Recently, scientists have introduced carbon monoxide (CO) into polyethylene polymer in the presence of a nickel catalyst to develop a new material that can undergo photodegradation.
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Plastics are the most abundantly used material. One of the biggest disadvantages of using plastics is that it does not degrade easily in the environment and result in enormous accumulation in soil and in the aquatic ecosystem.
Plastic pollution particularly affects aquatic organisms in the form of microplastics and also enters the food chain.
Polyethylene: A Commonly Available Plastic
Polyethylene is an abundantly found synthetic polymer that persists in the environment for a prolonged period. Some of the reasons why polyethylene is so commonly found include its low production cost, favorable mechanical properties, and immense applicability.
This type of polymer is a non-polar and hydrophobic material that does not adhere to polar materials, e.g., metal surfaces or wood. As hydrocarbon chains are chemically inert, polyethylene does not degrade easily and, therefore, pollutes the environment for a longer period.
Plastic Alternatives: Where Are We Now?
This type of plastic contains crystalline ordering of stretched inert hydrocarbon chain and this ordering is associated with the high-density polyethylene (HDPE). Scientists have developed various approaches to improve the properties of polyethylene such that it retains all its mechanical properties and can be degraded easily.
Modification of Polyethylene to Promote Photodegradation
In this regard, one of the prospective measures has been utilizing the presence of polar groups in the hydrocarbon chain to attach another element. The crystalline polyethylene materials have been subjected to catalytic copolymerization of ethylene using a very low ratio of CO.
This results in the formation of keto groups in the polyethylene chain that offers wide-ranging reactivity, such as photodegradability. Previous studies have reported that branched low-density polyethylenes (LDPEs) containing approximately 1 mol % of keto groups do not disturb the crystalline order and retain their tensile properties. However, scientists have pointed out that the addition of CO tends to diminish the ideal properties of plastics.
Ni(II) catalysts play an important role in promoting non-alternating catalytic copolymerization of ethylene and CO. This catalyst brings about temperature stability in the reaction process for the development of the keto-polyethylene film.
Nickel has been selected over other commonly used electron-deficient metal catalysts because the latter tends to get deactivated by CO. Additionally, the neutral active sites of the common cationic polymerization catalysts exhibit an insignificant preference for binding with CO over other ligands.
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The addition of nickel catalysts in bulky phosphinophenolate ligands results in the formation of phosphinophenolate-coordinated nickel complexes. This complex promotes catalytic reaction associated with ethylene polymerization which occurs when the polyethylene contains approximately 1% CO.
The resultant material comprises favorable low content of keto group in a high molecular weight polymer and exhibits greater tensile properties that are comparable to commercial HDPE.
Determination of the Keto Group in the Newly Developed Polymer
Generally, scientists study the infrared (IR) spectra of a polymer to determine the modification of polyketone. Nuclear magnetic resonance (NMR) spectroscopic methods revealed the presence of isolated ketone groups in the polyethylene chain.
These analytical tools characterized the newly developed polyethylene and revealed the non-alternating motifs of ketone groups to be in proximate positions to each other along the hydrocarbon chain. In this study, the overall content of the keto group was determined by quantitative NMR spectroscopy and confirmed by IR spectroscopy.
Wide-angle x-ray scattering (WAXS) diffractograms of both newly developed keto-polyethylene and HDPE revealed virtually identical structures.
Photodegradation of Newly Developed Plastic
Scientists revealed that keto-polyethylene films when exposed to sunlight undergo photodegradation. They conducted an experiment where the newly developed polymer was subjected to a water bath with a light intensity that corresponds to around five months of natural sunlight in Southern Europe.
In the study period, researchers observed the onset of degradation as the keto-polyethylene material became brittle and lost its mechanical properties. Additionally, researchers recorded weight loss of the samples which further proved photodegradation.
IR spectroscopic analysis of 13CO-labeled samples revealed the degradation process. Scientists conducted short-term degradation experiments where they found that keto groups were only partially consumed and new keto and ester groups were formed, which enabled continual chain degradation beyond the study period.
The control samples, i.e., keto-free ethylene homopolymer, did not undergo any degradation when subjected to the same experimental conditions.
Advantages of Copolymerization of Polyethylene
Previously, researchers had developed another strategy to alter the chemical structure of the polymer, i.e., post-polymerization oxidation of polyethylene. However, compared to the copolymerization method, this process required additional steps and was not specific to keto functional groups.
Another disadvantage of this method included the use of hazardous chemical reagents. Researchers pointed out that in the case of copolymerization, carbon monoxide established a stronger bond with the catalyst, compared with the ethylene monomer, which prevented it. Most importantly, these in-chain keto motifs restored all the desired material properties of polyethylene.
Another advantage was that this polymer possessed a ductile property, which was again comparable to commercially available HDPE. A previous study reported that crosslinking supported by keto motifs posed difficulties in the thermoplastic processing of commercial polyketone resins.
However, in the newly developed polyethylene, no evidence for unwanted cross-linking was observed that could obstruct melt processing. Scientists have highlighted the main advantage of this newly developed keto-polyethylene material to be its ability to undergo degradation in the presence of ultraviolet radiation.
References and Further Reading
Ali, S. S. et al. (2021) Degradation of conventional plastic wastes in the environment: A review on current status of knowledge and future perspectives of disposal. Science of the Total Environment. 771. 144719. https://doi.org/10.1016/j.scitotenv.2020.144719
Baur M. et al (2021) Polyethylene materials with in-chain ketones from nonalternating catalytic copolymerization. Science. 374(6567). pp. 604-607. https://www.science.org/doi/10.1126/science.abi8183
Morgen, T.O. et al. (2020) Photodegradable branched polyethylenes from carbon monoxide copolymerization under benign conditions. Nature Communication.11, 3693 https://doi.org/10.1038/s41467-020-17542-5
Ghatge, S. et al. (2020) Biodegradation of polyethylene: a brief review. Applied Biological Chemistry. 63, 27. https://doi.org/10.1186/s13765-020-00511-3
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