Polymerization mechanism of N‐carboxy‐α‐amino acid anhydride by organometallic compounds. IV. Stereospecific association of diisobutylaluminum 4‐methyloxazolidonate as a model compound of intermediate of the polymerization

Abstract

AbstractIn the copolymerization of D‐ and L‐alanine N‐carboxy anhydride (NCA) with an excess of L‐antipode by triethylaluminum, a polyalanine containing an equal amount of L‐ and D‐residues was obtained in the early stage of the polymerization.Initiation of the polymerization was the hydrogen abstraction from NH group of the NCA by the trialkylaluminum.Since sarcosine NCA (N‐methylglycine NCA) was not polymerized with trialkylaluminum, alanine NCA is considered to be polymerized through the form of the metalated NCA. A stereochemical study on diisobutylaluminum 4‐methyloxazolidonate (BAO) as a model compound of the metalated NCA showed that BAO was associated with each other forming preferentially a meso‐dimeric structure. From this result, the metalated NCA is also considered to form preferentially a meso‐dimeric structure in conformity with the observed fact that the NCA polymerization produced in the early stage the polyalanine consisting of practically the same amount of L‐ and D‐residues.