SignificanceWe report on a novel atmosphere-switching polymerization process (ASPP) that produces advanced polyolefin materials with high performance in a single polymerization reactor through unique monomer feeding policies. In the ASPP, each polymer particle is exposed to atmosphere A and atmosphere B in an alternating manner and time and again. As a result, polymer A and polymer B are formed within the polymer particle in an alternating manner and time and again too. Two types of polypropylene (PP) in-reactor alloys are prepared: impact polypropylene copolymers (IPCs) with an exceptional stiffness-toughness balance and soft polypropylene alloys (SPPs) with well-defined, high-rubber content sticking-free particles. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 4468-4473, 2013