AbstractViscoelastic properties of the BBL ladder polymer were determined by a dynamic mechanical technique for the temperature range −150°C to 250°C and at frequencies of 3.5, 11, and 110 Hz. A secondary loss dispersion centered at −50°C (at 110 Hz) was discovered which probably arises from an in‐chain relaxation mechanism. The loss dispersion resembles β‐transitions found in more conventional polymers in intensity (maximum tan δ = 0.035), activation energy(E= 19,000 cal/mole), and in its close relation to the high‐temperature mechanical properties of the polymers. Only slight changes in the low‐temperature loss peak cause significant increases in brittleness. It is proposed that such changes, produced by a variety of environments, result from random chain scission at imperfection points in the ladder structure. For this reason we conclude that improving the environmental stability of BBL is of prime importance in its further development for speci