Crystallization during polymerization with special reference to synthetic polypeptides

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 NCA in acetonitrile and the morphologies of the resultant polymers were studied. Electron microscopy revealed that the fibers formed in the beginning of polymerization thickened as the polymerization proceeded and that the growth mechanism of the polypeptide crystals is quite similar to that of polyoxymethylene crystals during the polymerization of trioxane in cyclo-hexane. X-ray diffraction patterns and IR spectroscopy revealed that the growing chain molecules, when they attained a certain degree of polymerization (DP = -3 to 4) beyond which they lost solubility in the system, 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 which seemed to be more stable than the β-conformation in the later stages. 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 loosely packed but extended α-helices in the crystals throughout the polymerization. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization.

The polymerization of N-carboxy anhydrides of glycine and β-alanine has also been studied in acetonitrile with n-butylamine as initiator. As was expected from the similarity of the backbone chain structure to nylons, the polymer precipitated in the form of a spherulite consisting of ribbon-like crystalline units just as in the case of nylons previously reported. The molecules in the ribbon-like crystals were arranged antiparallel, forming a hydrogen-bond sheet.