Multidimensional NMR Studies of Poly(ethylene-<i>co</i>-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 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

Synthesis, Solution Dynamics, and X-ray Crystal Structure of Bis(2,4,6-tris(trifluoromethyl)- phenyl)(1,2-dimethoxyethane)nickel

Reaction of dichloro(1,2-dimethoxyethane)nickel with 2 equiv of (tris(2,4,6-trifluoromethyl)phenyl)lithium, generated in situ, produced bis(2,4,6-tris(trifluoromethyl)phenyl)(1,2-dimethoxyethane)nickel. Significant nickel−fluorine interactions are revealed both in the solid state (X-ray crystal structure data) and in solution (variable-temperature 19F NMR spectral data)

Addition Polymerization of Norbornene-Type Monomers Using Neutral Nickel Complexes Containing Fluorinated Aryl Ligands

The strong Lewis acid B(C6F5)3 was found to activate complexes of nickel toward the polymerization of norbornene-type monomers. The active species in this reaction is created by the transfer of C6F5 from boron to nickel. As a result, a class of neutral, single-component nickel complexes was developed containing two electron-withdrawing aryl ligands that polymerize norbornene and norbornenes with functional pendant groups. Active complexes include Ni(C6F5)2(PPh2CH2C(O)Ph), (η6-toluene)Ni(C6F5)2, and Ni(2,4,6-tris(trifluoromethyl)phenyl)2(1,2-dimethoxyethane). In the case of (η6-toluene)Ni(C6F5)2, isolation and characterization of low molecular weight norbornene polymers, using ethylene, indicated that each polymer chain contained a C6F5 headgroup. This points to the initiation step as being the insertion of norbornene into the Ni−C6F5 bond. The polymer microstructure as revealed by 1H and 13C NMR spectrometry is entirely different from that produced using the cationic nickel catalyst, [(η-crotyl)Ni(1,4-COD)]PF6. This difference in microstructure led to improved mechanical properties for 80:20 copolymers of norbornene and 5-triethoxysilylnorbornene