Four Stereoisomeric Norbornadiene Dimers Containing a Cyclopropane Ring: ROMP, Polymer Properties, and Post-Polymerization Modification

Abstract

The efficient and selective introduction of functional groups to hydrocarbon polymers enables facile access to new polymer materials with various physical properties. In the present study, we have focused on cyclopropane-containing norbornadiene dimers (NBDDs) as bifunctional monomers and post-polymerization modification (PPM) for the synthesis of functionalized cyclic olefin polymers (COPs). The ring-opening metathesis polymerization (ROMP) of the four NBDD stereoisomers (exo-exo, exo-endo, endo-exo, and endo-endo) and the subsequent hydrogenation proceeded selectively to give the corresponding COPs (H-poly(NBDD)s) with reactive cyclopropane moieties. There are distinct differences between the four isomers in terms of polymerization rate and the physical properties of the resultant polymers. The endo-exo- and endo-endo-NBDDs show lower ROMP reactivities than the exo-exo- and exo-endo-NBDDs due to steric hindrance. All of the polymers before and after hydrogenation are amorphous, regardless of annealing (with the exception for the unannealed H-poly(exo-endo-NBDD)). Compared with the polymers of the exo-norbornenyl isomers, their endo-counterparts show lower solubilities, higher glass transition temperatures, sharper X-ray diffraction peaks, and larger d-spacings. The highly soluble H-poly(exo-exo-NBDD) was employed for the PPM via protic acid-catalyzed cyclopropane ring-opening to produce six new COPs bearing acyloxy, alkoxy, or aryl groups. Although rearrangements occur during ring-opening presumably through nonclassical carbocations, the polymer structures were determined with reference to the reactions of their corresponding monomeric model compounds. The PPM with m-xylene, for example, proceeds regioselectively while maintaining a narrow molecular weight distribution to produce a xylyl-substituted COP with good solubility and high thermal stability.

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