Ethylene−Norbornene Copolymers from Metallocene-Based Catalysts: Microstructure at Tetrad Level and Reactivity Ratios
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Ethylene−norbornene (E−N) copolymers were synthesized by catalytic systems composed of racemic isospecific metallocene or a constrained geometry catalyst (CGC) and methylaluminoxane. The following metallocenes were used: rac-Et(indenyl)2ZrCl2 (1), rac-Me2Si(indenyl)2ZrCl2 (2), rac-Me2Si(2-Me-[e]-benzindenyl)2ZrCl2 (3), and Me2Si(Me4Cp)(NtBu)TiCl2 (4). The copolymers were characterized by 13C NMR and the copolymer microstructures were analyzed in detail. A procedure for computing the molar fractions of the stereosequences that completely define the microstructure of an E−N copolymer at tetrad level, distinguishing between meso and racemic contributions to alternating and block sequences, was utilized. The information was converted into the complete tetrad distribution, which allowed us to determine the reactivity ratios, testing the first-order and the second-order Markov statistics. Here, examples of such an use of tetrad description of copolymers to test possible statistical models of copolymerization ar...