Polymerization process for preparation of acrylamide homopolymers with redox catalyst

There is disclosed a process for the aqueous polymerization of acrylamide to produce polymers of high molecular weight utilizing mixtures of water and monomer, at least one of which can be contaminated with small amounts of polymerization inhibiting components. In the process of the invention, a minimum amount of a redox pair catalyst is utilized in order to obtain the desired high molecular weight polymer. The minimum amount of catalyst is automatically provided to the polymerization mixture by adding a first member of a redox pair to the polymerization mixture and intimately combining a second member of the redox pair catalyst with an organic polymer capable of forming a colloidal dispersion in an aqueous medium and adding this intimate mixture to the mixture of water, acrylamide monomer, and a first member of the redox pair catalyst. The process disclosed provides for the slow release of one member of the redox pair catalyst from said intimate mixture into an aqueous solution or emulsion polymerization medium. As polymerization proceeds, an additional amount of catalyst is made available by the slow recess of one member of the redox pair into the aqueous polymerization medium. The polymerization process can be initiated and maintained at ambient temperatures and pressures. Thus polymerization is effected without providing additional heat or pressure to the reaction mass. Molecular weights of about 1 million to 10 million can be obtained where monomer concentrations are held between about 10 to about 50 percent by weight of the polymerization mixture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the production of high molecular weight polymers 
of acrylamide by a process of aqueous polymerization. 
2. Description of the Prior Art 
In U.S. Pat. No. 3,573,263 there is disclosed the preparation of 
water-soluble, high molecular weight synthetic polymers from monomers such 
as acrylic acid and acrylamide by the use of a redox system in conjunction 
with the use of an azo compound free-radical source. As indicated therein, 
the use of a redox catalyst only, without the additional azo compound 
catalyst, results in an exotherm during the process which causes greater 
initiator activity and thus, results in a branched polymer having a low 
molecular weight. 
The problems involved in the preparation of high molecular weight polymers 
utilizing solution or emulsion polymerization of ethylenically unsaturated 
monomers are extensively discussed in U.S. Pat. No. 4,103,080, 
incorporated herein by reference. In prior art solution polymerization 
processes, the polymerization temperature of about 60.degree. to about 
70.degree. C. has been used which causes the polymerization initiator to 
decompose rapidly. This results in too many polymer chains starting to 
form simultaneously with the end effect that too many chains which are 
short are produced. The resulting polymers are inadequately high in 
molecular weight. This has led to the use of polymerization initiators 
such as amines and ammonia. With the use of ammonia, generally extensive 
pH controls must be utilized to avoid the loss of ammonia and an 
uncontrolled course of reaction. 
The process of U.S. Pat. No. 4,103,080 involves the addition of catalysts 
in at least three steps at intervals during the course of the 
polymerization in order to maintain the initiator concentration low enough 
so as to obtain the very high molecular weight polymers desired. In U.S. 
Pat. No. 4,042,772, the problem of obtaining acrylamide and 
acrylamide-acrylic acid polymers utilizing contaminated acrylamide is 
solved by the use of urea as an additive during the polymerization. Use of 
a water-in-oil emulsion polymerization process is described as 
particularly suitable for the production of said polymers utilizing a 
redox polymerization catalyst. 
Other processes describing the polymerization of ethylenically unsaturated 
monomers utilizing redox polymerization catalysts are U.S. Pat. No. 
3,509,113 and 4,020,256. There is no indication in the prior art that very 
high molecular weight polymers can be obtained by the use of a minimum of 
a redox catalyst sufficient to initiate polymerization, said minimum being 
determined by the slow release of at least one member of a redox pair into 
the polymerization mixture containing a second member of the redox pair 
catalyst. The slow release of one member of the redox pair being 
controlled by the rate at which the organic polymer, capable of forming an 
aqueous colloidal dispersion, is solubilized in said aqueous 
polymerization medium. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide to the art a process for the 
preparation of high molecular weight polymers by the aqueous 
polymerization, particularly the aqueous solution polymerization of 
acrylamide utilizing a redox polymerization catalyst system wherein 
polymerization is initiated at ambient temperatures and pressures 
utilizing a minimum amount of redox catalyst. In the process of the 
invention, a first member of a redox pair catalyst can be admixed with 
water and acrylamide monomer. The second member of the redox pair is 
separately intimately mixed with an organic polymer capable of forming a 
colloidal dispersion in an aqueous medium. Subsequently, the intimate 
mixture of polymer and second member of a redox pair is added to the 
mixture of water, monomer, and first member of the redox pair catalyst. As 
the organic polymer is solubilized, i.e., forms a colloidal dispersion, in 
the aqueous polymerization medium, the two members of the redox pair react 
in situ, free radicals are formed, and polymerization is initiated. 
