Stereoregular and stereoirregular alternating ethylene-norbornene copolymers

Ethylene-norbornene (E-N) copolymers were synthesized with the C2 metallocene rac-Me2Si(2-Me-[e]-benzindenyl)2ZrCl2 (3) and with the constrained geometry Me2Si(Me4Cp)(NtBu)TiCl2 (4) in the presence of methylaluminoxane. The E-N copolymerizations were carried out using a variety of monomer feed compositions. Copolymers were fully characterized by 13C NMR spectroscopy, gel permeation chromatography, and differential scanning calorimetry. Copolymer microstructures were analyzed in detail, through a procedure which accounts for the stoichiometric requirements of the copolymer chain as well as for the measured areas of 13C NMR signals. This analysis, which quantifies the differences in sequence distribution and tacticity of the polymers, evidenced that mainly alternating stereoregular and stereoirregular copolymers were prepared with 3 and 4, respectively. The copolymer prepared with 4 contains both meso and racemic NEN sequences and small amounts of meso and racemic NN diads, while the alternating copolymer prepared with 3 contains only meso NEN sequences and small amounts of meso NN diads. The formation of NN diads is disfavored with both catalysts. Surprisingly, a significant amount of norbornene (up to ~10%) belonging to NNN triads (T) is obtained with the C2 catalyst.