Synthesis and Characterization of a High-Temperature Thermoset Network Derived from a Multicyclic Cage Compound Functionalized with Exocyclic Allylidene Groups

A heptacyclo­[6.6.0.02,6.03,13.04,11.05,9.010,14]­tetradecane (HCTD) complex with
terminal allylidene groups at the 7- and 12-positions (HCTD-7,12-diallylidene, 2) was generated at the multigram scale from norbornadiene
via an efficient six-step synthesis. Thermal polymerization of 2 at temperatures ranging from 160 to 240 °C yielded
a robust cross-linked material with thermal stability up to 488 °C
in air, a glass transition temperature of 377 °C, and a char
yield (600 °C) of 56% in air. This degree of thermal stability
is remarkable for a nonaromatic hydrocarbon polymer and is likely
due to the rigid multicyclic cages that make up the bulk of the material.
To elucidate the polymerization mechanism, a model compound, 7-allylidenenorbornane
(4), was synthesized and thermally cured. This resulted
in the formation of polymeric material, suggesting that the cross-linking
reaction of 2 proceeds via a free-radical reaction and
not through Diels–Alder cycloaddition. Addition of dibutylhydroxytoluene
(BHT) to compound 2 delayed the onset of cure, providing
further support for a radical mechanism. On the basis of these results,
it can be concluded that exocyclic allylidene groups represent a new
class of thermosetting end-cap capable of generating highly cross-linked
materials with thermal stabilities that rival those of high temperature
polyimides. Applications include heat resistant composites utilized
in the aerospace, electronic, automotive, and textile industries.