Stereoregular Poly(N-propargylcarbamates) Having Helical Conformation Stabilized by the Intramolecular Hydrogen Bonds

Abstract

N-Propargylcarbamates (2, HC⋮CCH2NHCO2R), where R = i-Bu (2a) or (S)-CH2CH(CH3)C2H5 (2b), were polymerized in the presence of a Rh catalyst, [(nbd)RhCl]2−Et3N, in CHCl3, giving stereoregular cis polymers as slightly yellow solids in good yields. The 1H NMR spectra of poly(2) showed very broad signals at ambient temperature, indicating the limited mobility of the main chain. IR spectroscopic study of the polymers in CHCl3 revealed that most of the pendant carbamate groups participate in intramolecular hydrogen bonds. The chiral polymer from 2b exists in a helical conformation with an excess of one-handed screw sense, which was supported by the very large optical rotation ([α]D = +777°) and intense CD effects ([θ]max = 81 700 deg cm2 dmol-1). A Rh-based polymer from the corresponding chiral carbonate monomer, (S)-HC⋮CCH2OCO2CH2CH(CH3)C2H5 (3), obtained as a viscous oil, showed much poorer chiroptical properties ([α]D = +13.8°), meaning that the intramolecular hydrogen bonds rigidify the polymer backbone and simultaneously induce and stabilize the helical conformation of poly(2). The contribution of the hydrogen bonds to the secondary structure was also supported by variation in the CD and UV−vis spectra of poly(2) upon the addition of methanol.