diluted salt was subjected to the gel permeation 
chromatography in a 0.15 M phosphate buffer solution (pH 7.0) at room 
temperature using an aqueous column G 4000 PW/G 2000 SW (a product of Toyo 
Soda Co.) to determine the number-average molecular weight. The light 
absorption was measured at 215 nm using a spectrophotometric detector 
SPD-1 (a product of Shimadzu Seisakusho). The above-mentioned methods were 
employed also in the following examples. In comparative examples, the 
molecular weight was not determined when the rate of polymerization was 
low. 
TABLE 1 
__________________________________________________________________________ 
Rate of 
Experi- 
Degree of neutralization of maleic acid 
polymer- 
Number average 
ment No. 
(molar ratio of MAN/NaOH) 
ization 
molecular weight 
(Note) 
Initial Final (%) (M.sub.N) 
M.sub.W /M.sub.N 
__________________________________________________________________________ 
1 0 1.0 93.1 390 1.32 
2 0 0 60.7 -- -- 
3 1.0 1.0 95.1 510 3.56 
__________________________________________________________________________ 
M.sub.W : weightaverage molecular weight determined from GPC, 
M.sub.N : numberaverage molecular weight determined from GPC. 
(Note) 
Experiment No. 1: example of the present invention 
Experiments Nos. 2 and 3: comparative examples 
EXAMPLE 2 
196 g (20 mol) of maleic anhydride and 60 g of ion-exchanged water where 
placed in a four-necked flask fitted with the same devices as in Example 
1. 117 g (1.4 mil) of 48% sodium hydroxide was added thereto under 
stirring to neutralize the same. 20.4 g (0.2 mol) of polyoxyethylene 
monoallyl ether (hereinafter referred to as "POA") having one ethylene 
oxide unit per molecule was added to the mixture and the temperature was 
elevated to 110.degree. C. 68 g of a 60% aqueous hydrogen peroxide 
solution and 20.4 g (0.2 mol) of POA were added dropwise thereto over 2 h. 
The maximum temperature of the reaction mixture reached 118.degree. C. 
After completion of the addition, the product was aged under reflux for 1 
h. 47 g of 30% sodium persulfate and 57 g of a 60% aqueous hydrogen 
peroxide solution were added dropwise thereto over 3 h. Simultaneously, 50 
g (0.6 mol) of 48% NaOH and 20.4 g (0.2 mol) of POA were added dropwise 
thereto over 2 h. After completion of the addition, the product was aged 
under reflux for 2 h. The mixture was cooled to below 40.degree. C. to 
obtain an aqueous copolymer solution having a solid content of 50% and pH 
of 4.86 (Experiment No. 4). For comparison, the same experiment as above 
was repeated except that the supplementation of 50 g of 48% NaOH to the 
polymerization system was omitted (Experiment No. 5). Further, 167 g of 
48% sodium hydroxide was added to a mixture of 196 g of maleic anhydride 
and 100 g of ion-exchanged water, to which was then added 30.6 g of POA. 
Additional 30.6 g of POA was added thereto dropwise over 4 h and 
simultaneously, 47 g of 30% sodium persulfate and 125 g of a 60% aqueous 
hydrogen peroxide solution were added dropwise thereto over h to obtain a 
polymer (Experiment No. 6). The test results of the polymerization 
products are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Rate of 
Experi- 
Degree of neutralization of maleic acid 
polymer- 
Number average 
Molar 
ment No. 
(molar ratio of MAN/NaOH) 
ization 
molecular weight 
rate of 
(Note) 
Initial Final (%) (M.sub.W) 
M.sub.W /M.sub.N 
MAN/POA 
__________________________________________________________________________ 
4 0.7 1.0 94.1 490 1.39 100/30 
5 0.7 0 80.1 -- -- " 
6 1.0 1.0 96.8 720 3.96 " 
__________________________________________________________________________ 
(Note) 
Experiment No. 4: example of the present invention 
Experiments Nos. 5 and 6: comparative examples 
EXAMPLE 3 
196 g (2 mol) of maleic anhydride and 150 g of ion-exchanged water were 
placed in a four-necked flask fitted with the same devices as in Example 
1. 117 g (1.4 mol) of 48% sodium hydroxide was added thereto under 
stirring to neutralize the same. 94.8 g (0.6 mol) of sodium 
methallylsulfonate was added thereto and the temperature was elevated to a 
reflux temperature. Thereafter, 68 g of a 60% hydrogen peroxide solution 
was added dropwise thereto over 2 h. After aging for 1 h, 47 g of 30% 
sodium persulfate and 57 g of a 60% aqueous hydrogen peroxide solution 
were added dropwise thereto over 3 h. Simultaneously 50 g (0.6 mol) of 48% 
NaOH was added dropwise over 2 h. After aging under reflux for 2 h, the 
reaction mixture was cooled to obtain a copolymer of maleic acid and 
methallylsulfonic acid (molar ratio of 100/30). 
The resulting copolymer had a rate of polymerization of 92.8%, a 
number-average molecular weight of 420 and M.sub.W /M.sub.N of 1.26.