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.