Oxidative degradation mainly includes thermal oxidative degradation, photooxidative degradation and biological oxidative degradation. Different oxidative degradation methods can be used alone or in series to achieve the best degradation effect^[9].

Thermal oxidative degradation can be divided into gas-solid reaction and liquid-solid reaction according to different reaction media. Gas-solid reaction refers to the oxidation reaction of polyolefin waste plastics by heating in the atmosphere of oxidizing gas (such as air, oxygen, nitrogen oxides, etc.). Different from the thermal cracking reaction, the temperature required for thermal oxidation degradation is lower, generally lower than 300 ℃^{[10⇓⇓~13]}. Liquid-solid reaction is a process of oxidative degradation of polyolefin waste plastics in oxidizing liquids such as acid, alkali and peroxide at high temperature. The microwave-assisted process can make the heating more uniform and increase the chemical reaction rate, thus accelerating the thermo-oxidative degradation process, and the introduction of oxygen element can convert polyolefin into various compounds with higher added value^[14]. In 2017, Hakkarainen et al. Used medium-concentration nitric acid as an oxidant and microwave irradiation to promote the complete degradation of LDPE. The degradation product is mainly dicarboxylic acid with succinic acid as the main component. The degradation product can be used to further synthesize plasticizers with high added value to realize the upgrading and recycling of polyethylene^[15]. The present study shows that the thermo-oxidative degradation process of polyolefin is a free radical reaction process (Fig. 1)^[13]. The tertiary carbon-hydrogen bond in polyolefins is the most active because of its small bond energy, and it is easy to be heated by oxidants to form peroxyl radicals by abstracting hydrogen atoms to form tertiary carbon-centered radicals, and then to form peroxyl radicals. The peroxyl radical can further abstract other hydrogen atoms from the polyolefin chain to form a peroxide group. The O — O bond of the peroxide group is then cleaved to form a ketone, and the carbon atom near the ketone is activated to promote the cleavage of the C — C bond.

Biological oxidative degradation refers to the degradation of polyolefin waste plastics into small molecules under the action of bacteria, fungi, algae, etc. Bacteria and fungi first adhere to the surface of polyolefin plastics to form a layer of biofilm. The oxidase released by microorganisms will oxidize the surface of polyolefin materials, and the temperature, humidity, ultraviolet radiation and pH of the external environment will accelerate the biological corrosion process^[16]. Oxidases further attack specific chemical bonds to produce intermediates such as long-chain aliphatic alkanes and alkenes, which are further degraded by microorganisms through metabolism until they are mineralized into small molecules such as carbon dioxide and water. This process is very slow and generally synergizes with other degradation methods to achieve better degradation effect.

Photooxidative degradation is a radical reaction process. The polymer absorbs ultraviolet rays with enough energy to break the chemical bonds in the polymer chain and generate hydroperoxidation groups, and then generate intermediate products containing carbonyl and vinyl groups. The carbonyl group is sensitive to light and promotes the main chain to undergo β-chain scission and gradual degradation^[17]. Free radicals with strong oxidizing properties, including · OH, {{\mathrm{·O}}_2}^{-}, H₂O₂, etc., were generated on the surface of the photosensitive catalyst after illumination, which accelerated the photooxidation degradation process (Fig. 2). Generally speaking, the density, branching degree and tertiary carbon content of polyolefin materials have a great influence on the photodegradation rate: the higher the density of polyolefin materials, the smaller the branching degree, the smaller the gas permeability, and the slower the photooxidation degradation rate; The energy of the carbon-hydrogen bond of the tertiary carbon is small, so it is easy to be oxidized, so the higher the content of the tertiary carbon, the faster the photooxidation rate. In addition, defects in the polymer chain can also accelerate photooxidation.

Although polyolefin plastics can degrade in nature, this process is very slow. With the increasing demand for disposable plastic packaging, it is particularly important to accelerate the degradation of polyolefin waste plastics. The oxidation process of polyolefin plastics can be accelerated by adding degradation AIDS. There are two types of degradation AIDS: transition metal salts and non-transition metal salts. Common transition metals include iron, which accelerates photooxidation, and manganese and cobalt, which accelerate thermal oxidation. The non-transition metal salt system introduces specific groups such as carbonyl, unsaturated alcohol or ester, etc., or adds additives such as benzophenone, beta-diketone, etc., into the polyolefin material to promote the degradation of the polyolefin^[18].

