Sulfur-crosslinked nitrile butadiene rubber (s-NBR) was found to be devulcanized when it was heated with nitrobenzene at 200°C for 3 h. The tetrahydrofuran (THF)-soluble fraction from s-NBR heated with nitrobenzene was purified by reprecipitation with THF/n-hexane, chloroform/ n-hexane, and THF/n-hexane systems and was then characterized by means of Fourier transform infrared (FTIR) spectroscopy, 1 H-NMR, gel permeation chromatography, dynamic thermogravimetry/differential thermal analysis (DTA), and differential scanning calorimetry (DSC). FTIR and 1 H-NMR results revealed that the THF-soluble fraction contained aromatic rings derived from nitrobenzene. Furthermore, the molecular weight of the THF-soluble fraction was much lower than that of the parent noncrosslinked poly(acrylonitrile-cobutadiene). Although the weight loss of THF-soluble fraction began at a lower temperature than that of the nonheated original nitrile butadiene rubber, the residual weight at 700°C tended to be higher for the former. This tendency became more marked with increasing time of heat treatment with nitrobenzene. The DSC-determined glass-transition temperature of the THF-soluble fraction was higher than that of the original s-NBR. To elucidate the devulcanization mechanism, we investigated two types of model reactions; one was the reaction of diphenyl disulfide with nitrobenzene, and the other was the reaction of polybutadiene with nitrobenzene. The former reaction, carried out at 250°C in diphenyl ether, yielded diphenyl sulfide with a loss of diphenyl disulfide and nitrobenzene. The use of a higher molar ratio of nitrobenzene to diphenyl disulfide resulted in a depression of diphenyl sulfide formation. The reaction of p-chloronitrobenzene with diphenyl disulfide also gave diphenyl sulfide. The reaction of polybutadiene with nitrobenzene at 200°C resulted in the backbone scission of the polymer. The THF-soluble solid product of the latter model reaction was found by FTIR and 1 H-NMR to contain aromatic rings derived from nitrobenzene. The devulcanization mechanism is discussed on the basis of a comparison of the results of the model reactions with those of the s-NBR devulcanization.

The crosslinking reaction of 1,2‐polybutadiene (1,2‐PB) with dicumyl peroxide (DCPO) in dioxane was kinetically studied by means of Fourier transform near‐infrared spectroscopy (FTNIR). The crosslinking reaction was followed in situ by the monitoring of the disappearance of the pendant vinyl group of 1,2‐PB with FTNIR. The initial disappearance rate (R0) of the vinyl group was expressed by R0 = k[DCPO]0.8[vinyl group]−0.2 (120 °C). The overall activation energy of the reaction was estimated to be 38.3 kcal/mol. The unusual rate equation was explained in terms of the polymerization of the pendant vinyl group as an allyl monomer involving degradative chain transfer to the monomer. The reaction mixture involved electron spin resonance (ESR)‐observable polymer radicals, of which the concentration rapidly increased with time owing to a progress of crosslinking after an induction period of 200 min. The crosslinking reaction of 1,2‐PB with DCPO was also examined in the presence of vinyl acetate (VAc), which was regarded as a copolymerization of the vinyl group with VAc. The vinyl group of 1,2‐PB was found to show a reactivity much higher than 1‐octene and 3‐methyl‐1‐hexene as model compounds in the copolymerization with VAc. This unexpectedly high reactivity of the vinyl group suggested that an intramolecular polymerization process proceeds between the pendant vinyl groups located on the same polymer chain, possibly leading to the formation of block‐like polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4437–4447, 2004

ABSTRACT:The crosslinking reaction of liquid carboxylated poly(acrylonitrile-co-butadiene) [or nitrile rubber (NBR); acrylonitrile ϭ 10 wt %] with dicumyl peroxide (DCPO) was studied in dioxane by means of Fourier transform near-infrared spectroscopy (FT-NIR) and electron spin resonance spectroscopy (ESR). Among the three butadiene units (1,2, cis-1,4, and trans-1,4 units) of NBR, only the pendant vinyl group of the 1,2 unit showed an absorption at 6110 cm Ϫ1 from the FT-NIR examination of dioxane solutions of NBR, 1-octene, 3,3-dimethyl-1-butene, trans-2-octene, cis-5-octen-1-ol, poly-cis-1,4-butadiene, and poly-1,2-butadiene. The crosslinking reaction was followed in situ in dioxane by the monitoring of the disappearance of the pendant vinyl double bond with FT-NIR. The initial disappearance rate (R 0 ) of the vinyl group was expressed by R 0 ϭ k [DCPO] 0.9 [NBR] Ϫ0.2 (120°C). The overall activation energy of the reaction was calculated to be 20.7 kcal/mol. This unusual rate equation suggests unimolecular termination due to degradative chain transfer and depressed reactivity of the vinyl group caused by crosslinking. ESR study of the reaction mixture revealed that an allyl-type polymer radical was formed in the reaction, and its concentration increased with time and was then saturated.