Dynamic Disorder and Conformer Exchange in the Crystalline Monomer of Polycarbonate

Direct measurement of chemical exchange events in the crystalline polycarbonate monomer 4,4‘- isopropylidenediphenol (bisphenol A, BPA) via 2D 13C solid-state NMR reveals slow, large-amplitude aromatic
ring reorientations. X-ray diffraction, however, indicates a static crystalline structure. Experiments with multiple
exchange times show that ring flips occur in all of the three unique conformers found in the crystalline unit
cell of this compound, but in specific cases, two of the three unique molecules actually switch conformations.
Kinetic analysis of the exchange data indicates that the average rate constant kex = 0.01 s-1 for ring flips and
conformer interchange at room temperature. Differential scanning calorimetry and variable-temperature powder
diffraction studies indicate a systematic volume expansion that accompanies this motion but no first-order
phase transition. All room-temperature exchange events may be quenched at 213 K, at least on the time scale
(up to several seconds) probed in this work. Simulation of the potential-energy surface of BPA molecules
reveals that the lowest-energy pathway for ring flips (maximum energy of 1.9 kcal/mol) involves +110° flips
of one ring coincident with +70° flips of the second. The mechanism of the ring dynamics in the crystalline
monomer is unique relative to those previously reported for the polycarbonate in that the collective motion
of both rings in a monomer unit is 180°, whereas single-ring flips of 180° occur in the polymer.