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

Abstract

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 13C NMR, 13C DEPT (distortionless enhancement by polarization transfer), pulsed-field-gradient (PFG) 1H−13C 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 T2 (spin−spin) relaxation at ambient temperature. The combined information from 2D 1H−13C gHSQC, gHMBC, and quantitative 13C 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 [(C5Me4)SiMe2N(t-Bu)]TiMe2/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 13C NMR data analysis.