Potentiometric titration of syndiotactic methacrylic acid/dimethacryloylamine copolymers

Abstract

AbstractThe potentiometric titration of syndiotactic methacrylic acid/dimethacryloylamine (MMA‐MAI) copolymers is investigated. In contrast to copolymers studied before, MAA‐MAI copolymers possess a highly syndiotactic backbone structure, a compositional statistics known from NMR‐measurements, and a cyclic structure as a part of the chain. The statistics of the MAA‐units and MAI‐rings is nearly random with respect to the formation of imide rings, the rings introducing a trans and a gauche conformation for each ring. Plots of the apparent pK (pKapp) versus the degree of ionization α, or versus log [(1‐ α)/ α], indicate a change from a relatively dense coil (a‐state) to an extended coil (b‐state) when α is increased. The decisive parameter for pKapp in the b‐state and for the conformational transition of the a‐state to the b‐state is the charge density on the chain β. The transition takes place for all copolymer compositions in an interval of Δβ ≈1 but the interval is situated at lower absolute values of β for the copolymers with higher contents of MAI‐rings. The Δβ at lower β may be either due to a decrease in intramolecular attraction or to a higher electrostatic repulsion. The free enthalpy of the conformational transition is, however, approximately the same for copolymers with different MAI contents. This constancy of the free enthalpy points to a conformational transition which is energetically not much different for the MAA‐ and the MAI‐units in the chain.