Helix conformation in helical polycarbodiimides studied by solid state 13C NMR

The helix conformation in helical polycarbodiimides as a function of the side chains were studied by C-13 CP/MAS NMR. From these results, the structures of the all polycarbodiimides were defined, and the C-13 spin-lattice relaxation times in the rotating frame were measured. We discuss the mobility, the correlation time, and the activation energy for each carbon of the polycarbodiimides as a function of the side chains. The activation energies of main-chain carbons are strongly dependent on the side chains. From these results, the activation energies of the main-chain carbons for polycarbodiimides with directly bonded aromatic rings are distinctly different from those for polycarbodiimides with non-bonded aromatic rings. Based on our findings, we know that the polycarbodiimides with directly bonded aromatic rings have high activation energy and high helix reversal barriers. (C) 2001 Published by Elsevier Science B.V.