Synthesis of Polycyclic\nOlefinic Monomers from Norbornadiene\nfor Inverse Vulcanization: Structural and Mechanistic Consequences

The preparation of high-sulfur content organosulfur polymers\nhas\ngenerated considerable interest as an emerging area in polymer science\nthat has been driven by advances in the inverse vulcanization polymerization\nof elemental sulfur with organic comonomers. While numerous new inverse\nvulcanized polysulfides have been made over the past decade, insights\ninto the mechanism of inverse vulcanization and structural characterization\nof the high-sulfur-content copolymers remain limited in scope. Furthermore,\nthe exploration of new molecular architectures for organic comonomer\nsynthesis remains an important frontier to enhance the properties\nof these new polymeric materials. In the current report, the first\ndetailed study on the synthesis and inverse vulcanization of polycyclic\nrigid comonomers derived from norbornadiene was conducted, affording\na quantitative assessment of polymer microstructure for these organopolysulfides\nand insights into the inverse vulcanization polymerization mechanism\nfor this class of monomers. In particular, a stereoselective synthesis\nof the endo-exo norbornadiene cyclopentadiene adduct (Stillene) was\nachieved, which enabled direct comparison with the known exo-exo norbornadiene\ndimer (NBD2) previously used for inverse vulcanization. Reductive\ndegradation of these sulfur copolymers and detailed structural analysis\nof the recovered sulfurated organic fragments revealed that remarkable\nexo-stereospecificity was achieved in the inverse vulcanization of\nelemental sulfur with both these polycyclic dienyl comonomers, which\ncorrelated to the robust thermomechanical properties associated with\norganopolysulfides made from NBD2 previously. Melt processing and\nmolding of these sulfur copolymers were conducted to fabricate free-standing\nplastic lenses for long-wave infrared thermal imaging.