Abstract
Copolymerization of dibenzofulvene (DBF) with (−)-menthyl methacrylate ((1R,2S,5R)-5-methyl-2-(1-methylethyl)cyclohexyl methacrylate) (MnMA) prepared from L-(-)-menthol ((1R,2S,5R)-(-)-5-methyl-2-(1-methylethyl)cyclohexanol) and methacrylic chloride was conducted under free radical conditions in toluene using α,α′-azobisisobutylonitrile (AIBN). DBF exhibited much higher reactivity compared with MnMA under all conditions examined in this work, leading to the products comprised mainly of DBF units. NMR, UV, and fluorescence spectral analyses indicated that higher-molar-mass copolymers have completely π-stacked DBF sequences with high conformational regularity while lower-molar-mass copolymers possess both π-stacked and unstacked DBF units in the chain. The ratios of π-stacked and unstacked side-chain fluorene groups of DBF units in excited states were determined by fluorescent emission spectra. The chiral copolymers exhibited circular dichroism (CD) spectra in the wavelength range of absorbance of side-chain fluorene units, suggesting that a preferred left-handed helical conformation was induced by the chirality of MnMA units. In addition, a conformational transformation leading to a loss of optical activity was induced by photo irradiation while the chirality of the copolymers was stable in tetrahydrofuran solution on heating up to 60 °C. The stereomutation was suggested to be a helix-to-random transition.
Published Version
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