We present first-principles simulations of cis-polyacetylene fragments under tensile stress. The barrier of a cis-trans isomerization of conjugated carbon-carbon double bonds is lowered by tensile stress. However, it does not seem likely that the barrier can be sufficiently reduced to achieve a cis-trans isomerization in an atomic force microscopy experiment. In particular, the transition state of the reaction cannot be converted into a stable or metastable minimum by mechanical stress. We devise a general model for the calculation of bond rupture probabilities.