Three Polypropylene/Poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys produced by a two-stage slurry/gas polymerization had different ethylene contents and mechanical properties, which were achieved by controlling the copolymerization time. The three alloys were fractionated into five fractions via temperature rising dissolution fractionation (TRDF), respectively. The chain structures of the whole samples and their fractions were analyzed using high-temperature gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), 13 C nuclear magnetic resonance ( 13 C NMR), and differential scanning calorimetry (DSC) techniques. These three in-reactor alloys mainly contained four portions: ethylenepropylene random copolymer (EPR), ethylene-propylene (EP) segmented and block copolymers, and propylene homopolymer. The increased copolymerization time caused the increased ethylene content of the sample. The weight percent of EPR, EP segmented and block copolymer also became higher. The more EPR content indeed improves the toughness of the alloy but lowers its stiffness. Increasing the ethylene content in the EPR fraction and EP segmented and block copolymer, as well as the suitable content of EPR, is believed to be the key factors resulting in the excellent toughnessstiffness balance of in-reactor alloys.KEY WORDS: PP/EPR In-Reactor Alloy / Fractionation / Microstructure / Isotactic polypropylene (iPP) is a thermoplastic material widely used as it offers interesting combinations of good mechanical performance, heat resistance, and fabrication flexibility. However, it has relatively poor impact resistance, especially at low temperatures. Its toughness could be improved by a variety of elastomers, 1-3 by adding a nucleating agent that reduces the average dimensions of the spherulites.
4The toughness could also be improved by copolymerization of propylene with ethylene or other olefins, 5-10 among which the copolymerization with ethylene is one of the most useful and effective methods. Polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys have been industrialized on a large scale.The in-reactor blending technology was developed by Montell Company, hence opening up new horizons for polyolefin materials. The technology involves bulk polymerization of propylene and gas-phase copolymerization of ethylene and propylene using spherical superactive TiCl 4 /MgCl 2 based catalyst systems.11-14 The use of a spherical catalyst allows a wider range of rubber content in the alloy and better control over phase structure to be achieved. The resulting spherical granules can be directly processed, eliminating the need for pelleting.The mechanism of the two-stage in-reactor blending technology is very complicated, depending on the nature of active species in the catalysts and polymerization process. Xu et al. had proposed that the iPP with active center still can copolymerized with propylene and ethylene in the second stage.6 A previous investigation of the composition and chain structure of the PP/EPR in-reactor ...