Because the rate of solubilization of the organic polymer is generally 
less than instantaneous, and is usually accomplished at ambient 
temperatures and pressures over a period of about one minute to about two 
hours, the members of the redox pair do not combine instantaneously but do 
so over a period of time thus allowing a minimum amount of catalyst to be 
present for the initiation of polymerization and so as to provide 
controlled additional amounts of catalyst to be formed in situ as 
polymerization progresses.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The invention is concerned with the preparation of acrylamide homopolymers 
by an aqueous polymerization process utilizing redox catalysis in which a 
first member of a redox pair catalyst is slowly released into the aqueous 
medium and allowed to react with a second member of the said redox pair 
catalyst. The slow release is obtained by intimately combining one member 
of a redox pair catalyst with an organic polymer capable of forming a 
colloidal dispersion in an aqueous media. Acrylamide-acrylic acid 
copolymers can be formed from the acrylamide polymers prepared by the 
process of the invention by hydrolysis, as is well-known in the art. The 
direct preparation of high molecular weight copolymers of acrylic acid and 
acrylamide by the process of the invention would not be possible since 
acrylic acid would act as a chain stopper resulting in undesirably low 
molecular weight copolymers. 
Useful organic polymers capable of forming a colloidal dispersion in 
aqueous media include natural and synthetic polymers such as 
cellulose-derived polymers modified for water dispersibility, starch, 
modified starch, natural gums, and water-dispersible proteins. Examples of 
useful synthetic polymers include alkylene oxide polymers, polyvinyl 
alcohol, polyvinyl pyrrolidone, polyacrylics, polyacrylamides, 
polyethylene imine, starch ethers, carboxy methyl cellulose, hydroxy ethyl 
cellulose and hydroxy propyl cellulose. Useful natural polymers include 
natural gums, starch, dextrin and water-dispersible proteins. Examples of 
useful natural gums include gum arabic, guar gum, alginic acid, alginates, 
Locust bean gum, Locust bean kernal gum, gum Tragacanth, gum Karaya, 
Iceland moss, and various seed extracts such as flax seed, psillium seed 
and quince seed extracts. Examples of useful starches include corn, 
potato, and tapioca starch. Useful dextrins include those derived from 
said starches. Examples of water-dispersible proteins include gelatin, 
casein, soya bean protein, albumen, hide glue, bone glue, and fish glue. 
Generally about 1 percent to about 10 percent by weight, based upon the 
weight of the acrylamide monomer, are utilized of these organic polymers. 
Preferably about 1 percent to about 5 percent by weight and most 
preferably, about 2 percent to about 3 percent by weight are used, all 
based upon the weight of acrylamide monomer. 
Where the melting point of said organic polymer permits, the intimate blend 
with one member of the free-radical-generating redox pair catalyst is 
accomplished by melt-blending. Alternatively, said intimate mixture is 
made by forming an aqueous solution of said redox pair member with an 
aqueous colloidal dispersion of said organic polymer and subsequently 
evaporating the mixture to dryness. 
