Molecular motion of polycarbonate included in gamma-cyclodextrin

Journal of Polymer Science Part B: Polymer Physics (2007) Pages 1283 - 1290

ABSTRACT
Molecular motions of single polycarbonate (PC) chains threaded into crystalline gamma-cyclodextrin (gamma-CD) channels were examined using solid-state 13C NMR and molecular dynamics simulations. The location of PC within the channels was confirmed by spin diffusion from a PC 13C label to natural-abundance 13C of the gamma-CD. Rotor-encoded longitudinal magnetization (RELM) (under 7-kHz magic-angle sample-spinning conditions) was combined with multiple-pulse 1H-1H dipolar decoupling to detect large-amplitude phenyl-ring motion in both bulk PC and polycarbonate -cyclodextrin inclusion compound (PC-gamma-CD). The RELM results indicate that the phenyl rings in PC-gamma-CD undergo 180-degree flips faster than 10 kHz just as in bulk PC. The molecular dynamics simulations show that the frequency of the phenyl-ring flips depends on the cooperative motions of PC atoms and neighboring atoms of the gamma-CD channel. The distribution of protonated aromatic-carbon laboratory and rotating-frame 13C spin-lattice relaxation rates for bulk PC and PC-gamma-CD are similar but not identical. The distributions for both systems arise from site heterogeneities. For bulk PC, the heterogeneity is attributed to variations in local chain packing, and for PC-gamma-CD the heterogeneity arises from variations in the location of the PC phenyl rings in the gamma-CD channel.

Molecular structure and models of (a) polycarbonate and (b,c) an inclusion compound PC--CD, side and top views. The 13C-labeled sites of polycarbonate are marked by stars. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Schematic representation of the pulse sequence for 1H-13C rotor-encoded longitudinal magnetization experiments with MREV-8 homonuclear dipolar decoupling. The sequence consists of six parts: a preparation period for building up S-spin polarization, two REDOR dipolar recoupling periods, a wait period for removing unwanted antiphase coherence signals, a rotor encoding period in an indirect time dimension (t1), and a detection time dimension (t2) with heteronuclear decoupling. The inset (a) shows the insertion of a proton 180¡Æ REDOR pulse at the center of a rotor period between two MREV-8 cycles. The inset (b) depicts incrementation of t1 by the dwell time (tdw) in the dipolar time dimension.

Stereoviews of the four PC-gamma-CD inclusion compounds used in the molecular dynamics simulations.

(a) Dipolar rotational spin-echo 13C NMR spectra of ring-13C-labeled PC-gamma-CD. The centerband from the labeled protonated aromatic carbons of PC is at 120 ppm and spinning sidebands are marked by stars. The centerband from the 2, 3, 5-13C sites (natural abundance) of the cyclic glucose units of gamma-CD is at 82 ppm. The time step between each slice in t1 corresponds to one MREV-8 cycle (tc = 33.6 s). (b,c) Evolution of the spectral intensities of the 120- and 82-ppm peaks (including their sidebands). (d,e) Comparison of the experimental dipolar sideband patterns obtained by 12-point discrete Fourier transforms of the time-domain signals shown in (b,c), with dipolar sideband patterns calculated using SIMPSON.

(a) Rotor-encoded longitudinal magnetization 13C NMR spectra of ring-13C-labeled-PC-gamma-CD acquired with MREV-8 homonuclear dipolar decoupling and magic-angle sample spinning of 7440 Hz. The centerband from the labeled protonated aromatic carbons of PC is at 120 ppm with spinning sidebands marked by stars. The centerband from the 2, 3, 5-13C sites (natural abundance) of the cyclic glucose units of gamma-CD is at 82 ppm. Small peaks from various carbon sites at natural abundance are also observed (not assigned). The spectra of only the first half of one rotor period in the t1 dimension is shown. (b,c) Evolution of the spectral intensities of the 120- and 82-ppm peaks (including the sidebands). (d,e) Comparison of the experimental dipolar sideband patterns obtained by 32-point discrete Fourier transforms of the time-domain signals shown in (b,c), with dipolar sideband patterns calculated using SIMPSON.

Molecular dynamics simulation at 900 K of 450 ps duration for the four PC-gamma-CD inclusion compounds of Figure 3. Five full PC repeat units are shown for each inclusion compound. The gamma-CD atoms and H atoms of PC are not shown for clarity. Each color represents a time progression of 150 ps in the simulation. Carbon atoms on one side of each phenyl ring are shown as balls. If a single ring flip has occurred (circle A), solid balls appear on both sides of the ring. The circled site at the lower right (circle B) had two flips within 150 ps. At some sites, solid balls are distributed in space as the result of a series of small-amplitude steps. Averaged over all sites, the solid balls are not uniformly (isotropically) distributed, but tend to lie in a single plane. The insets at the bottom of the figure show the time evolution of the dihedral angles defined by (CO)OCarCar for four selected sites on the PC chains. These dihedral angles in combination with the ones for the other side of a ring, CarCar[C(CH3)2]Car, were used to detect phenyl-ring flips.