We selected EHA as the amine for this reaction, because of the shorter chain and the flexibility originating from the branched structure. TDT and EHA were reacted for 4 h at room temperature under a nitrogen atmosphere, and the corresponding thiol was quantitatively produced in situ
via a nucleophilic ring-opening of the cyclic dithiocarbonate structure as reported. Then, Zn(OAc)2
(1.5 equivalents with respect to SH) in methanol was added, and reacted with the thiol generated in situ
for 24 h under a nitrogen atmosphere (Scheme 1
, Table 1
). For example, the reaction in DMSO with the polycondensation at 60 °C gave a corresponding polymer in a 45% yield (run 1). Whereas the product produced via the previous polycondensation using the purified thiol was insoluble in any common solvents [11
], the products in this one-pot method were soluble in THF, DMSO, and DMF. This improved solubility may be ascribed to the suppressed cross-linking with oxidative coupling of the thiol moieties. The 1
H-NMR and IR spectra were analogous to those of the soluble polymer obtained by the previous method. For example, the IR absorptions of the C=S bonds were observed at 1172 and 1168 cm−1
for the isolated thiol monomer and the polymer obtained by this one-pot method, respectively, indicating the ligation of the C=S moieties onto the zinc atom [11
]. The 1
H-NMR spectrum of the polymer obtained in run 1 is shown in Figure 1
with the peak assignments. Signals assignable to the 2-ethylhexyl groups, the ring-opened structure originated from TDT, and the NH moieties were observed. The integral ratio of the signals assigned to the two methyl protons in the 2-ethylhexyl groups to that assigned to the methyne proton in the ring-opened unit is 6.0/0.9, which almost agree with the theoretical value (6/1). Other signals were overlapped, and detailed calculation was difficult.
Various conditions were investigated for optimization. First, the effect of solvents was examined using DMSO, THF, and DMF (runs 1–3). The yields were higher with polar DMSO and DMF, and the molecular weight became highest with DMSO. The atom ratios of Zn/S were estimated by energy dispersive X-ray (EDX) analysis, and those of the products obtained in DMSO and THF almost agreed with the theoretical value (0.25). The Zn/S may contain 20% to 30% of errors in a similar manner with the previous products. This result indicates that DMSO is the best solvent among the examined solvents. Then, the temperature for the polycondensation process was varied. As the temperature increases, the polydispersity indices (Mw/Mn) became broadened, probably due to the cross-linking reaction, although the yields and the Zn/S ratios were identical (runs 4 and 5). We also examined the reaction using an excess amount of EHA for neutralization of the eliminated acetic acid to tilt the equilibrium to the product (run 6). As a result, the yield was improved, but the product became partially insoluble in THF. We presumed that the lower solubility originated from the accelerated oxidative coupling of the thiol moieties under basic conditions. These experiments show the appropriate conditions using DMSO as the solvent and the equivalent amount of EHA to the cyclic dithiocarbonate moieties and polycondensation at moderate 30 °C.
One-pot synthesis of organic-sulfur-zinc hybrid via sequential ring-opening addition of EHA and TDT and polycondensation of the in situ generated thiol with Zn(OAc)2.
H-NMR spectrum (400 MHz, 5:1 DMSO-d6
COOH) of the polymer obtained in run 1 in Table 1