Of the current worldwide production of synthetic polymers, nearly 90% is represented by full-carbon-backbone macromolecular systems (polyvinylics and polyvi-nylidenics),49 and 35 to 45% of production is for one-time-use items (disposables and packaging). Therefore, it is reasonable to envisage a dramatic environmental impact attributable to the accumulation of plastic litter and waste constituted by full-carbon-backbone polymers, which are recalcitrant to physical, chemical, and biological degradation processes.
In contrast to the "hydro-biodegradation" process of natural and synthetic polymers containing hetero atoms in the main chain (polysaccharides, proteins, polyesters, polyamides, polyethers), the mechanism of biodegradation of full-carbon-backbone polymers requires an initial oxidation step, mediated or not by enzymes, followed by fragmentation, again mediated or not by enzymes, with substantial reduction in molecular weight. The functional fragments then become vulnerable to microorganisms present in different environments, with production (under aerobic conditions) of carbon dioxide, water, and cell biomass. Figure 5.2 outlines the general features of environmentally degradable polymeric materials, which are classified as hydrobiodegradables and oxobiodegradables. Typical examples of the so-called oxo-biodegradable polymers are represented by polyethylene, poly(vinyl alcohol), and lignin (a natural heteropolymer).50 The major biodegradation mechanism of PVA in aqueous media is represented by the oxidative random cleavage of the polymer chains, the initial step being associated with the specific oxidation of methylene carbon bearing the hydroxyl group, as mediated by oxidase- and dehydrogenase-type enzymes, to give P-hydroxyketone as well as 1,3-diketone moieties. The latter groups are susceptible to carbon-carbon bond cleavage promoted by specific P-diketone hydrolase, leading to the formation of carboxyl and methyl ketone end groups.51,52
The ultimate biochemical fate of partially hydrolyzed PVA samples has been recently described by using Pseudomonas vesicularis PD strain, a specific PVA-assimilating bacterium.53 This bacterium metabolizes PVA by a secondary alcohol oxidase throughout the oxidation of the hydroxyl groups followed by hydrolysis of the formed P-diketones by a specific hydrolase. Both enzymes are extracellular, and the polymer chains are cleaved by repeated enzyme-mediated reactions outside the cells into small fragments, which are further incorporated and assimilated inside the bacterial cytoplasm and metabolized up to carbon dioxide (Figure 5.3).53
The initial oxidation step of PVA macromolecules can also be promoted by ligninolytic enzymes (lignin peroxidase [Lip] and laccase) produced by white-rot fungal species such as Phanerochaete crysosporium54,55 and Pycnoporus cinnabar-inus.56 The monoelectronic enzymatic oxidation reactions lead to formation of free radicals along with the formation of carbonyl groups as well as double bonds, thus increasing the macromolecule unsaturation.54
Similarly, the oxidative instability of polyolefins in the environment is due to physical-chemical radical reactions enhanced by the presence of sensitizing impurities in the polymer chain. To counter the mechanisms of physical aging (e.g., thermal and photolytic degradation) of poly(ethylene) (PE), heat and light stabilizers
H2O - Uptake Enzyme mediated or not
> Polyamides Polysaccharides
O2 - Uptake Catalyst
> Polyolefins Polyvinylalcohol • Lignin
FIGURE 5.2 General classification of environmentally degradable polymers.
FIGURE 5.3 Biodegradation pathway of partially acetylated PVA.
FIGURE 5.3 Biodegradation pathway of partially acetylated PVA.
CO2, H2O, biomass
CO2, H2O, Cell biomass have been used to improve the resistance of PE to environmental oxidation since the 1960s. Today, there is an opposite strategy aimed at accelerating the degradation rate of PE to overcome its intrinsic recalcitrance to biological attack. In this connection, copolymerization with a small amount of monomers containing carbonyl groups (carbon monoxide, methyl vinyl ketone) or the incorporation of transition metal compounds (dithiocarbamates) as photoinitiators or photosensitizers, and the addition of pro-oxidants (fatty acids and salts) constitute the major strategies for the introduction of functional groups and substances capable of promoting the degradation of macromolecules. When exposed to light and temperature, these additives generate free radicals that react with molecular oxygen to produce peroxides and hydroperoxides.57
Further, the hydroperoxides decompose in the presence of heat, light, and metallic ions, leading to the formation of macroalkoxy radicals, and the autooxidation of polyethylene proceeds through classical free radical chain reactions.58 59 As a result, chain scission and cross-linking are the major consequences of thermal oxidation of polyolefins.60,61 In the presence of oxygen, however, chain scission and oxidation of macromolecules are the predominant reactions,62 leading to (a) a significant reduction of both Mn (Average Number Molecular Weight) and molecular weight of poly(ethylene) samples containing pro-oxidant after thermal degradation63 and (b) to the formation of oxidation products, including carboxylic acids, ketones, lactones, and low-molecular-weight hydrocarbons.62
The rate and extent of radical-induced oxidation of polyolefins are also affected by structural parameters such as chain defeats and branching, these latter being representatives of weak links liable to bond cleavage and production of free radicals for succeeding chain fragmentation. The hierarchy in the oxidation susceptibility of polyolefins is: iPP (isotactic polypropylene) > LDPE (low-density PE) > LLDPE (linear low-density PE) > HDPE (high-density PE).6465
By considering the same carbon skeleton and the identical requirement of an initial oxidation step, studies on the environmental fate of Environmental Degradable Polymers (EDPs) based on PVA and PE may represent an interesting tool for identifying similarities or differences between the degradation mechanisms and particularly the influence of different environmental parameters in accelerating the biodegradation of these two large-volume carbon-backbone synthetic polymers.
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