Synthesis of Polypropylene/poly(ethylene-co-propylene) In-Reactor Alloys by Periodic Switching Polymerization Process: Dynamic Change of Gas-Phase Monomer Composition and Its Influences on Polymer Structure and Properties

Using a MgCl2-supported Ziegler–Natta catalyst, a series of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys with high EPR content (>20 wt %) were prepared by a periodic switching polymerization process (PSPP) in which the monomer feed was periodically switched between pure propylene and an ethylene/propylene mixture. The change of gas-phase monomer composition with time in the PSPP process was investigated. Transition periods were identified when switching the monomer feed from propylene to ethylene/propylene mixture (or vice versa). Because the gas-phase ethylene content in the transition periods was lower than that of the monomer mixture feed, copolymer formed in these periods has lower ethylene content than those formed in the steady periods of the copolymerization stage. As a result, the transition periods led to formation of more segmented ethylene/propylene copolymer (EPS) and decreased its ethylene content. When the switching frequency was altered in an adequate range, the cont...