Condensation polymers of long-chain functional monomers are different from their short-chain homologues in solid state structure and properties due to their longer methylene sequences, which are between traditional condensation polymers and polyethylene^[20]. At present, there is no precise definition of "long chain", and it is generally believed that long chain monomers are composed of 14 or more carbon atoms. In general, the biodegradability of long-chain functional monomer polycondensates decreases with the increase of the chain length between functional groups, which is in conflict with their physical properties such as crystallinity, hydrophobicity, and mechanical properties. In order to balance the biodegradability and polyethylene-like properties of long-chain polycondensates, it is essential to carefully design their chain structure.

Long chain aliphatic polyester is a kind of polymer with biodegradable potential, which has the structure and properties similar to polyethylene. Mecking et al. Extracted long-chain 1,18-octadecanediol from vegetable oil raw materials to obtain renewable long-chain polycarbonate and polyester by polycondensation. The low-density functional groups in the chain can be used as the breaking point of the polyethylene chain, which can be recovered by the chemical method of solvent dissolution, with a recovery rate of more than 96%^[21]. At the same time, the functional groups in the chain have little effect on the solid-state structure and properties of the obtained polymer, which is comparable to the properties of commercial HDPE. Wu et al. Synthesized a series of long-chain polyester {{\mathrm{PE}}_{\mathrm{s}}}_{xy} (X = 2 – 4,6, y = 10 – 16) by melt polycondensation from C_2-4,6 long-chain α, ω-diols and {{{\mathrm{C}}_{10}}_{~}}_{16} long-chain α, ω-diacids (Fig. 3)^[22]. It is found that their crystallization temperature and melting temperature increase with the increase of diacid chain length, and they have obvious odd-even effect. Their crystallinity, tensile modulus, tensile strength, ductility and oxygen barrier properties are equivalent to those of LLDPE. Huang et al. Prepared degradable high molecular weight polymers by melt polycondensation or solid phase polycondensation of existing polymers containing reactive functional groups at the end group or in the side group of the chain^[23]. The obtained copolymer has excellent performance, functional groups such as ester groups, amide groups and the like can be used as degradation sites, and the obtained long-chain degradation product can be subjected to polycondensation again to realize closed-loop recovery.

In addition to polyesters, monomers such as long-chain acetals and phosphate esters can also be polycondensed to obtain degradable polyolefin analogs, which retain many of the characteristics of PE but have poor environmental durability^[20]. Compared with polyethylene, long-chain polyacetals can be synthesized from sustainable biomass and have hydrolytic degradability, showing great potential^[24,25]. At the same time, the physical properties of polyacetals are closely related to their crystallization properties, so it is very important to pay attention to the crystallization behavior of polyacetals with different chain lengths to control their physical properties (polyethylene-like properties).

Alamo et al. Studied the rapid melt crystallization behavior of polyacetals with precise spacing of 12, 18, 19, and 23 methylene backbone carbons^[26]. It was found that the polyacetal crystal showed two reorganizations and structural transitions at a low heating rate. In addition to polyacetal, polyphosphate is also water-degradable or enzyme-degradable, and its similarity to nucleic acid makes it biocompatible, which is an important class of polymers for degradable polyolefin analogues. Mecking et al. Used bio-based long-chain diols as raw materials to esterify dimethyl phosphate with bio-based long-chain diols and further hydrogenate to obtain aliphatic polyphosphate esters. Experiments showed that with the increase of methylene sequence length, the crystallinity and melting point increased significantly (the highest T_m=110℃)^[27]. The degradation rate decreased with increasing length of the methylene sequence.

The non-polar chain structure of polyolefin inhibits its compatibility with other materials, and the compatibility of polyolefin can be improved by copolymerization of olefin monomer with other polar monomers. Previous copolymerization methods often introduce functional groups into the molecular chain end or molecular side chain, which can not insert effective degradation sites into the polymer molecular chain. Recently, Coates et al. Developed a new catalyst which can effectively control the selective copolymerization of propylene and butadiene, and the obtained high molecular weight unsaturated isotactic propylene copolymer underwent olefin metathesis reaction with 2-hydroxyethyl acrylate.Finally, the degradable polypropylene material is obtained by polymerization, the polymer shows thermodynamic properties equivalent to those of linear low density polyethylene, and the far helical macromonomer generated after depolymerization can be effectively chemically recycled^[28].

