Crystallization during polymerization with special reference to synthetic polypeptides

Abstract

In order to pursue the possibilities of the extended chain crystals formed in the course of polymerization, kinetics of the heterogeneous polymerization of L- and DL-alanine N-carboxy anhydrides (NCA) in acetonitrile and the morphologies of the resultant polymers were studied. It was revealed that the growing chain molecules, when they attained a certain degree of polymerization (DP = 3 to 4), crystallized into the form of ribbon-like fibers in which molecules, with the β-conformation, were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends. The grown polypeptide chains were extended α-helices in the crystals throughout the remainder of the polymerization. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization. The polymerization of NCA's of glycine and β alanine has also been studied.