In the process of the invention, at least one free-radical generating 
initiator of the redox type is used. Preferably, these include, as one 
member of the redox pair, at least one organic or inorganic peroxy 
compound. As the second member of the redox pair catalyst, there is used a 
peroxy compound activator, or reducing agent. Useful peroxy compounds 
include ammonium persulfate, potassium persulfate, hydrogen peroxide, 
diisopropyl peroxydicarbonate, bis(2,4-dichlorobenzoyl)peroxide, caprylyl 
peroxide, lauroyl peroxide, acetyl peroxide, tert-butyl peroxyisobutyrate, 
benzoyl peroxide, bis(p-chlorobenzoyl)peroxide, hydroxyheptyl peroxide, 
cyclohexanone peroxide, di-tert-butyl peroxyphthalate, tert-butyl 
peroxyacetate, tert-butyl peroxybenzoate, dicumyl peroxide, methyl ethyl 
ketone peroxide, di-tert-butyl peroxide, p-methane hydroperoxide, pinane 
hydroperoxide, cumene hydroperoxide, and 2,5-dimethyl-2,5-dihydroperoxide. 
These peroxy compounds are used in accordance with the process of the 
invention preferably in conjunction with trace, or co-catalytic, amounts 
of a metal ion such as an iron (ferrous) salt, for instance, ferrous 
sulphate or iron pyrophosphate. Corresponding amounts of a peroxy compound 
activator, the second member of the redox pair, function as a reducing 
agent. The ferrous ion can be added in many forms, for instance, as 
colloidal dispersions of an iron pyrophosphate complex. Reducing sugars 
can be added to insure presence of the ferrous state and to act as 
chelating agents. The sodium formaldehyde sulfoxylate-iron-Versene complex 
is advantageous in that much less iron is required than is the case with 
the iron pyrophosphate system. Versene is a trademark for 
disodiumethylenediaminetetraacetate dihydrate, a chelating agent, which 
acts to keep the iron in a water-soluble state. Other metal ions can also 
be used as co-catalysts, particularly when complexed with salts of 
ethylene-diaminetetracetic acid or salicylic acid. Copper is preferred 
over the ferric ion since the cupric ion will not act as a chain 
terminator. 
Useful peroxy compound activators include sodium sulfite, sodium bisulfite, 
sodium metabisulfite, and combinations of sodium bisulfite with 
ethylenediaminetetraacetic acid. Typical useful redox pairs and coupling 
metal salts are ammonium persulfate and 3,3',3"-nitrilotrispropionamide; 
sodium bromate and sodium sulfate; sodium bromate, sodium persulfate, and 
sodium sulfite or sodium bisulfite; potassium persulfate and sodium 
metabisulfite; ammonium persulfate and sodium bisulfite; ammonium 
persulfate and sodium metabisulfite; and hydrogen peroxide and thiourea. 
In accordance with the process of the invention, the redox pair member 
activator can be added to the reaction vessel in combination with water 
and the acrylamide monomer. The proportion of said activator compound is 
generally about 0.05 to about 0.3 percent by weight based upon the weight 
of the monomer, preferably about 0.05 percent to about 0.2 percent by 
weight upon the weight of the monomer. Comparable proportions of peroxy 
compound can be used. 
In the process of the invention, water is utilized as a reaction medium. 
Preferably, the water is deionized but the process of the invention is 
sufficiently flexible to utilize water contaminated with small amounts of 
various ions which would normally interfere with polymerization. In 
addition, acrylamide monomer can be used in the process of the invention 
in the unpurified form. Thus, the impure acrylamide utilized by Ballweber 
et al, in the polymerization which is disclosed in U.S. Pat. No. 
4,042,772, can be utilized without the expedient of polymerization in the 
presence of urea. 
As is well known, the molecular weight of polymers produced by a process of 
polymerization such as contemplated herein varies directly with the 
monomer concentration used in making the polymer. If higher molecular 
weight polymers are desired, the monomer concentration should be about 10 
to about 50 percent by weight, preferably about 15 to 35 percent by 
weight, of the total polymerization reaction mixture. When monomer 
concentration rises above 50 percent by weight of the reaction mixture, 
the polymeric products formed generally contain a large amount of low 
molecular weight components. Generally, the polymers of the invention have 
a molecular weight within the range of about 1,000,000 to about 
10,000,000. 
