Synthesis and Photoisomerization of Poly(1-methylpropargyl ester)s Carrying Azobenzene Moieties

Optically active 1-methylpropargyl esters bearing azobenzene groups, namely, (S)-(-)-3-methyl-3-{4-[4-(n-butyloxy)phenylazophenyl]carbonyl}oxy-1-propyne (1), (S)-(-)-3-methyl-3-{4-[4-(n-hexyloxy)phenylazophenyl]carbonyl}oxy-1-propyne (2), and (S)-(-)-3-methyl-3-{4-[-4-(n-octyloxy)phenylazophenyl]carbonyl}oxy-1-propyne (3) were synthesized and polymerized with Rh+(nbd)[eta(6)-C6H5B-(C6H5)(3)] (nbd, norbornadiene) as a catalyst to afford the corresponding poly(1-methyloropargyl ester)s with moderate molecular weights (M-n = 24,000-31,300) in good yields (79-84%). Polymers were soluble in common organic solvents including toluene, CHCl3, CH2Cl2, THF, and DMSO, whereas insoluble in diethyl ether, n-hexane, and methanol. Large optical rotations and strong CD signals demonstrated that all the polymers take a helical structure with a predominantly one-handed screw sense. The helical structure of the polymers changed with the addition of MeOH and heat. The trans-azobenzene of the polymer side chains isomerized. into cis on UV irradiation, which was accompanied with drastic helical conformational changes of the polymer backbone. The cis-azobenzene moiety reisomerized into trans on visible-light irradiation, which induced the recovery of chiral geometry of azobenzene moieties in the side chain. Conformational analysis revealed that the polymers form a tightly twisted right-handed helical structure with a dihedral angle of 70 degrees at the single bond of the main chain. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4749-4761, 2009