二酸化窒素によるポリエチレンの酸化反応について.赤外線吸収スペクトルによって反応生成基の確認と反応機構についての研究を行なった.フィルム試料と二酸化窒素を封入した反応容器を温度調節したシリコン浴中に浸し,反応を行なった.反応後,フイルム試料を赤外線分光器によって4000～400cm^<-1>の範囲のスペクトルを測定した.酸化ポリエチレンに新しく出現した吸収帯の帰属を行なうために,いくつかの有機硝酸エステル,亜硝酸エステルなどを合成し,その赤外線吸収スペクトルを測定した.これらのスペクトルと,酸化反応後の試料フイルムを種々な条件で処理した際に生ずるスペクトル変化などの比較によって,ニトロ基,亜硝酸基,硝酸基,カルボニル基,水酸基の生成を確認した.以上の反応生成基のある一定温度(100℃)における量的な時間変化を測定すると,反応初期にはまずニトロ基と亜硝酸基が現われ,その生成量の比は約2:1である.ニトロ基は反応時間の経過と共に単調に増加するが,亜硝酸基はある時間後に極大値に達し,減少し始める.それと同時に硝酸基.カルボニル基,水酸基の吸収が現われ,増加し始める.これらの事実より,次の反応機構が結論される.酸化反応は,ポリエチレン内に生成した反応活性点に二酸化窒素のN原子が付加することによってニトロ基が,O原子が付加することで亜硝酸基が生成することから開始する.反応後期においては,ニトロ基は安定で,亜硝酸基が更に分解を受けて硝酸基,カルボニル基,水酸基などを生成する.反応初期に生成する反応活性点は,二酸化窒素によるポリエチレン主鎖からの水素原子の引き抜き反応によって生じた遊離基である.しかし,室温下の反応ではこの水素引き抜き反応は起らず,ポリエチレン内に残存する二重結合,遊離基などと二酸化窒素との付加反応が主反応である.水素引き抜き反応に対する測定された活性化エネルギーは,14kcal/moleであった.

The oxidative degradation of polyethylene has been studied by many researchers. But most of their works were made under the presence of oxygen or air and the similar studies under the atmosphere of the other oxidants such as nitrogen dioxide, nitric oxide and nitrous oxide were very rare. However, the oxidation reaction by these rather special oxidants is very important for the understanding of the combustion of rocket fuels. In this report, we have studied the oxidative degradation of polyethylene in nitrogen dioxide atmosphere. The process of chemical changes or the molecular structural changes of polyethylene were followed by the infrared spectrophotometry. A polymer film spread on a glass plate is put into a reaction vessel filled with a definite pressure of the purified nitrogen dioxide gas. Extreme cautions are taken to avoid the migration of oxygen and water vapor. This vessel is heated in a silicone bath and kept at 100±0.05℃. After a certain time, this specimen is picked out from the reaction vessel and the polymer film is separated from a glass support. Ordinarily, an infrared spectrum of this oxidized film is measured, but sometimes special such as the spectral measurement of a hot film and the rapid quenching procedures are also adopted to analyze the reaction process. For the application of the infrared spectrometry to the chemical problems, it is quite necessary to determine the assignment of the observed bands to the probable chemical groups as accurately as possible. However, unfortunately we have not enough data to discuss the absorption bands of nitrite and nitrate ester groups which are expected to be formed during the oxidation process by nitrogen dioxide. Then, some organic nitrite and nitrate esters are synthesized and their infrared spectra are also measured. Referring to these spectra, we can assign the most of the observed bands of the oxidized specimen. The formation of nitro, nitrite ester, nitrate ester, carbonyl and hydroxyl groups is identified. We have measured the intensity changes of these definitely assigned absorption bands with the variations of the experimental conditions, e.g., heating time, cooling method, partial pressure of nitrogen dioxide, reaction temperature, etc. In the early stage of the reaction, the absorption bands of nitro and nitrite ester groups are observed and their intensities increase with reaction time. But the absorption intensity of nitrite ester group reaches a maximum value after a certain reaction time and begins to decrease. In the same time, the absorption intensities of nitrate ester, carbonyl and hydroxyl groups become larger. On the basis of these findings, we have concluded that nitro and nitrite ester groups are formed by the addition of nitrogen dioxide to the activated sites of polyethylene in the first step of the reaction, and that lately some of these groups, especially most of nitrite ester group are changed to nitrate ester, carbonyl and hydroxyl groups by the process similar to the decomposition. In the last section, the formation of the activated sites of polyethylene by nitrogen dioxide has been discussed. Since nitrogen dioxide has an odd electron and is a highly reactive molecule, it abstracts hydrogen atom from polyethylene chain and forms -CH_2-CH-CH_2- radical in the temperature range higher than 80℃. The radical formed undergoes the association reaction with nitrogen dioxide to produce nitro and nitrite ester groups. At room temperature, however, the aforementioned hydrogen abstraction reaction does not occur, and nitro and nitrite ester groups are formed by the addition of nitrogen dioxide to double bonds or some structural irregularities in the polyethylene specimen used here. The activation energy of the hydrogen abstraction reaction by nitrogen dioxide is found to be about 14 kcal/mole.

資料番号: SA4135039000