A method for preparing reversibly cross-linked isotactic polypropylene having a degree of crystallinity substantially identical to a degree of crystallinity of a starting polypropylene by mixing in the presence of heat, a polymer of isotactic polypropylene homopolymer; isotactic polypropylene copolymer; mixtures of isotactic polypropylene homopolymer and copolymer with an elastomer, an organic peroxide, and an accelerator, forming macro-radicals; the simultaneously coupling the macro-radicals with a sulfur obtaining an optimum cross-linking degree and a product made by the method.