To study the network structure generated by the polysulfide polymer [[(CH2)2S4]p as a crosslinking agent, model compound vulcanization with 2,3-dimethyl-2-butene as the model alkene was carried out. It was found that the polysulfide polymer generates hybrid bridges of the general constitution Sn−(CH2CH2−Sm)k(n=1−4,m=1−4,k=1−6). To a lesser extent the polysulfide polymer behaves as a sulfur donor generating conventional sulfur bridges Sx(x=1−5). Polysulfidic bridges (x=4, 5) were only detected in the very early stage of network formation. The process of network formation and the development of the bridges during prolonged heating was monitored by 1H NMR spectroscopy and compared with that of a sulfur cure. In the case of the polysulfide polymer cure the early network contained a relatively high amount of mono-and disulfidic bridges and a stable network was formed after at least 60 min of heating (150 °C). This effect results from the higher thermal stability of hybrid bridges in comparison to conventional sulfur bridges. The higher stability of the hybrid bridges also leads to a suppression of the formation of 3,4-dimethylthiophene which has been identified in the sulfur cure as a product of reversion processes. Thus, the increased reversion stability of polysulfide polymer cures, which has been claimed earlier, can be related to the formation and the increased stability of the hybrid bridges.
Model compound vulcanization in combination with reversed-phase high-performance liquid chromatography (RP-HPLC), NMR spectroscopy, and mass spectrometry was used to study the properties of the polysulfide polymers [Sx(CH2)y]p (x=2,3,4, y=1,2,3,6) as crosslinking agent in the vulcanization process. It was found that the tetrasulfidic polymers [S4(CH2)y]p are capable of forming hybrid bridges of general constitution Sn-[(CH2)y-Sm]k. Besides, they react as sulfur donors producing ordinary sulfur bridges. The yield of hybrid bridges strongly increased when the alkanediyl unit ((CH2)y) of the polymer was changed from methanediyl (y =1) to ethanediyl (y =2) and remained nearly constant for longer alkanediyl segments (y =3, 6). In general, the repeating unit k also increased with increasing length of the alkanediyl segment. The only exception was observed for poly(tetrathiopropanediyl) [S4(CH2)3]p. Compared to [S4(CH2)2]p it gave a slightly lower portion of hybrid bridges and also the maximum value of k appeared to be somewhat reduced. When the number of sulfur atoms in the poly(polythioethanediyl) polymers [Sx(CH2)2]p(x=2–4 was reduced from 4 to 3 the yield of bridged molecules diminished slightly. But in the case of the disulfidic polymer nearly no crosslinked products were observed. The disulfidic polymer could be activated by the addition of sulfur into the vulcanization mixture. Hybrid bridges Sn-[(CH2)6-Sm]k of low k-values were obtained when 1,6-bis-(N,N-dibenzyl-thiocarbamoyldisulfido) hexane was used as curing agent. In addition to RP-HPLC, we also used gel permeation chromatography (GPC) in the characterization of the vulcanizates. In contrast to RP-HPLC, where the number of sulfur atoms has the strongest influence on the retention time, GPC separates the vulcanization products according to the number k of alkanediyl units present in the hybrid bridge.
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