Composition Control Through pH and Ionic Strength During Acrylic Acid/Acrylamide Copolymerization Ahmet Paril, Ahmet Giz, Huceste Catalgil-Giz ITU Fen-Edebiyat Fakultesi, 34469 Maslak, Istanbul, Turkiye Correspondence to: H. Catalgil-Giz (E-mail: catalgil@itu.edu.tr) ABSTRACT: The possibility of controlling the composition of acrylic acid/acrylamide copolymers by controlling the pH and the ionic strength of the reaction medium is investigated. The reactivity ratios of charged monomers depend on the pH of the medium, acrylic acid is the more reactive monomer below pH 3 and acrylamide above pH 4. The working pH was set at 3.6, a candidate for the crossover point, where no composition drift is expected. Copolymerization kinetics is investigated at this pH at various ionic strengths and a reaction without composition drift up to 80% conversion was achieved. All the chains produced in this reaction con- tain 30% 6 3% acrylic acid. Copolymer conversions, molecular weights, and composition distributions were measured through Auto- matic Continuous Online Monitoring of Polymerization (ACOMP) system. The copolymerization data were analyzed by a recent error in variables method (EVM) and reactivity ratios are calculated. The results show that in salt free conditions, the reactivity ratios depend on the ionic monomer concentration (ionic strength) in addition to the pH of the reaction medium. The effect of polyelec- trolytic interactions on the reactivity ratios and the resulting composition drift during the reaction, sequence length distribution, and Stockmayer bivariate distribution are discussed in detail. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: polyelectrolyte; copolymerization; kinetics (polym); reactivity ratio; water-soluble polymer; bivariate distribution Received 30 March 2011; accepted 7 March 2012; published online 00 Month 2012 DOI: 10.1002/app.37644 INTRODUCTION Differences in the reactivity ratios result in composition drift during copolymerization reactions. Early production material is richer in the more active monomer than the material produced late in the reaction. When this effect is severe, composition and properties of polymers produced at the beginning of the reac- tion are very different from those produced late in the reaction. At the end of the reaction homopolymer chains of the less active monomer can be produced. 1 Microstructure properties are especially important in the coat- ings and adhesives industry. 2,3 When copolymers with specific properties are desired, product homogeneity is important and composition drift has to be eliminated. Effect of composition drift on the strength of polymer phase boundaries was investi- gated by Kramer. 4,5 In research, composition drift is usually avoided by working at a low conversion (less than 5–10%) but this measure is out of question for industry. Composition control can be achieved by controlling the addition rate in emulsion polymerization. 6,7 Var- ious on-line and off line methods and algorithms have been developed for this purpose. 7,8 Another method is to arrange physical characteristics to compensate for the reactivity ratio differences. For example, using a solvent where the more active monomer is less soluble than the less active one is used to obtain homogenous products. 9 In polyelectrolyte systems the reactivity ratios may differ widely, the composition drift can be more severe and its undesirable effects more significant. In these systems, the reactivity ratios depend on the pH of the medium and composition control can be achieved by controlling the medium characteristics. The acrylic acid (Aac)/acrylamide (Aam) system has many fields of applications and new and exciting applications are continu- ally introduced. 10–13 For this reason, this system has been exten- sively studied. 10–23 The considerable scatter of the published reactivity ratio results is probably due to the effect of various experimental conditions. In the comprehensive study by Wan- drey et al., 22,23 the monomer reactivity ratios were found to be closely correlated with the pH of the medium and r Aac was seen to decrease and r Aam to increase monotonically as pH increased from 1.8 to 12, sequential sampling and the basic method of Kelen and Tu ¨d€ os method were used. V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.37644 1