Polymerization of N‐CARBOXY‐α‐amino acid anhydride by organometallic compounds III. Mechanism of asymmetric selective polymerization of n‐carboxy alanine anhydride by organoaluminum catalysts

Copolymerizations of D ‐ and L ‐N carboxy alanine anhydride by Al(C 2 H 5 ) 3 and ( i ‐C 4 H 9 ) 2 AlN(C 2 H 5 ) 2 were studied in order to elucidate the mechanism of stereospecific polymerization of the N‐carboxy‐α‐amino acid anhydride (NCA) with the two organoaluminum compounds. The enantiomer involved in higher concentration in the starting monomers was polymerized preferentially in the propagation step by these catalysts. The mechanism of the asymmetric selective polymerization was concluded to be the growing chain end control, neither the enantiomorphic catalyst sites control nor the control by the helical conformation of the growing polymer chains. It was found in Al(C 2 H 5 ) 3 initiated polymerization system that almost perfect stereoselective reaction took place at an early stage of the polymerization; that is, about five mole of the NCA initially reacted with one mole of Al(C 2 H 5 ) 3 and the amounts of L ‐ and D ‐enantiomer of the NCA reacted were always equal independently of enantiomer ratio in the starting NCA.