Conformations in partly ionized poly(methacrylic acid): 1. Ionic hydrogen bonds in syndiotactic chains

Abstract

Comparisons of u.v. spectrophotometric titrations of some dicarboxylic acids, ‘conventional’ poly(methacrylic acid) and poly(acrylic acid) suggest that ionic or acid-salt hydrogen bonds form between adjacent carboxyl groups in racemic dyads in partly ionized forms of poly(methacrylic acid). The ionic hydrogen bonds form in tt conformations of the dyads. The maximum concentration of ionic hydrogen bonds occurs at 50% ionization in pure syndiotactic chains in the presence of electrolyte to give a chain having four-bond repeating units. Bonds 1 and 2 are presumed to be fixed in trans-trans conformations while bonds 3 and 4 retain some conformational freedom although this is somewhat reduced as a consequence of side group rotations necessary for hydrogen bonding. While ionic hydrogen bonds may form across meso dyads in tḡ conformations, isotactic chains incorporating ionic hydrogen bonds are predicted to be less stable than the corresponding syndiotactic chains.