Composite materials were synthesized by direct polymerization of propylene on the surface of natural or synthetic graphite particles. The method ensures grafting of polymer to a part of filler particle surface, while the other part remains open to physical contact between the particles, the resulting properties of the material becoming favorably different from previously known composites. Coefficient of strain sensitivity K equals (Delta) R/R(epsilon) (R is resistance of the sample, (epsilon) is tensile strain) was measured at different concentrations of filler and different temperatures. There is a broad maximum of K around the percolation threshold (4.5 vol.% for natural graphite) with a peak value of 100 - 150, which is much higher, compared to conventional wire resistors. A slight hysteresis is observed at unloading due to plasticity of the matrix. Hysteresis disappears, when temperature is lowered by 20 - 50 degrees, or (epsilon) is less than 1%, but previously high value of K remains. Below the glass transition temperature K is very low. The results are explained by the change of current- carrying chains in loading-unloading cycles. Temperature dependence of resistance is presented, thermal conductivities were calculated for different models and compared with experimental values.