Clustering of Poly(methacrylic acid) around Appended Binaphthyl Labels as Reflected by the Disruption of γ-Cyclodextrin Complexation and Racemization Kinetics

A derivative of 1.1′-binaphthyl (BN) was appended to poly(methacrylic acid) (PMA), and its reversible complexation with γ-cyclodextrin (CyD), which leads to an induced circular dichroism signal (ICD), was studied as a function of pH. The ICD was observed at pH above 6 but disappeared sharply at lower pH's where clustering of the chain led to encapsulation of the BN by the polyelectrolyte and prevented the approach of the CyD. Kinetic work based on the atropisomeric character of the BN showed the capsule to exert a strong and pH dependent restriction on the racemization of the BN. In syndiotactic PMA the racemization followed first-order kinetics at all pH values while, with atactic PMA as the carrier, the racemization at low pH was biphasic. Remarkably, the most restrictive relationship between the polyelectrolyte and the racemization of the probe is found at a degree of ionization just slightly below the point where the chain charge density disrupts the capsules. The behavior of the BN label appended to PMA was compared with its behavior when attached to poly(acrylic acid), poly(2-ethylacrylic acid), and an alternating maleic acid-butyl vinyl ether copolymer.