Stereocontrol in DL-Leucine N-Carboxyanhydride Polymerizations

L-Leucine and DL-leucine N-carboxyanhydride were polymerized in 1,4-dioxane at 25°C with either benzylamine or L-a-methylbenzylamine as initiator, using a constant volume reactor for rate studies. A rapid initiation period was followed by two pseudo-first-order propagation periods for the racemic (DL) monomer and by three pseudo-first-order propagation periods for the L-monomer. The ratio of rate constants of the L- to the DL-polymerization is about 2 for any given polymerization period, independent of initiator type. Such a behavior is expected for stereoselective propagations without crossover, leading to blends of isotactic polymers.