Abstract
AbstractPolymerization of triphenylmethyl crotonate with a 9‐fluorenyllithium/(S,S)‐(+)‐2,3‐dimethoxy‐1,4‐bis(dimethylamino)‐butane complex in toluene at −78°C gave a highly threodiisotactic polymer. The polymer had a trimodal distribution of molecular weight (MW); the highest‐MW part showed a small negative optical rotation and the other two showed positive rotation. The middle‐MW fraction (DP=21.5, M̄w/M̄n=1.07) had an [α]D of +53.3°. The optical rotation of this fraction decreased gradually in chloroform at 60°C while the GPC curve of the fraction remained unchanged. The loss of optical activity was ascribed to the racemization through helix sense reversal of the polymer chain. The rate of the decrease in optical rotation obeyed first‐order kinetics. From the rate constants at 50 and 60°C, the activation enthalpy for the helix reversal was determined to be 28.4 kcal/mol.
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