| The small molecule2-methyl-2-butene, with similar molecular chain structure to natural rubber (NR), was chosen as the reference model for the bromination of natural rubber. The aim is to simplify the research system, use modern means of analysis, systematically research the main factors of affecting the microstructure of small molecule brominated products, and explore the bromination mechanism of small molecules. This project research will help understand the bromination behavior of natural rubber from microcosmic angle, ascertain the bromination mechanism, especially the generation mechanism of crosslinking structure. After the bromination process and method was changed, we hope the structure of brominated natural rubber products can be effectively regulated, so as to improve its performance, and ultimately promoting its development and application in various fields.The bromination of small molecule2-methyl-2-butene from solution and latex was carried out respectively; furrier infrared spectroscopy, nuclear magnetic resonance and gas chromatography-mass spectrometry were used to characterize the microstructure of small molecule brominated products. The effects of bromine content, pH value and brominated temperature on the microstructure of small molecule brominated products were investigated, and the bromination mechanism was discussed.The results showed that, the bromination of2-methyl-2-butene small molecule from solution can be carried out immediately, and no induction period occurred. The purified brominated products of small molecule are colored transparent liquid, and its density increased with the increment of bromine content in general. When the molar ratio is1.25, the density of small molecule brominated product increases up to1.73g/ml. The effect of temperature on the color and density of small molecule brominated product has no obvious regularity.The analysis results of FTIR,1H-NMR and13C-NMR showed that the C=C double bond of small molecule can be reduced to C-C single bond and the bromide substitution occurred on the methyl hydrogen during the bromination process. With the increasing of the bromine content, the degree of C=C double bond reducing to C-C single bond and methyl hydrogen substitution by bromide both gradually increased. The C=C double bond was completely reduced to C-C single bond as the molar ratio was0.75. It was disadvantageous for the reduction of C=C double bond to C-C single bond when the temperature was reduced, but the substitution reaction of methyl hydrogen by bromide was little affected by temperature.The analysis results of GC-MS showed that the dibromo products were largely presented in the small molecule brominated products,3-methyl-1,3-dibromo-butane was69.43%when the molar ratio was0.25. The content of3-methyl-l-bromo-2-butene tends to decrease and disappear completely with the increasing of liquid bromine. Moreover, the tribromo products increased gradually and the monobromo products decreased with the increasing of liquid bromine. The content of dibromo product3-methyl-1,3-dibromo-butane decreased but the monobromo product of2-methyl-2-bromo-butane increased with increasing temperature. However, the content of tribromo products about2-methyl-1,2,4-tribromo-butane and3-methyl-1,2,3-tribromo-butane was little affected by the temperature change.The bromination of2-methyl-2-butene small molecule from solution is performed according to the free radical reaction mechanism. The reaction at the early stage was generated mainly of3-methyl-l-bromo-2-butene, but it was added with hydrogen bromide or it transferred to other products with the advance of reaction, which makes the content decreased gradually. Hydrogen bromide gas can participate in olefin Markovnikov addition reaction. With the increasing amount of bromine, the reaction system tends to generate multi-bromo products due to the increasing of bromine free radical concentration.The bromination of2-methyl-2-butene small molecule from latex can also be carried out instaneously, and no induction period occurred. The purified brominated products of small molecule are colored transparent liquid, and its density tends to increase with the increment of bromine content. When the molar ratio is1.5, the density of small molecule brominated product increases up to1.76g/ml. The acidity has a certain effect on the color and density of small molecule brominated product. The color of brominated product from acidic latex fades, and its density slightly decreases.The analysis results of FTIR showed that the reduction of C=C double bond of small molecule to C-C single bond and the bromide substitution on the methyl hydrogen did occur during the latex bromination process, and no aldehydes generated. With the increasing of the bromine content, the degree of C=C double bond reducing to C-C single bond and methyl hydrogen substitution by bromide both gradually increased. The degree for substitution reaction of methyl hydrogen by bromide decreased slightly under the acidic condition.The analysis results of GC-MS showed that the dibromo products from latex under the neutral condition were largely presented in the small molecule brominated products, but no monobromo products were detected.3-methyl-1,3-dibromo-butane was74.74%when the molar ratio was0.5. The content of dibromo products decreased and tribromo ones increased slightly with the increasing of liquid bromine. In addition, the acidic condition was more conducive to the formation of dibromo products, but the content of tribromo ones decreased accordingly. The bromination of small molecule from latex is also a free radical reaction process, which is specifically similar to the one from solution. But hydrogen bromide generated from the emulsion is easily soluble in water and tends to add with the monobromo alkene, thereby will increase the content of dibromo and tribromo products. In addition, no hypobromous acid was detected involving in the reaction process. |
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