The copolymerization of ethylene and CO can introduce carbonyl groups into the main chain of polyethylene, and the carbonyl groups are sensitive to light, so the obtained copolymer can undergo Norriish II photochemical degradation reaction under the illumination with the wavelength of more than 340 nm. Branched low-density polyketones obtained by radical copolymerization of ethylene and CO were developed in the 1950s^[29,30]. Later, Drent et al. Obtained linear E/CO alternating copolymers using coordination-insertion copolymerization of group 10 metal complexes^[31]. In 2002, Drent et al. Synthesized non-alternating copolymers of E/CO with carbonyl content up to 40.8 mol% using a palladium catalyst containing a phosphine-sulfonic acid ligand (P/S)^[32]. The structure and properties of high carbonyl content copolymer are far from those of polyethylene, so it can not be used as a substitute for polyethylene. It is of great significance to develop polyethylene materials with low carbonyl content to retain the material properties of polyethylene to the maximum extent, while endowing the materials with more properties, such as photodegradability. In the later study of Sen et al., the carbonyl insertion rate could be reduced to 1.5 mol% by increasing the ratio of ethylene to CO in the monomer. However, due to the formation of strong chelates after CO insertion, CO and ethylene were continuously and alternately inserted, which were degraded into polymer fragments with higher molecular weight after light irradiation, and the subsequent degradation process was slow^[33]. Therefore, it is of great significance to reduce the insertion rate of carbonyl groups in polyethylene materials, and to make the carbonyl units uniformly dispersed in the polymer chain, so as to maximize the degradation of the polymer into smaller molecular weight fragments. Recently, Mecking et al. Reported that nickel (Ⅱ) complexes coordinated by phosphonophenolate could catalyze the non-alternating copolymerization of ethylene and carbon monoxide to obtain high molecular weight polyethylene with low density of isolated intrachain carbonyls^[34]. Exposure of the ketone-PE film to simulated sunlight conditions confirmed its photodegradability. Nozaki et al. Synthesized linear high molecular weight polyethylene chains by palladium-catalyzed copolymerization of ethylene and metal carbonyls^[35]. Unlike the conventional ethylene/CO copolymerization, this reaction has excellent non-alternating selectivity. The copolymer retains the properties of polyethylene, and the degradation rate of the copolymer under ultraviolet irradiation is faster than that of polyethylene. Therefore, the synthesized material can be used as a more environmentally friendly plastic alternative than the traditional polyethylene material.

In 1987, Schrock et al. Found that tungsten and molybdenum catalysts could be used to catalyze olefin metathesis to synthesize high molecular weight and low dispersion polyolefins^[36,37]. Subsequently, Grubbs et al. Synthesized ruthenium-containing late transition metal catalysts, which improved the tolerance of olefin metathesis to functional groups and greatly expanded the types of functionalized polyolefins^[38]. Grubbs et al. Further optimized the ligand of the catalyst, which not only improved the tolerance of the catalyst to water and oxygen, but also accurately controlled the molecular weight of the polymer^[39⇓~41]. Olefin metathesis is broadly divided into two types: acyclic diene metathesis polymerization (ADMET) and ring-opening metathesis (ROMP).

ADMET is obtained by the polycondensation of α, ω-dienes, during which the removal of ethylene shifts the equilibrium toward the product^[42]. Due to the characteristics of ADMET stepwise polymerization, high purity monomer is required for the polymerization reaction. Early studies on the ADMET reaction focused on symmetric α, ω-dienes, through which the spacing between pendant functional groups can be precisely controlled^[43]; Recently, the range of monomers suitable for ADMET has been extended to α, ω-dienes containing heteroatoms (Table 1), and new α, ω-dienes connected by heteroatoms have been designed and synthesized, which can be used to synthesize polyethylene-like materials with more degradable functional groups through ADMET reaction^[44⇓~46].

