Abstract
A structural study of epoxidized natural rubber (ENR-50) and its cyclic dithiocarbonate derivative was carried out using NMR spectroscopy techniques. The overlapping 1H-NMR signals of ENR-50 at δ 1.56, 1.68–1.70, 2.06, 2.15–2.17 ppm were successfully assigned. In this work, the 13C-NMR chemical shift assignments of ENR-50 were consistent to the previously reported work. A cyclic dithiocarbonate derivative of ENR-50 was synthesized from the reaction of purified ENR-50 with carbon disulfide (CS2), in the presence of 4-dimethylaminopyridine (DMAP) as catalyst at reflux temperature. The cyclic dithiocarbonate formation involved the epoxide ring opening of the ENR-50. This was followed by insertion of the C–S moiety of CS2 at the oxygen attached to the quaternary carbon and methine carbon of epoxidized isoprene unit, respectively. The bands due to the C=S and C–O were clearly observed in the FTIR spectrum while the 1H-NMR spectrum of the derivative revealed the peak attributed to the methylene protons had split. The 13C-NMR spectrum of the derivative further indicates two new carbon peaks arising from the >C=S and quaternary carbon of cyclic dithiocarbonate. All other 1H- and 13C-NMR chemical shifts of the derivative remain unchanged with respect to the ENR-50.
Highlights
Epoxidized natural rubber (ENR) is a modified natural rubber (NR) [1]
These overlapping signals may be due to the methylene protons of the ENR-50 chain
The overlapping 1H-NMR signals of purified ENR-50 at δ 1.56, 1.68–1.70, 2.06, 2.15–2.17 ppm was successfully assigned to their respective triad sequences using the 2D NMR; heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond coherence (HMBC) and correlation spectroscopy (COSY) techniques
Summary
Epoxidized natural rubber (ENR) is a modified natural rubber (NR) [1]. A typical formation of ENR from NR, cis-1,4-isoprene, employing peracetic acid is shown in Scheme 1a [2]. The isoprene (C) and epoxidized isoprene (E) act as monomer units that are randomly distributed along the polymer chain [1]. Various degree of epoxidation of NR is commercially available. The isoprene units in the polymer chain are 25%, 50% and 75% epoxidized in ENR-25, ENR-50 and ENR-75, respectively. For the purpose of nomenclature, the general structure and the numbering of carbon atoms in ENR-50 is shown in Scheme 1b
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