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

As the need for engineeredsoft materials continues togrow, blockpolymers possessing tailored chain architectures and chemical sequencesare being upscaled from the laboratory bench to commercial production.Of particular interest here are controlled-distribution styrenic blockcopolymers (SBCs), i.e., SBCs with the terminal regions adjacent tothe monoalkenylarene blocks having a greater than average fractionof conjugated diene units, or one or more regions not adjacent tothe monoalkenylarene blocks with a greater than average fraction ofmonoalkenylarene units. Here, we are specifically interested in applyingspectroscopic methods to differentiate their chemical microstructureand 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 magneticresonance (NMR) techniques, including proton (H-1) NMR,carbon-13 (C-13) NMR, distortionless enhancement by polarizationtransfer (DEPT) NMR, and heteronuclear single quantum coherence (HSQC)NMR, and cross-correlate the resultingspectra. The impact of styrene in the midblock is clearly discerniblefrom C-13 NMR spectra, and precise carbon multiplicity assignmentsare made on the basis of DEPT NMR and 2D-HSQC NMR results. The informationgleaned from C-13 NMR, DEPT, and 2D-HSQC NMR spectroscopyprovides unambiguous chemical assignments from possible triad andtetrad sequences. High-resolution C-13 NMR spectra can thusbe used as "fingerprints" to compare the chemical sequencingof different controlled-distribution hydrogenated SBCs and advancethe identification of significant sequencing-property relationshipsin this class of tailored copolymers.