Chapter 7
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Copolymers of Acrylamide and a Novel Sulfobetaine Amphoteric Monomer Luis C. Salazar and Charles L. McCormick Department of Polymer Science, The University of Southern Mississippi, Hattiesburg, M S 39406-0076 The free radical copolymerization of acrylamide (AM) with 3-(2acrylamido-2-methylpropyldimethylammonio)-1-propanesulfonate (AMPDAPS) has been studied i n the range from 25 to 90% A M in the feed. The value of r r has been determined to be 0.60 for the A M - A M P D A P S pair. The copolymer compositions have been determined from elemental analysis and C N M R . Molecular weights were determined using L A L L S and were found to vary from 1.7 to 10.2 x 10 g/mol. The copolymer microstructures, including run numbers and sequence distributions, were calculated from the reactivity ratios. The solution properties of the A M - A M P D A P S copolymers, as well as the A M P D A P S homopolymer, have been studied as a function of composition, p H , and added electrolytes. The solutions of these polymers show increased intrinsic viscosities i n the presence of sodium chloride and/or calcium chloride. The solution behavior of the homopolymer of A M P D A P S is independent of pH. The observed properties are consistent with the charge density of the polymers and the sulfobetaine structure of the A M P D A P S monomer. 1
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Polyampholytic polymers have been under investigation i n our laboratories for the past several years (1,2). These polymers are polyions which have both positive and negative charges bound along the backbone (3). The solution properties of these polymers are of interest because their viscosities increase with the addition of electrolytes (4-7). This behavior has become known as the "antipolyelectrolyte" effect. Polymers which display this type of behavior have promise as absorbent materials and viscosifiers i n the presence of electrolytes. The first polyampholytes studied i n our laboratories consisted of copolymers made from cationic and anionic acrylamido monomers (8). These monomers were copolymerized i n varying feed ratios to obtain copolymers of different compositions. Antipolyelectrolyte behavior was observed when the copolymers contained an equivalent amount of each monomer (9). We next studied polyampholytes with low-to-medium charge density (McCormick, C. L . and Johnson, C. B., Polymer, i n press). The charge density could be varied by forming cationic-anionic monomer pairs and then polymerizing them with varying amounts of acrylamide (AM). These terpolymers exhibited
0097-6156/91/0467-0119$06.00/0 © 1991 American Chemical Society
In Water-Soluble Polymers; Shalaby, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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WATER-SOLUBLE POLYMERS
antipolyelectrolyte behavior i n solution when the ratio of each oppositely charged monomer was approximately one-to-one (McCormick, C. L . and Johnson, C. B., J . Macromol Sci. Chem.. i n press). In all cases it was important that the net charge of the polymer be zero regardless of the charge density. The antipolyelectrolyte effect was compromised i f a charge imbalance existed. If aie polymers contained an excess of either charged monomer, the solution behavior reflected that of a typical polyelectrolyte. The importance of having an equivalent amount of each cationic and anionic moiety i n the polymer led us to our work with the amphoteric monomer 3-(2-acrylamido-2-methylpn)pyldimethylammonio)-lpropanesulfonate (AMPDAPS). The use of an amphoteric monomer insures that a one-to-one ratio of positive to negative charges is maintained (10.11). A monomer with both charges can be incorporated i n varying amounts into a copolymer to produce a series of polymers i n which the charge density of polymer can be varied from low to high while the net charge remains zero. Other polyampholytes i n which different monomers provide the negative and positive charges do not guarantee this. A M P D A P S has the added advantage of having charged groups which are stable to hydrolysis. The positive charge is provided by an ammonium quaternized by alkyl groups and the negative charge is provided by a sulfonate group which is difficult to protonate. This chapter reports the synthesis and characterization of a series of copolymers of acrylamide and the new amphoteric monomer A M P D A P S . The methods used to synthesize these polymers and their dilute solution behavior will be discussed. Experimental Details Monomer Synthesis. 3-(2-Acrylamidc-2-methylpropyldimethylammonio)-lpropanesulfonate (AMPDAPS) was synthesized by the ring opening reaction of 1,3-cyclopropanesultone (PS) with 2-aciylamido-2-methylpropanedimethylamine (AMPDA), Figure 1. 1,3-Cyclopropane-sultone was obtained from the Aldrich Chemical Co. and was used without further purification. The synthesis A M P D A has been previously reported (12). In a typical monomer synthesis, 0.144 mol A M P D A and 0.156 mol of PS were reacted i n 500ml propylene carbonate (PGC) under N at 55°C for 4 days. During this period the product formed as a white precipitate. This was then filtered and washed with diethyl ether until all the propylene carbonate was removed. Acrylamide (AM) from Aldrich Chemical Co. was recrystallized twice from acetone and vacuum-dried at room temperature. Potassium persulfate from J.T. Baker Co. was recrystallized twice from deionized water. 2
Copolymer Synthesis. The homopolymer of A M P D A P S and the copolymers of A M P D A P S with A M , the D A P S A M series, were synthesized free radically i n a 0.512-M NaCI aqueous solution under nitrogen at 30°C using 0.1 mol % potassium persulfate as the initiator. The monomer concentration was 0.45 M with a p H of 7.0 ± 0.1. A low conversion sample was analyzed to allow reactivity ratio studies. The reaction was terminated at
