Synthesis and Chiroptical Properties of Helical Polyallenes Bearing Chiral Amide Pendants

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Abstract

Two allene derivatives, l- and d-N-(1-(octylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)benzamide (l-1 and d-1), bearing chiral amide pendants were designed and synthesized. Living polymerizations of l-1 and d-1 with allylnickel complex as a catalyst afforded poly-l-1m and poly-d-1m with controlled molecular weights and narrow molecular weight distributions. These polymers were found to possess a stable helical conformation with a preferred handedness in aprotic solvents on the basis of their circular dichroism (CD) spectra and specific rotation as well as computer simulation. The helical conformation of the polymers was revealed to be stabilized by elongation of the repeating unit until the degree of the polymerization reaches 80. The slightly influence of temperature on the CD spectra of poly-l-1100 in CHCl3 indicated the helical conformation was quite stable at least in the range of 0–55 °C. Although poly-l-1100 showed similar CD spectra in different aprotic solvents, remarkable decrease was observed upon the addition of protic solvents such as methanol due to the weakened hydrogen bonding interactions between the adjacent repeating units. The poly-l-1100 behaves as a pH-responsive property; the helical structure of the main chain can be transformed to random coil by addition of trifluoroacetic acid to the THF solution which again switches back to helical conformation by neutralization with triethylamine. It was confirmed that the copolymerization of l-1 and d-1 obeyed the majority rule as supported by the nonlinear correlation between the enantiomeric excess of monomer 1 with the CD intensities of the generated copolymers. Atomic force microscope (AFM) and scanning electron microscope (SEM) studies revealed poly-l-1100 self-assembled into well-defined helical fibrils with distinct handedness.

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