A high-pressure mixing apparatus for polymeric components suitable for providing a reactive mixture for a polymerizable resin, provided with a self-regenerating seal. An annular sealing element is housed in a circular seat inside a cross hole of a bush transversely extending to and in communication with the mixing chamber, in a sealing zone downstream the injection holes for the polymeric components, between the mixing chamber and the same delivery bush; worn and/or torn parts of the annular sealing element are automatically regenerated by the reactive mixture delivered during operation of the mixing apparatus.