C-13 NMR study on helix inversion barrier in polyguanidines

The activation energy of the main-chain carbons for copolymerization of bot h enantiomers in polyguanidines has been studied using measurements of C-13 T-1p relaxation times. The activation energies of main-chain carbons depen d on the ratios for copolymerization of both enantiomers. The activation en ergies of the carbons in three polyguanidines are distinctly different from those in pure [R] and [S] polyguanidines previously reported. It is worth noting that the carbon of 24.6% EE has a larger activation energy than 48.8 and 81.7%EE polymers. This large activation energy means that the backbone mobility is hindered; we think that this prevents a smooth precession of t he chain around the helix axis. (C) 1999 Elsevier Science Ltd. All rights r eserved.