A heterophasic polymer composition comprises a propylene polymer phase, an ethylene polymer phase, and a compatibilizing agent. The compatibilizing agent comprises (i) at least one tertiary carbon-hydrogen bond having a bond-dissociation energy of about 380 kJ/mol or less and (ii) at least one acyclic carbon-carbon double bond. A method of making a heterophasic polyolefin polymer composition comprises the steps of (a) providing a propylene polymer phase and an ethylene polymer phase, (b) providing a compatibilizing agent, and (c) mixing the propylene polymer phase, the ethylene polymer phase and the compatibilizing agent, in the presence of free carbon radicals. The compatibilizing agent reacts with propylene polymers and ethylene polymers thereby bonding propylene polymers to ethylene polymers.