Two novel systems were developed to selectively crosslink dispersed unsaturated elastomer particles in saturated polyolefin matrices under dynamic crosslinking conditions. One system comprises N,N’-m-phenylene-bis-maleimide (PM) as a crosslinker and 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline or polymerized- (2,2,4-trimethyl-1,2-dihydroquinoline) as an activator. They are believed to form charge-transfer-complexes to produce PM radicals, which accelerate the crosslinking reaction of the elastomer. The other comprises PM as a crosslinker and 9,10-dihydrophenanthrene as an activator. These are believed to act as a hydrogen acceptor and a hydrogen donor, respectively, involving transference of a hydrogen radical between them to produce a PM radical. These crosslink systems convert the unsaturated elastomer particles in the polyolefin matrices to crosslinked particles selectively without causing degradation or crosslinking of the matrices, because the radicals produced from the systems are carbon radicals and abstract only hydrogen radicals at the allylic positions of the unsaturated elastomers and not a hydrogen radical at the secondary or tertiary carbons of the polyolefin. The reactions and their confirmed selectivity were conducted on PP/EPDM blend, PP/SBS blend, PP/SIS blend, PP/1,2-PB(1,2-polybutadiene) blend, and PP/PE/EPDM blend, respectively.