Water-soluble free radical addition polymerizations: polyacrylamides: acrylamide conform high-molecular-weight polymers and react with a variety of co-monomers.
In 2009, an estimated four billion metric tons of natural, semisynthetic, and synthetic water-soluble polymers were consumed globally for use in the production of food, clean water, and energy; and for personal care, pharmaceutical, and industrial applications. (1) Synthetic water-soluble polymers synthesized by free-radical addition polymerization methods account for half of this volume. They include polyacrylamides, polyacrylates, polydiallyl-dimethylammonium chloride (polyDADMAC), polyvinyl alcohol, and polyvinylpyrrolidones. Depending on the target applications, these polymers can have different molecular weights, charge, and architecture.
Several reviews that cover the synthesis, characterization, and application of polyacrylamides are available. (6-9) Acrylamide has the unique ability to form high-molecular weight polymers (> 1 million Da), to react with a variety of co-monomers to produce cationic, anionic, nonionic, amphoteric, or zwitterionic polymers, and to produce specialized polymers available through derivatization. Monomers commonly used in the synthesis of water-soluble polymers of commercial interest are shown in Figure 1.
Polyacrylamides of high commercial interest are copolymers of acrylamide (1, In Figure 1) with sodium or ammonium salts of acrylic acid (4) or 2-acrylamldo tert-butylsulfonic acid (ATBS) (5) to produce anionic polymers or with acryloyloxyethyltrimethylammonlum chloride (AETAC, 6), to produce cationic polymers. Although not produced in large volume, copolymers of acrylamide and DADMAC (10) are used to produce cationic polymers for use In papermaking applications.
Polyacrylamides with a variety of charge densities (obtained by varying monomer ratios), molecular weights, and architecture (linear, branched, and structured) are available commercially in dry and liquid form. Branched and structured polyacrylamides are obtained by including multifunctional monomers In the polymerization mix or through cross-linking following polymerization. (10,11)
Polyacrylamides are produced commercially in an aqueous environment by free radical polymerization, which follows the classical vinyl polymerization model with initiation, propagation, and termination processes. In this model, the propagation/termlnatlon ratio [([k.sub.p]/[k.sub.t]).sup.1/2] and chain transfer to monomer, polymer, Initiator, and other small molecules Impact the molecular weight of the formed polymer. High molecular-weight polyacrylamides are possible because of the high [([k.sub.p]/[k.sub.t]).sup.1/2] ratio for acrylamide and the low chain-transfer activity to monomer and polymer in an aqueous environment. Polymer molecular weights can be lowered and customized through the use of chain-transfer agents. Initiation with water and oil-soluble azo compounds, redox couples, peroxides, or photochemical initiators is common. (6,12,14)
Common cationic polyacrylamides produced industrially Include copolymers of acrylamide (1) with AETAC (6), MAETAC (7), MAPTAC (9), or DADMAC (10) (Figures 7 and 2). Among them, acrylamide/AETAC copolymers are the most common due to the relatively low AETAC monomer cost and similar reactivity with acrylamide ([r.sub.1] (AM) = 0.61, [r.sub.2](AETAC) = 0.47) producing copolymers with a roughly uniform sequence distribution of co-monomers.
In general, cationic methacrylates and methacrylamides are more reactive than their acrylate and acrylamide counterparts. In batch copolymerlzations of acrylamide with MAETAC ([r.sub.1](AM) = 0.25, [r.sub.2](MAETAC) = 1.71) or MAPTAC ([r.sub.1] (AM) = 0.57, [r.sub.2](MAPTAC) = 1.13), the more reactive MAETAC and MAPTAC monomers react faster with their own monomers than with acrylamide. This leads to compositional drift and often poor performance. (6)
There are more significant differences in the reactivity ratios of AM and DADMAC (([r.sub.1](AM) = 6.4, [r.sub.2](DADMAC) = 0.06). (14) DADMAC (10) Is the least-expensive cationic monomer. The molecular weight of DADMAC-containing polymers is limited due to the high chain transfer activity from the DADMAC allylic moiety. The low molecular weight, compositional heterogeneity, and branching have limited the use of acrylamide/DADMAC copolymers to predomlnantlyjust a few papermaking applications (coated broke, pitch and stickles control).
