Inclusion of poly(ethylene glycol)s by crystalline (R)-(1-naphthyl)glycyl-(R)-phenylglycine

Crystallization of (R)-(1-naphthyl)glycyl-(R)phenylglycine [(R,R)-1] in the presence of oligo(ethylene glycol) dimethyl others 2(n) or poly(ethylene glycol)s (PEGs, 3(M-n)) afforded inclusion compounds. The ratio of (R,R)-1/the guest polymer (2 or 3) was proportional to the length of the polymer chain. The crystal structure of a hepta(ethylene glycol) dimethyl ether-included compound was disclosed by X-ray crystallography which showed that (R,R)-1 molecules form a sheet and the guest molecule penetrates the crystal lattice of (R,l;l)-1 through a one-dimensional channel on the sheet. Powder X-ray analysis revealed that, regardless of the length of the guest polymer, the distance between the neighboring sheets remains unchanged (12.0-12.3 Angstrom) in these inclusion crystals. By thermal analysis, it was shown that the decomposition points of those inclusion compounds became higher with the longer PEG included. The inclusion phenomenon enabled the fractionation of PEGs with various molecular weights, among which longer PEG was preferably included.