In this article, an extension of quantum scattering methods based on propagative R-matrix techniques to deal with negative imaginary potentials is presented. Reactive probabilities can be then obtained, considering only the reactants arrangement channel and Jacobi coordinates; It has been necessary to generalize the R-matrix propagation method, in order to consider the complex-valued nature of the interaction matrix. The new formulation has been particularized, in the present case, to the Infinite-Order Sudden Approximation, for which several results, focusing on the reliability and numerical performances of the method, will be shown.