Synthesis and chiroptical properties of amino acid-derived poly(methylpropargyl ester)s

Chiral methylpropargyl ester monomers containing amino acid moieties, N-(tert-butoxycarbonyl)-l-alanine (R)-1-methylpropargyl ester (1), N-(benzyloxycarbonyl)-l-valine (R)-1-methylpropargyl ester (2), and N-(tert-butoxycarbonyl)-l-O-cyclohexyl-l-glutamic acid (R)-1-methylpropargyl ester (3) were synthesized and polymerized with (nbd)Rh+[η6-C6H5B−(C6H5)3] to give the corresponding polymers with number–average molecular weights ranging from 32 700 to 64 100 in 84–85% yields. The polymers were soluble in toluene, CHCl3, CH2Cl2, THF, DMF, and DMSO, but insoluble in hexane, diethyl ether, and MeOH. The specific rotation and circular dichroism (CD) spectroscopic studies revealed that poly(1)–poly(3) took predominantly one-handed helical structures in the solvents, wherein the degree of one-handedness and tightness of the helix depended on the solvent. The helical structure of the polymers was stable to the addition of MeOH and heat.