Conformations in partly ionized poly(methacrylic acid): 2. Unperturbed dimensions of syndiotactic chains

Abstract

A rotational isomeric state model for the half-ionized syndiotactic polymethacrylate chain is described. The coupling of adjacent carboxylate groups by acid-salt hydrogen bonds suggests a model having a structural unit of 4 skeletal bonds. Energy calculations suggest that the skeletal bonds 1 and 2 between the coupled carboxylate groups adopt displaced tt locations at ∼(−8°, −8°) and the carboxylate groups are rotated ∼30° from the plane which bisects the skeletal angle at the C α atom. The carboxylate rotations give rise to displacements of about 15° in the t and g states of bonds 3 and 4 although they do not affect the gg location which remains at (120°, 120°). Agreement between the computed characteristic ratio, C ∞, for the syndiotactic chain and the experimental value of ∼15 for the half-ionized ‘conventional chain’ is obtained if the energies of tg and gg states in bonds 3 and 4 exceed tt by 1.0–1.5 kcal mol −1. It is assumed that the unionized polyacid in aqueous solution is of similar conformational character to poly(methyl methacrylate) and has C ∞ = ∼8. The computed dimensions of copolymers of unionized and ionized units prescribe a sigmoidal form for C 8 vs. α over the range 0 < α < 0.5. It is suggested that hydrophobic interactions are not required to account for the conformational transformation.