By the process of the invention, acrylamide homopolymers can be prepared at 
ambient temperatures and pressures. Certain of the polymerization 
conditions required are those under which polymers have been prepared in 
the prior art. Thus, the aqueous solution of monomer containing one member 
of a redox pair as activator is purged with nitrogen or carbon dioxide or 
other inert gas in order to remove entrained oxygen which would interfere 
with the polymerization. In the process of the invention, the 
polymerization is initiated at ambient temperatures and at atmospheric 
pressure. No additional heat need be added to the polymerization mixture 
since, once the polymerization is initiated by the generation of 
sufficient free radicals, the heat released is sufficient for 
polymerization. Generally, the reaction temperature is not allowed to rise 
above a maximum of about 60.degree. C., preferably a maximum temperature 
of about 50.degree. C., and most preferably a maximum temperature of about 
40.degree. C. 
Molecular weight regulators, such as those disclosed in the prior art, are 
generally not included in the reaction mixture since very high molecular 
weights are desirable. Thus, compounds such as acetone, methanol, ethanol, 
isopropyl alcohol, polyethylene glycol, etc., are unnecessary in the 
process of the invention. The control of the pH of the reaction medium is 
also generally unnecessary. This is not to imply that the pH of the 
reaction medium is not important, since hydrolysis of amide groups can 
take place at high pH and imidization is favored at low pH and at high 
temperatures. A pH range of about 3 to about 8 is satisfactory, preferably 
about 3 to about 6. Generally the tap water available has a pH within this 
range and therefore is acceptable for use in the process of the invention. 
The polymerization reaction of the invention also can be carried out in the 
presence of a salt or a buffer system involving the use of one or more 
salts in combination. Such buffer systems can include an alkali metal or 
an ammonium acetate, carbonate, bicarbonate, chloride, phosphate, sulfate, 
bisulfate, or borate. Alkali metal and ammonium salts of other weak acids 
are also useful. The amount of buffer salt which can be used is about 0.1 
percent to about 2 percent by weight, preferably about 0.2 percent to 
about 1 percent by weight of the reaction mixture. The pH can be buffered 
in the range of about 3 to about 8, preferably about 5 to about 7. 
The process of the invention can be carried out with or without the 
addition of one or more other materials commonly added to the reaction 
mixtures of the prior art. For instance, a surfactant can be employed 
where it is necessary to reduce the chance that polymeric materials 
produced in the process will adhere to or build up on the walls of the 
reactor and other equipment used. The use of surfactants during 
polymerization to reduce deposits on the walls of the reactor is well 
known in this art. The amounts of surfactants utilized are also well known 
and are not critical to the process of the invention. 
Generally, the polymerization is conducted utilizing agitation during at 
least the initial stages of polymerization. As polymerization progresses, 
the viscosity can become so great that effective agitation of the mixture 
is no longer possible. In contrast to the acrylamide polymers produced in 
the prior art, the process of the invention results in high molecular 
weight polymer solutions without the presence of the insoluble particles 
found in aqueous acrylamide solutions prepared using dry powders obtained 
by drying an aqueous solution or emulsion of an acrylamide polymer. The 
viscosity of the polymerization mixture is directly proportional to the 
concentration of monomers utilized and the molecular weight obtained. 
Isolation of the polymers produced by the process of the invention can be 
accomplished, if desired, in conventional ways. 
The polymers of the invention are obtained in aqueous solution and are 
suitable for many uses simply upon dilution to a lower solids content. 
Thus, the polymer solutions of the invention are useful in the mining and 
process industries, the paper industry, oil well applications and drinking 
water and waste-water treatment. In the mining and process industries, 
solids must be separated from water in process streams in which they are 
suspended. The separation process can be one involving settling, 
filtration, centrifuging or combinations thereof. The objective of the 
separation being the recovery of a valuable, relatively water-free cake or 
filtrate or the production of solid or liquid products suitable for 
disposal. The addition of a high molecular weight polyacrylamide as a 
flocculant ties together the colloidal particles in the process stream, 
thus forming heavy agglomerates which settle out rapidly leaving a 
supernatant liquid free of solids. Similar applications for the polymers 
of acrylamide prepared by the process of the invention exist in the paper 
industry and in water and wastewater treatment. In oil well applications, 
high molecular weight polyacrylamides improve the efficiency of aqueous 
hydraulic fracturing and water-flood treatment of oil reservoirs. The use 
of dilute solutions of polyacrylamides in the hydraulic fracturing 
operation reduces the friction loss of the fracturing fluid and the 
horsepower requirement of the pumping units by reducing the extremely high 
pressure drops which develop during the hydraulic fracturing operation 
where polyacrylamides are not ulitized as additives. 
