The second one is through the liquid-phase radical pathways. Both the liquid-phase carbon-center and oxygen-center (mostly peroxyl) radicals react with solid-bound carboncarbon double-bond reactive sites (CDB) to cause film growth. There are few solid-bound conjugated diene reactive sites (CD) as they are brought in by the slow propagation of bisallylic radicals, so the propagation of liquid-phase radicals with CD is not significant.