Characterization of Controlled-Distribution Hydrogenated Styrenic Block Copolymers by Nuclear Magnetic Resonance Spectroscopy

As the need for engineered soft materials continues to grow, block polymers possessing tailored chain architectures and chemical sequences are being upscaled from the laboratory bench to commercial production. Of particular interest here are controlled-distribution styrenic block copolymers (SBCs), i.e., SBCs with the terminal regions adjacent to the monoalkenylarene blocks having a greater than average fraction of conjugated diene units, or one or more regions not adjacent to the monoalkenylarene blocks with a greater than average fraction of monoalkenylarene units. Here, we are specifically interested in applying spectroscopic methods to differentiate their chemical microstructure and derive relevant chemistry-property relationships. In this spirit, we examine the chemical microstructures of fully hydrogenated polystyrene-b-polybutadiene-b-polystyrene and polystyrene-b-poly(styrene-co-butadiene)-b-polystyrene thermoplastic elastomers by a variety of nuclear magnetic resonance (NMR) techniques, including proton (1H) NMR, carbon-13 (13C) NMR, distortionless enhancement by polarization transfer (DEPT) NMR, and heteronuclear single quantum coherence (HSQC) NMR, and cross-correlate the resulting spectra. The impact of styrene in the midblock is clearly discernible from 13C NMR spectra, and precise carbon multiplicity assignments are made on the basis of DEPT NMR and 2D-HSQC NMR results. The information gleaned from 13C NMR, DEPT, and 2D-HSQC NMR spectroscopy provides unambiguous chemical assignments from possible triad and tetrad sequences. High-resolution 13C NMR spectra can thus be used as "fingerprints" to compare the chemical sequencing of different controlled-distribution hydrogenated SBCs and advance the identification of significant sequencing-property relationships in this class of tailored copolymers.