The mechanical properties and fracture toughness of thin films of a series of poly(n-pentyl-n-alkylsilanes) were investigated. Poly(n-butyl-n-pentylsilane) is the strongest of these polymers with an elastic modulus of 2.96 x 10(8) Pa and a fracture strain of 85% at 25 degrees C. The hexagonal mesophases of these polymers generally show elastic moduli on the order of 10(7) Pa and are often quite extensible. A J-integral analysis of the ductile tearing of thin films of poly(n-butyl-n-pentylsilane) and poly(n-propyl-n-pentylsilane) using an Instron tensile testing machine and specimens in the single edge notch (SEN) geometry yielded plane stress J(1c) (critical value of J for fracture initiation) of 1745 J/m(2) and 205 J/m(2), respectively. Both values are significantly higher than the plane stress G(1c) (critical energy release rate) value of 109 J/m(2) obtained for poly(di-n-hexylsilane) with a residual stress analysis using the same apparatus and testing procedure.