Because of the simplicity of the process of the invention and its relative 
freedom from the need to provide adjustments in pH and since the process 
does not require the use of deionized water or even purified monomer, 
i.e., acrylamide, the process of the invention is particularly suited for 
the preparation of high molecular weight polyacrylamide homopolymers at 
the site of an oil well. Production on site eliminates the necessity of 
isolating as a dry powder the polyacrylamide homopolymer. Shipping costs 
and the costs of preparing an aqueous solution of the polymer prior to use 
are thus eliminated. Similar advantages would result by the utilization of 
the process of the invention for the preparation of polyacrylamide 
homopolymers on site in paper mills and in water and waste-water treatment 
plants. 
Where it is desired to isolate the polymers produced by the process of the 
invention, it is desirable to utilize a solution-precipitation process of 
polymerization or alternatively, conduct the reaction in a so-called 
inverse emulsion in which the acrylamide polymer is present in a 
concentrated aqueous solution which is dispersed in an organic medium as 
small droplets. Generally, a surface active stabilizer is used to prevent 
coagulation of the emulsion. Both solution-precipitation polymerization 
and polymerization utilizing an inverse emulsion are procedures well known 
in the prior art. One skilled in the art would know how to utilize 
teachings herein to produce polymers in accordance with these prior art 
polymerization procedures. It is noted that, in the solution-precipitation 
procedure of polymerization, the monomer is soluble in the reaction medium 
but the resulting polymer is not. Thus, the medium never gets very viscous 
as is the case with the solution-polymerization process exemplified herein 
and the polymer is relatively easy to isolate and dry. Similarly, 
conducting the polymerization process by the inverse emulsion process does 
not result in a significant rise in viscosity of the medium. Thus, the 
polymer is relatively easy to isolate and dry as compared with a solution 
of the polymer. 
The following examples illustrate the process of the invention. Where not 
otherwise specified throughout the specification and claims, temperatures 
are given in degrees centigrade and parts, percentages and proportions are 
by weight. 
EXAMPLE 1 
A polymer of acrylamide was prepared by the process of the invention in 
accordance with the following procedure. At ambient temperature and 
pressure, a glass bottle containing 142.8 grams of water was charged with 
35.7 grams of acrylamide. The mixture was sparged with nitrogen and then 
0.035 gram of sodium metabisulfite was added followed by one drop of a 0.1 
percent by weight aqueous solution of ferrous sulfate. In a separate 
container, 0.18 gram of ammonium persulfate was melt-blended with 15 grams 
of polyethylene glycol having a molecular weight of about 1500. This 
mixture was then added to the water, acrylamide and sodium 
metabisulfite-ferrous sulfate mixture in the amount of 1.67 grams. 
Polymerization to the gel stage thereafter took place in 30 minutes and 
resulted in an exotherm to a temperature of 30.degree. C. Upon dilution of 
the mixture to a five percent by weight solids basis, a viscosity of 100 
centipoise was obtained as measured by a Brookfield viscometer LVT spindle 
No. 2 at 25.degree. C. and 60 rpm. 
EXAMPLES 2-11 
Example 1 is repeated utilizing the following polymers to replace the 
polyethylene glycol: partially hydrolyzed polyvinyl alcohol, polyacrylate, 
polyacrylamide, polyethylene imine, polyvinyl pyrrolidone, gum arabic, 
casein, gelatin, hydroxyethyl cellulose, and corn starch. Where 
melt-blending as a means of incorporating the ammonium persulfate with 
said polymers is not feasible, mixtures are made of aqueous solutions of 
the polymers. These mixtures are subsequently dried before use. A viscous 
gel is obtained. 
While this invention has been described with reference to certain specific 
embodiments, it will be recognized by those skilled in this art that many 
variations are possible without departing from the scope and spirit of the 
invention.