Multidimensional NMR Studies of Poly(ethylene-co-1-butene) Microstructures

The microstructures, including tacticity (triad and tetrad stereoconformation) and comonomer sequence distribution, of poly(ethylene- co -1-butene) copolymer with different 1-butene contents were investigated by high-temperature (120 °C) two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy at 750 MHz. The microstructures of these copolymers were analyzed by the combination of different NMR techniques including quantitative 13 C NMR, 13 C DEPT (distortionless enhancement by polarization transfer), pulsed-field-gradient (PFG) 1 H− 13 C heteronuclear single quantum coherence (gHSQC), and heteronuclear multiple bond coherence (gHMBC). High temperature, along with multidimensional PFG NMR, facilitates the study of poly(ethylene- co -1-butene) copolymers by improving the resolution of resonances which otherwise have short T 2 (spin−spin) relaxation at ambient temperature. The combined information from 2D 1 H− 13 C gHSQC, gHMBC, and quantitative 13 C NMR experiments provided unambiguous resonance assignments from triad, tetrad, and, in a few cases, pentad comonomer sequence distributions of poly(ethylene- co -1-butene) with improved resolution in the regions of the spectrum containing the resonances of structures associated with ethyl branches. The copolymer with 41% 1-butene content synthesized using the metallocene catalyst system [(C 5 Me 4 )SiMe 2 N( t -Bu)]TiMe 2 /MAO shows a mixture of both meso and racemic diads with predominantly racemic configuration, while another commercially available copolymer with 12% 1-butene content shows predominantly meso diads. Quantitative analysis of comonomer sequence distributions was determined from 13 C NMR data analysis.