Synthesis and photophysical characterization of aryl-substituted polynorbornenediol acetal and ketal multiblock copolymers

A series of homopolymers and di-, tri-, and tetrablock copolymers (MW = 3600-28 000) synthesized by ring-opening metathesis polymerization of exo,cis-norbornenediol monomer units bearing various aryl chromophores (naphthalene, phenanthrene, anthracene, pyrene, dimethylaniline, dicyanobenzene, and pentamethylbenzene) have been examined by steady-state emission. Ketal-appended naphthyl, phenanthryl, and anthryl homopolymers show unperturbed monomer emission indicative of a rigid polymer backbone (at least on the time scale of the excited-state lifetimes of the chromophores), whereas acetal-appended naphthyl, anthryl, and pyrenyl homopolymers show substantial excimer formation. Molecular modeling of four possible polymer conformations for both the acetal- and ketal-derived polymers qualitatively explains the differences in their fluorescence emissions. 1 H- and 13 C-NMR and absorption spectroscopy as well as retention behavior in gel permeation chromatography (polydispersity indices 1.1-1.4) show this family to exist as rigid, living, well-defined, narrow-polydispersity polymers.