A polymeric peroxide comprising a dilvalent recurring unit having the formula: ##STR1## wherein the recurring unit of Formula I has a 10 hour half-life temperature of at least 80.degree. C. and optionally a divalent recurring unit having the formula: ##STR2## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.5, X, X.sup.1, Y, Z, y and z are as defined in the Detailed Description of the Preferred Embodiments and the polymeric peroxide has a molecular weight of 1,000 to 200,000. The polymeric peroxides are derived from hydroxy-hydroperoxides of dihydroxy dialkyl peroxides. These polymeric peroxides can be used for preparing block copolymers and compatibilizing polymer blends and alloys.