The probability that any isoprene unit becomes part of a cross-link node is proportional to the ratio of the concentrations of the cross-linker molecules (e.g., dicumyl-peroxide) to the isoprene units: p x = 2 [cross-link] [isoprene] The factor of two comes about because two isoprene units (one from each chain) participate in the cross-link. The probability for finding a chain containing N isoprene units is given by: