Helical Nylons 3. Synthesis and Crystal Structure of Poly(β-l-aspartate)s with Branched Alkyl Side Chains

Abstract

A set of poly(β-l-aspartate)s bearing branched alkyl side groups of varying shapes and sizes has been synthesized, and the crystal structures of these polymers have been investigated by X-ray diffraction and computational methods. The anionic ring-opening polymerization of 4-((S)-alkoxycarbonyl)-β-lactams (the alkyl groups being isopropyl, isopentyl, and neopentyl) afforded the corresponding poly(α-alkyl β-l-aspartate)s with high molecular weights in good yields. The thermal behavior of these poly(β-l-aspartate)s parallels that described for their linear alkyl side chain homologues with a melting−decomposition process taking place above 250 °C. The analysis of fiber X-ray diffraction diagrams complemented with molecular mechanics modeling and LALS refinement revealed that helical conformations of α-helix type were adopted in the three cases. Whereas the isopentyl derivative crystallized in a quasi-hexagonal form with the chains in the right-handed 13/4 helical conformation characteristic of this family of compounds, a hexagonal array of 16/5 helices was found to be the arrangement preferred by the isopropyl and neopentyl derivatives. These two polymers displayed in addition the second crystal form common in poly(β-l-aspartate)s, which consists of a tetragonal structure of right-handed 4/1 helices. Energy calculations showed negligible energy differences between the two crystal forms in both systems. Conformation and crystal packing of linear and branched poly(β-l-aspartate)s were compared and their features discussed with regards to the architecture of the alkyl side group.