This paper reviews current understanding of the role of free radicals in the oxidation of polyethylene induced by high-energy irradiation (gamma or e-beam). To evaluate the reactivity, stability and mobility of different macroradicals, their reactions after gamma irradiation of different polyethylenes (LDPE, LLDPE, VLLDPE, HDPE, UHMWPE) are considered. Macroradicals are formed in all phases of PE (crystalline, amorphous and interphase). Their overall determination is possible only if both irradiation and detection are carried out at 100 K, or below. At this temperature, the most abundant macroradical formed is the secondary alkyl macroradical (R’’°). On gradually raising the temperature, the macroradicals decay with formation of vinylene double bonds and molecular hydrogen. At room temperature, the macroradical concentration is about 4% of the original quantity at 100 K for LDPE and around 15% for HDPE. In HDPE and UHMWPE; the macroradicals are mainly present in the crystalline phase and in short times (hours) they migrate to the amorphous phase. In LDPE and LLDPE macroradicals are mainly allylic, present in minimal amounts at the crystalline-amorphous interphase. In the nearly fully amorphous VLDPE no residual macroradicals can be detected at room temperature.

The mobility of R’’° is variable and is a function of the mobility of the polymer backbone, slow in the crystalline phase, relatively fast in the amorphous phase. Kinetic stability, or persistence, is often more important than thermodynamic stability in determining radical lifetimes, in particular for radical processes in the solid phase. Alkyl macroradicals react with chain imperfections, additives and oxygen, in a cyclic process involving initiation, propagation and termination reactions. The initiation reactions form macroradicals by cleavage of the C-H bonds of PE, induced by irradiation or by photo- or thermal-scission of peroxides formed during processing of the polymer. Propagation involves the reaction of R’’° with vinyl or vinylidene double bonds, with the formation of a new radical, in competition with their reaction with oxygen to form various oxidation products (ketones, hydroperoxides acids, alcohols and esters). It is notable that the formation of ketones does not necessarily require decomposition of the hydroperoxides. In the presence of stabilizing additives, radicals react with the additive (ADH), with the formation of a more kinetically stable radical (AD°), which considerably decreases the propagation rate, but a termination reaction between R’’° and AD° may also be envisaged.

It is observed that radiation-induced oxidation has a constant rate during irradiation. Post-irradiation, the oxidation occurs via transfer of the macroradicals from the crystalline phase and the interphase to the amorphous phase, where oxygen is available, and the rate decreases, approaching zero. The occurrence of termination is apparent. Termination must occur through the reaction of two macroradicals. The reaction between two R’’° is difficult, due to steric hindrance. The most probable reaction is that between peroxy macroradicals (ROO°), fixed in position on the polymer chain and R’’°, which migrate through the polymer bulk. This reaction is difficult to confirm experimentally because of the lack of reliable analytical methods for ROOR species in the presence of ROOH.

The formation reactions of the different oxidation products are reported and the branching reactions occurring in thermal and photo-oxidation are also discussed.

本文回顾了当前对自由基在高能辐照（γ或电子束）诱导的聚乙烯氧化中的作用的理解。为了评估不同大基团的反应性，稳定性和迁移率，考虑了它们在不同聚乙烯（LDPE，LLDPE，VLLDPE，HDPE，UHMWPE）进行γ射线辐照后的反应。在PE的所有相（结晶相，非晶相和中间相）中均形成大自由基。仅当在100 K或更低的温度下进行照射和检测时，才有可能对它们进行总体确定。在此温度下，形成的最丰富的大自由基是仲烷基大自由基（R''°）。随着温度的逐渐升高，大分子自由基随着亚乙烯基双键和分子氢的形成而衰减。在室温下，对于LDPE，在100 K时，宏观自由基浓度约为原始量的4％，对于HDPE，约为15％。在HDPE和UHMWPE中；大自由基主要存在于结晶相中，并在短时间内（数小时）迁移至非晶相。在LDPE和LLDPE中，大基团主要是烯丙基的，在结晶-非晶相间存在的量很少。在几乎完全非晶的VLDPE中，在室温下无法检测到残留的大自由基。

R”°的迁移率是可变的，并且是聚合物主链迁移率的函数，在结晶相中较慢，在非晶相中相对较快。在确定自由基寿命时，尤其是对于固相中的自由基过程，动力学稳定性或持久性通常比热力学稳定性更重要。烷基大自由基与链缺陷，添加剂和氧在涉及引发，扩散和终止反应的循环过程中发生反应。引发反应通过辐照或通过在聚合物加工过程中形成的过氧化物进行光致断裂或光致断裂而引起的PE的CH键断裂而形成大自由基。传播涉及R''°与乙烯基或亚乙烯基双键的反应，并形成新的自由基，与它们与氧气反应形成各种氧化产物（酮，氢过氧化物酸，醇和酯）竞争。值得注意的是，酮的形成并不一定需要氢过氧化物的分解。在稳定添加剂存在下，自由基与添加剂（ADH）反应，形成动力学上更稳定的自由基（AD°），这大大降低了传播速率，但R''°和AD°之间的终止反应可能也可以设想。

可以看出，辐射诱导的氧化在辐照期间具有恒定的速率。辐照后，氧化是通过将大自由基从结晶相和中间相转移到无定形相（其中有氧气）而发生的，并且速率降低，接近零。终止的发生是显而易见的。终止必须通过两个宏观基团的反应而发生。由于空间位阻，两个R”°之间的反应很困难。最可能的反应是在聚合物链上固定的过氧大自由基（ROO°）与R''°之间发生反应，后者通过聚合物本体迁移。由于缺乏在存在ROOH的情况下对ROOR物质的可靠分析方法，因此难以通过实验确认该反应。

报告了不同氧化产物的形成反应，并讨论了在热氧化和光氧化中发生的支化反应。