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

Abstract

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 1 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.

Colloidal and rheological properties of natural rubber latex concentrated with hydroxyethyl cellulose and sodium dodecyl sulphate

“…(2) Interaction between HEC and NRL particles, also by hydrogen bonding interaction between polar of HEC NRL particle (especially with the α-terminal groups) 10 this gives rise to a depletion flocculation phenomenon due to weak attractions in particles that increases creaming velocity. Flocculation grade depends on low, medium, or high HEC concentration.…”

“…2,[5][6][7][8] Due to the negative charge of long-chain fatty acids, the ionization of carboxylic groups, and positive charges from amino groups of proteins, the whole system have a negative net charge. 2,9,10 Colloidal stability on NRL can be measured by means of zeta potential due to its electrostatic repulsions of their negative net charge. Normally, a colloidal system as NRL particles has a stability range between À30 and À70 mV, 11 being the most negative the most stable value.…”

Silk/Natural Rubber (NR) and 3,4-Dihydroxyphenylalanine (DOPA)-Modified Silk/NR Composites: Synthesis, Secondary Structure, and Mechanical Properties

“…This secondary structure change should contribute to the enhancement of the mechanical properties of DOPA-silk 20 /NR 80 , as well as the higher adhesion properties of the DOPA structure. The stronger interaction between DOPA and the hydrophilic α-terminus of NR [65] might also help to improve the mechanical properties of the formed composite, although no clear evidence for this speculation has been obtained so far. After the stretching deformation, the WAXD 1D profiles of silk 20 /NR 80 -2 and DOPA-silk 20 /NR 80 demonstrated the clear strain-induced crystallization of NR, according to previous studies and assignment [66,67].…”

The Role of Non-Rubber Components on Molecular Network of Natural Rubber during Accelerated Storage

“…On the basis of analyses mentioned above, we tentatively proposed a novel model for natural network structure of NR, as illustrated in Figure 4. NR molecules chains comprise of long-chain isoprene units, as well postulated that α-terminal links with mono-or di-phosphate groups that associate with phospholipids, whereas the ω-terminal is a dimethylallyl group that interacts with proteins [12,[43][44][45]. Meanwhile, these molecules chains could interact with each other via their terminal groups.…”