Long-chain aliphatic polyesters have been widely studied because of their excellent mechanical properties and biodegradability^[47]. Aliphatic polyesters are usually synthesized by metal/organocatalyzed ring-opening polymerization of lactide/lactone or free radical ring-opening polymerization of polyketal. ADMET reaction greatly expands the structural diversity of aliphatic polyesters with different physicochemical properties^[54,55]. The thermal and degradation properties of the polyester can be further adjusted by the post-polymerization modification reaction. Li et al. Reported the synthesis, functionalization, and controlled degradation of high molecular weight polyesters based on itaconic acid, and 10-undecenol (Figure 4A)^[48]. In this work, the ADMET reaction was used to provide a more convenient strategy for the synthesis of high molecular weight polyesters, and the Michael addition reaction of different monomers such as benzyl mercaptan, dodecyl mercaptan and piperidine with itaconic acid vinyl was used to modify the polymer and endow the polyester with high temperature degradability. Similarly, in 2016, Li et al. Reported a new method for the synthesis of thermally degraded polyesters by post-polymerization modification of unsaturated aliphatic polyesters via Michael addition reaction^[49]. Nevertheless, the above studies have not examined the performance of the polymer, and whether it has similar comprehensive properties as polyolefin materials and thus becomes a suitable substitute for polyolefin still needs further study. Through metathesis polymerization of D-xylose derivatives, followed by post-modification reactions such as olefin hydrogenation, the mechanical properties and gas barrier properties of the polymer can be comparable to those of commercial polyolefins (Figure 4B)^[50]. From a system point of view, an ideal circular economy should meet two conditions: first, raw materials are abundant and can be obtained from nature; Secondly, the comprehensive performance of the material can be comparable to that of traditional polyolefin materials, and can be degraded into recyclable small molecules. This paper provides us with a polymerization method of competitive materials from sustainable raw materials, but the degradation and recycling performance of the materials has not been investigated, and whether the degradation products can be recycled or polymerized again to complete the recycling system still needs further study.

With the development of ruthenium-containing late transition metal catalysts and heteroatom-containing α, ω-dienes, especially the discovery of a new generation of Ru-NHC alkylethyl catalysts, more polyethylene materials with degradable functional groups can be synthesized by ADMET reaction. Sulfonate groups can be pyrolyzed and hydrolyzed, and can become suitable degradation sites in polyethylene-like materials. Simon et al. Synthesized sulfonates containing α, ω-diene monomers with different numbers of methylene groups, and obtained aliphatic polysulfonates by ADMET polymerization^[51]. It was found that the chain length of α, ω-diene monomer with shorter chain length not only affected the comprehensive properties of the polymer, but also had a great influence on the polymerization. Dienes with shorter chain length can be observed to have a significant negative neighbor effect in polymerization, inhibiting polymerization or producing oligomers. The incorporation of sulfonate groups into the polymer backbone further expands the functional groups available for "degradable polyolefins" and helps to further understand the effects of these defects on the overall properties of the polymer.

In nature, phosphate is an important component of nucleotide and plays an important role in the energy transfer process of adenosine triphosphate. The degradation rate of polyphosphate is effectively increased due to the lactone exchange of RNA-like molecules. Through ADMET polymerization, Wurm et al. Obtained a PE-like polymer with a molecular weight of up to 38 400 g·mol^-1. There are 20 methylene groups between each phosphate group carrying an ethoxy hydroxyl side chain, which can exchange with the main chain for a molecular lactone similar to RNA hydrolysis to accelerate degradation^[52]. Due to the low molecular weight of the polymer, its mechanical properties still need to be improved. In general, mimetic biopolymers provide a new strategy for the synthesis of novel degradable polyolefins.

Since Cassidy systematically discussed redox polymers in 1949, these polymers have attracted much attention because of their degradability and electrochemical energy storage^[56]. Trimethyl locked benzoquinone (TMBQ) is a relatively new functional group that shows sensitivity to redox environments. Mutlu et al. Used the ADMET reaction to synthesize long-chain aliphatic polymers containing TMBQ as a redox reaction functional unit^[53]. The polymer can be degraded to small molecules with the addition of a reducing agent, Na₂SO₄, and the TMBQ unit can be recovered in high purity, thereby facilitating the synthesis of recyclable redox polymers.

Compared with condensation polymerization, ring-opening polymerization of monomers avoids the disadvantages of removing or adding other small molecules, and can better realize the chemical cycle of monomers. Ring-opening polymerization has certain requirements for the size of the monomer ring: for small rings (three-membered and four-membered rings), ΔH is dominant, which can not selectively reverse the equilibrium at a reasonable temperature, and can not achieve chemical recycling; For macrocyclic systems, ΔH is usually not sufficient to achieve high turnover rapidly, and the dominant role is played by the increased conformational entropy and mixing entropy during polymerization. The ring size also directly affects the number of methylene groups between the functional groups of the polymer, thereby affecting its physical properties and degradation performance. Therefore, it is very important to design the ring size of the monomer reasonably. According to different ring-opening mechanisms, ring-opening polymerization can be divided into radical ring-opening polymerization, cationic/anionic/metal/organic catalyzed ring-opening polymerization, and ring-opening metathesis polymerization (ROMP) (Fig. 5).

