Multi-center nature of heterogeneous Ziegler-Natta catalysts: TREF confirmation

A new approach to detailed Tref analysis of ethylene/alpha-olefin copolymers prepared with multi-center polymerization catalysts is developed. It is based on resolution of complex Tref curves into elemental components described with the Lorentz distribution function. This approach was applied to the study of a series of ethylene/ 1-butene copolymers prepared with a supported Ti-based catalyst. The analysis showed that the copolymers, which, on average, contain from 6.5 to 3.5 mol % of 1-butene, consist of seven discrete components with different compositions, ranging from a completely amorphous material with a 1-butene content of > 15-20 mol %, to two highly crystalline components with 1-butene contents < 1 mol %. A comparison of these Tref results with the data on the molecular weight distribution of the copolymers (based on resolution of their GPC curves) shows that Tref and GPC data provide complimentary information on the properties of active centers in the catalysts in terms of the molecular weights of the material they produce and their ability to copolymerize alpha-olefins with ethylene. Tref analysis of copolymers produced at different reaction times showed that the active centers responsible for the formation of various Tref components differ in the rates of their formation and in stability. The centers that produce copolymer molecules with a high 1-butene content are formed rapidly but decay rapidly as well whereas the centers producing copolymer molecules with a low 1-butene content are formed more slowly but are more stable. (c) 2005 Wiley Periodicals, Inc.