Abstract
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 are given. The first-order r1 and r2 reactivity ratios of copolymers prepared with all catalysts depend on the monomer concentration. The products r1r2 were found in the range between 0 and 0.177. The tendency to alternate ethylene and norbornene is 4 > 3 > 1 > 2. The root-mean-square deviations between experimental and calculated tetrads demonstrate that penultimate (second-order Markov) effects play a decisive role in E−N copolymerizations. Our first results show clues for more complex effects depending on the catalyst geometry in copolymers obtained at high N/E feed ratios.
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