Structural Analysis of the Terminal Groups in Commercial Hevea Natural Rubber by 2D-NMR with DOSY Filters and Multiple-WET Methods Using Ultrahigh-Field NMR.

The terminal groups of natural rubber (NR) are widely believed to play a crucial role in defining the excellent mechanical and other physical properties of processed NR products. Despite their presumed importance, the chemical structures of the terminal groups are elusive in widely used NR species with a high degree of polymerization, such as Hevea natural rubber (H-NR). In previous studies, structural analysis by solution NMR has been carried out on the terminal units of NR after chemical treatment involving chemical alterations, such as deproteinization with enzymes and other chemicals. However, there is concern that such chemical treatments may alter the properties of the terminal units. In this study, we established an NMR-based approach to analyze the structures of the terminal units in commercial H-NR without any chemical treatments, or with only a mild treatment of some samples, such as acetone extraction for removing the impurities. To suppress the signals of low-molecular-weight impurities, we have developed methods combining DOSY-based diffusion filters with multiple-WET (MWET) 2D-NMR, which we introduced previously to suppress strong signals from main-chain of polymer and solvents (Tanaka et al. Macromolecules, 2016, 49, 5750-5754). Using the new method and MWET 2D-NMR methods with high-field NMR at a H frequency of 900 MHz, we observed NMR signals of the terminal units of chemically untreated commercial H-NR for the first time. The NMR results for eight commercial H-NR samples consistently demonstrated the presence of at least five kinds of terminating-end (α-terminus) units of the H-NR polymer chain in addition to NMR signals for the initiating-end (ω-terminus) units. Our NMR analyses revealed for the first time that none of the α-terminal groups form a phosphate ester.