HT5, HT8
The carborane phases were originally developed as very high thermal stability phases for high temperature work to 460 °C. The functionality of the carboranes is diffi cult to explain – they end up with pentavalent bonds with shared sigma bonds rather than π – bonds. The bonds are transient like a benzene with a ball of shared electrons.
HT5 and HT8 are low π – bonding purely due to the low concentration of carborane in the polymer, otherwise it would be high.