Semibatch strategies have been employed where the more reactive monomer is withheld from the initial reaction mixture and fed into the reactor during polymerization to obtain copolymers with similar co-monomer sequence distributions. (15,16) Extensive reactivity ratio data of acrylamide with cationic and anionic monomers under various conditions are summarized elsewhere. (6,7,14)
Hydrolytic stability and pH have to be considered when selecting a cationic monomer. Cationic acrylate esters are susceptible to base hydrolysis above pH 6, resulting in the loss of cationic charge on the polymer. The rate of hydrolysis is concentration- and temperature-dependent. In contrast, cationic acrylamide copolymers containing amide monomers such as APTAC and MAPTAC are reasonably stable up to a pH of 9-10. Acrylamlde/MAPTAC copolymers and acrylamide/MAPTAC/acrylate terpolymers can be found as conditioners and deposit aids in hair-care formulations.
Common commercial anionic polyacrylamides include copolymers of acrylamide with sodium and ammonium salts of acrylic acid (4) and ATBS (5) (in Figure 2). Criteria for polymer selection include cost, molecular weight, polymer solubility, application pH, and salt tolerance. Acrylic acid is a rather inexpensive monomer and yields very high molecular weight acrylamide/acrylate copolymers. Sodium or ammonium acrylates are typically formed prior to polymerization by reaction of acrylic acid with sodium hydroxide or ammonia.
Acrylamide/acrylate copolymers have poor solubility at low pH (acrylic acid pKa = 4.3) and have poor salt tolerance. The high molecular weights available for anionic flocculants render them useful in mining and papermaking applications. In contrast, acrylamide/Na-ATBS copolymers have good solubility, maintain their charge at low pH (pKa ATBS = 1.7), and have a high tolerance for salt (including many divalent cations). These features render them useful for oil field applications and as flocculants in phosphate production. Reactivity ratios of acrylamide with acrylic acid salts vary with pH. At pH 4 [r.sub.1](AM) = 0.57, [r.sub.2](acrylic acid) = 0.32, and at pH 8 [r.sub.1](AM) = 0.12, [r.sub.2](acrylic acid) = 0.63. It was reported that a random copolymer is formed at a pH of about 5. (7)
A variety of commercially useful polyacrylamides obtained via the derivatization of polyacrylamide are reported. These include the formation of anionic polyacrylamide through hydrolysis, (17) the synthesis of sulfomethylated derivatives from the reaction of polyacrylamide with formaldehyde and sodium bisulfate, (18) the formation of aminomethylated polyacrylamide from the reaction of polyacrylamide with formaldehyde and dimethylamine (Mannich reaction), (19) and the generation of various derivatives by transamidation reactions including hydroxamated polyacrylamides used as a flocculant in the Bayer process. (20,21)
Fluorescent polyacrylamide or polyacrylate derivatives, useful for monitoring polymer concentration in industrial applications, are obtained by the incorporation of a small amount of fluorescent monomer into the polymer backbone. (22)
Polyacrylamide Manufacturing Process
Liquid polyacrylamides are available as solutions, inverse (water-in-oil) emulsions, or dispersions. Dry polyacrylamides are available as powders from a dried gel or as beads from a water-in-oil suspension process. Polyacrylamides are formed from the radical chain polymerization of acrylamide with cationic or anionic monomers, which is a highly exothermic reaction (Figure 2). (7)
Reactions may be run isothermally or adiabatically. In an isothermal process, the rate of heat generation can be controlled by adjusting the initiator concentration, reaction temperature, or initiator feed rate. If the heat generation exceeds the rate at which heat can be removed from the reactor, a runaway reaction will result, which could be potentially dangerous in large-scale reactions. For an adiabatic process, temperature rise can be calculated for given monomer concentrations and the final temperature estimated based on the initial temperature. Formula adjustments can be made to avoid exceeding the pressure rating of the reaction equipment. (6)
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Note: For information about Monitoring Polymerization Reactions: From Fundamentals to Applications, go to www.wiley.comA/VileyCDA/.
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|Author:||Whipple, Wesley L.; Zheng, Hua|
|Date:||Sep 1, 2014|
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