Stereoregular and Stereoirregular Alternating Ethylene−Norbornene Copolymers

Ethylene−norbornene (E−N) copolymers were synthesized with the C 2 metallocene rac -Me 2 Si(2-Me-[ e ]-benzindenyl) 2 ZrCl 2 ( 3 ) and with the constrained geometry Me 2 Si(Me 4 Cp)(N t Bu)TiCl 2 ( 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 13 C 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 13 C 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 C 2 catalyst.