Carborane transition temperatures

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

More interestingly perhaps, the polyketone 7, derived from bis(4-phenoxyphenyl)-p-carborane (3) and biphenyl-4,4 -dicarboxylic acid (4), is essentially amorphous on isolation from trifluoromethanesulfonic acid, but crystallises on heating above its glass transition temperature (267 °C) before finally melting at around 390 °C. 

Fig. 1. The glass transition for poly(carborane siloxane)—longitudinal sound speed and absorption vs temperature at 1 MHz. Adapted from Ref 49.

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