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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective Figure 2. Photo-oxidative degradation of PE containing carbonyl impurities, via (a) a Norrish type I mechanism or (b) a Norrish type II mechanism. (c) Radical recombination to form cross-linked chains.38 R, R′, and R′′ are polymer chains of variable length. Figure 3. Small molecule products of the three common degradation routes in the environmental degradation of polyethylene terephthalate. R and R′ are polymer chains of variable length. The mechanism of photo-oxidative degradation begins when an excited state (localized at a carbonyl or other structural defect) in PE abstracts a hydrogen atom from the polymer backbone, generating a reactive carbon-based alkyl radical. The first step in the subsequent radical chain mechanism is the reaction of the alkyl radical with O2 to form a peroxy radical, which abstracts a hydrogen atom from another polymer chain (or from a distant site on the same polymer chain) to form a hydroperoxide and a new alkyl radical. Subsequent O−O bond scission in the hydroperoxide leads to alkoxy and hydroxyl radicals, each of which can abstract another hydrogen atom and generate a new alkyl radical. Finally, termination occurs through bimolecular radical recombination.24 Carbonyl defects introduced into polyethylene via oxidative reactions can also lead to Norrish Type I reactions (Figure 2a), in which photochemically induced homolytic cleavage leads to free radical intermediates, or Norrish type II reactions, in which intramolecular γ-H abstraction generates ketones and vinylidenes (Figure 2b).45,46 Although HDPE, LDPE, and LLDPE all have nominally the same chemical compositions, they have very different degrees of crystallinity. The rate of degradation depends strongly on the amorphous fraction of the polymer. Thus, degradation is far slower for crystalline HDPE, whose lower chain mobility promotes radical recombination at the expense of radical propagation reactions.47 Polyethylene Terephthalate (PET). The chemical structure of polyethylene terephthalate (PET) consists of alternating ethylene glycolate and terephthalate subunits, linked via ester bonds. It therefore belongs to the class of polymers known as polyesters. In the natural environment, PET can degrade by thermal oxidation, but hydrolytic cleavage and photo-oxidation initiated by UV light are more common under ambient conditions.48 In particular, the low temper- atures typical of the marine environment mean that floating plastics degrade primarily by slow, photo-oxidative degrada- tion. When PET is landfilled, or sinks below the upper regions of the ocean penetrated by sunlight, the buried polymer obviously cannot undergo photodegradation. Under these conditions, slow thermal oxidative degradation and hydrolysis may occur together, or sequentially. If PET is landfilled in an oxygen-poor environment, anaerobic degradation is unlikely to occur naturally due to the high temperatures required (≥200 °C).36 The chemical products resulting from each of these processes are compared in Figure 3. Hydrolysis of PET forms shorter carboxylic acid-terminated and alcohol-terminated chains, leading ultimately to tereph- thalic acid and ethylene glycol. In the near-neutral pH of the marine environment, hydrolytic cleavage of PET is very slow,49,50 but the rate is strongly enhanced under acidic conditions.51 In landfills, ester hydrolysis can induce a local drop in pH if the amount of moisture is insufficient to dilute the carboxylic acid products, resulting in autocatalytic acceleration.24 In the presence of O2, thermal degradation may proceed via a free-radical mechanism, initiated when the α-H of the ester is abstracted by an excited carbonyl group. The resulting carbon- centered radical reacts rapidly with O2 to give a peroxy radical which abstracts another α-H to form a new hydroperoxide and https://dx.doi.org/10.1021/acssuschemeng.9b06635 3496 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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