Crystal topology prediction in alternating amphiphiles: Aromatic polyethers and the role of volume fraction

The crystal structures of alternating aromatic polyethers in the Cambridge Structural Database have been analyzed. Alternating aromatic polyethers are chain or ring-like molecules in which there is a regular alternation between the aromatic and the polyether moieties. In all structures examined, a clear interface is present between the hydrophobic aromatic and the hydrophilic polyether moieties. An examination of the 21 structures where the ratio of the hydrophobic-to-total volume ratio is 40-50% shows that 14 of the structures are lamellar (i.e., have alternating hydrophobic and hydrophilic layers within the structure). These results suggest that other as yet unsynthesized aromatic polyethers will also adopt lamellar crystal topologies. We prepared two such systems 1,5-bis(1,3-dioxolan-2-ylmethoxy)-naphthalene, 1 and 1,5-bis(2-methoxyethoxy)-naphthalene, 2 (Scheme 1). Solution of their single-crystal structures revealed that both crystallize in a lamellar topology. The connection between our findings and results from the block and regular copolymer literature are made. For all classes considered: block copolymers, regular copolymers, regular oligomers, and small molecule alternating amphiphiles, we find whenever the hydrophobic-to-total volume ratio is between 40 and 50% that the lamellar topology is the dominant one. On the basis of this finding, we make predictions as to the crystal topology for a number of specific alternating amphiphilic molecules and polymers.