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

Abstract

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.

Chemically Induced Dynamic Electron Polarization Studies of a pH-Dependent Free Radical Cage Formed in a Photoinitiator Labeled Poly(methacrylic acid)

“…Furthermore and importantly, the inflection point in our measured ratio of RPM/TM occurs within the pH range where the polymer capsule has been proposed to open (pH ∼ 6.5) as independently judged by previous work in which the capsule and cyclodextrin competed for complexation with a binaphthyl group appended to PMA. 4 Opening of the capsule allows complexation of the cyclodextrin with the appended binaphthyl group, which is revealed by a large induced circular dichroism. In a parallel study we have studied the pH dependence of the induced circular dichroism associated with the photoinitiator group in 2, which again signals the opening of the capsule in the identical pH range.…”

“…This is evidenced by the large change in the photophysical characteristics of fluorescent dyes reflecting their separation from the aqueous medium [1][2][3] and the inability of cyclodextrin to complex with binaphthyls clustered within the PMA. 4 Within the cluster, molecular motion is impeded as shown by the large extension of the rotational relaxation time of the attached dye 3 and by the large reduction of the rate of racemization of a binaphthyl atropisomer attached to PMA. 4 The above phenomena are clearly related to the titration behavior of PMA.…”

“…4 Within the cluster, molecular motion is impeded as shown by the large extension of the rotational relaxation time of the attached dye 3 and by the large reduction of the rate of racemization of a binaphthyl atropisomer attached to PMA. 4 The above phenomena are clearly related to the titration behavior of PMA. The polymer chain does not expand with increasing ionization below a critical change density.…”

“…We have found that the mechanism of polarization changes as a function of pH, in a manner that correlates with the opening of the hydrophobic cluster consistent with earlier studies using other kinds of probes. [2][3][4]9,10 …”

Polyelectrolyte Solutions: Phenomena and Interpretation

“…5, drops precipitously in the pH range where the pK exhibits a plateau (21). Another evidence for PMA clustering was provided by the reduction of the conformational mobility of binaphthyl residues attached to the polymer chain (22).…”