Free radical ring-opening polymerization combines the advantages of free radical polymerization and ring-opening polymerization, that is, unsaturated bonds or other functional groups can be introduced into the main chain of the polymer under mild polymerization conditions, and the representative monomers are cyclic ketene, cyclic ketene acetal, etc. Due to the low conversion of the cyclic ketene acetal monomer, the obtained polymer has an uncontrolled ester group content and a low molecular weight. Recently, You et al. Combined radical ring-opening polymerization with RAFT, and dithiocarbamate-mediated hybrid copolymerization of ethylene and cyclic ketal was carried out, and the incorporation of ester group and molecular weight of the copolymer were easy to adjust^[57]. In addition, lipases can efficiently degrade ester-containing polyvinyl copolymers into short fragments. The properties of the copolymer were not studied in this paper, and the crystallinity of the copolymer decreased significantly with the increase of the ester content.

Conventional cationic ring-opening polymerization produces polymers with low molecular weight and lack of effective tensile strength due to uncontrolled current. Coates et al. Used a commercial halomethyl ether initiator and an indium bromide catalyst to catalyze the reversible deactivation reaction of cyclic acetals to synthesize poly (1,3-dibenzofuran) (PDXL), which has a tensile strength comparable to that of some commercial polyolefins and can be depolymerized under a strong acid catalyst to achieve quantitative chemical recovery^[58]. However, the number of methylene groups between PDXL functional groups is small, which affects the crystallinity of the polymer. Therefore, the rational design of monomers and the optimization of polymerization conditions are essential for the development of circular plastics economy. Small ring systems can release more ring stress during ring-opening polymerization, which can often achieve rapid ring-opening polymerization, while there are few entropy-driven studies on ring-opening polymerization of large ring systems. Odelius et al. Realized the ultrafast ring-opening polymerization of macrocyclic carbonates in two steps by using tert-butyl alcohol anion catalysis. The polymerization rate is related to the difference of molecular conformation of carbon nanotubes. The obtained polymer can regenerate the original cyclic carbonates with high selectivity (95 ~ 99 mol%) and high yield (70% ~ 85%), and realize closed-loop recovery^[59]. Williams et al. Studied the polymerization of ω-pentadecanolide, nonanolide, and trilactone using yttrium phosphide catalyst^[60]. Compared with the previous aluminum complex catalyst, the catalyst has higher activity and improves the conversion rate of the polymer. Therefore, in addition to monomer design, the development of new catalysts is of great significance for ring-opening polymerization.

In the past few decades, thanks to the development of catalysts, ROMP has a high tolerance to functional groups, and its tolerance to water and oxygen has also been improved. People have used ROMP to obtain polyolefins with degradable groups in the main chain, and have developed many new degradable polymers that cannot be obtained by other polymerization methods^[61,62]. Compared with stepwise polymerization, ring-opening metathesis polymerization has better control over the molecular weight of the obtained polymer, higher molecular weight and narrower distribution. Wurm et al. First synthesized degradable polyethylene containing orthoester group by olefin metathesis polymerization. The melting point and crystallization temperature of the material were affected by the content of orthoester in the copolymer.In addition, the orthoester substituent affects the hydrolysis rate of the polymer in solution, and the non-hydrogenated copolymer with high orthoester content is biodegraded by microorganisms in the activated sludge of the local sewage treatment plant^[63]. Toste et al. Designed and synthesized cyclic acylsilane monomers that can be used in ROMP reaction, and achieved efficient copolymerization with cyclooctene (Fig. 6)^[64].

Copolymers can be photodegraded by an α-siloxycarbene intermediate generated by 1,2-Brook rearrangement intercalating into water and dissociating into hemiacetals, which highlights the potential of acylsilanes as photocleaving groups. In addition to the design of comonomers, the modification of existing catalysts or the development of new catalysts can expand the types of comonomers, which is very important for the development of new degradable polyethylene materials. Chen et al. Added a cocatalyst, sodium tetrakis (3,5-bis (trifluoromethyl) phenyl) borate (NaBArF), to various commercially available ruthenium catalysts to extract chloride ions, significantly improve their reactivity toward electron-deficient internal olefins, and synthesize far-hydroxy polyethylene analogs that can be recycled by ring closure^[65].