Growth mechanism of crystals of glycine/S‐methyl‐ L ‐cysteine copolymer in the course of polymerization

In order to examine the growth mechanism of copolymer crystals formed during polymerization, the heterogeneous copolymerization of glycine N‐carboxy anhydride (2,5‐dioxo‐1‐oxa‐3‐azacyclopentane) and S‐methyl‐L‐cysteine N‐carboxy anhydride (4‐methylthiomethyl‐2,5‐dioxo‐1‐oxa‐3‐azacyclopentane) was studied in acetonitrile. In all the polymerization systems, the crystal growth occurs through formation of the cross‐β‐type structure. The high conversion in the glycine‐rich systems is accounted for by the widening of the cross‐section of the β‐chain in the backbone crystal, due to the introduction of S‐methyl‐L‐cysteine residues into the polyglycine chain. On the other hand, the narrowing of the cross‐sectional area per chain in the skeleton crystals of the oligomer gave rise to the lowering of the conversion in the S‐methyl‐L‐cysteine‐rich systems. This shows the importance of the effect of the cross‐sectional area of the oligomer chain (of the β structure) in the crystal formed in the begining relative to that of the growing (α‐helical) chain. The glycine residue seems to be incorporated into the crystalline lattice of the S‐methyl‐L‐cysteine and vice versa.