Recent Developments in Synthesis, Properties, Applications and Recycling of Bio-Based Elastomers.

In 2021, global plastics production was 390.7 Mt; in 2022, it was 400.3 Mt, showing an increase of 2.4%, and this rising tendency will increase yearly. Of this data, less than 2% correspond to bio-based plastics. Currently, polymers, including elastomers, are non-recyclable and come from non-renewable sources. Additionally, most elastomers are thermosets, making them complex to recycle and reuse. It takes hundreds to thousands of years to decompose or biodegrade, contributing to plastic waste accumulation, nano and microplastic formation, and environmental pollution. Due to this, the synthesis of elastomers from natural and renewable resources has attracted the attention of researchers and industries. In this review paper, new methods and strategies are proposed for the preparation of bio-based elastomers. The main goals are the advances and improvements in the synthesis, properties, and applications of bio-based elastomers from natural and industrial rubbers, polyurethanes, polyesters, and polyethers, and an approach to their circular economy and sustainability. Olefin metathesis is proposed as a novel and sustainable method for the synthesis of bio-based elastomers, which allows for the depolymerization or degradation of rubbers with the use of essential oils, terpenes, fatty acids, and fatty alcohols from natural resources such as chain transfer agents (CTA) or donors of the terminal groups in the main chain, which allow for control of the molecular weights and functional groups, obtaining new compounds, oligomers, and bio-based elastomers with an added value for the application of new polymers and materials. This tendency contributes to the development of bio-based elastomers that can reduce carbon emissions, avoid cross-contamination from fossil fuels, and obtain a greener material with biodegradable and/or compostable behavior.

Recent developments in bio-based polyethylene: Degradation studies, waste management and recycling.

Nowadays, the tendency to replace conventional fossil-based plastics is increasing considerably; there is a growing trend towards alternatives that involve the development of plastic materials derived from renewable sources, which are compostable and biodegradable. Indeed, only 1.5 % of whole plastic production is part of the small bioplastics market, even when these materials with a partial or full composition from biomass are rapidly expanding. A very interesting field of investigation is currently being developed in which the disposal and processing of the final products are evaluated in terms of reducing environmental harm. This review presents a compilation of polyethylene (PE) types, their uses, and current problems in the waste management of PE and recycling. Particularly, this review is based on the capabilities to synthesize bio-based PE from natural and renewable sources as a replacement for the raw material derived from petroleum. In addition to recent studies in degradation on different types of PE with weight loss ranges from 1 to 47 %, the techniques used and the main changes observed after degradation. Finally, perspectives are presented in the manuscript about renewable and non-renewable polymers, depending on the non-degradable, biodegradable, and compostable behavior, including composting recent studies in PE. In addition, it contributes to the 3R approaches to responsible waste management of PE and advancement towards an environmentally friendly PE.

Synthetic Polyisoprene Rubber as a Mimic of Natural Rubber: Recent Advances on Synthesis, Nanocomposites, and Applications.

Up to now, rubber materials have been used in a wide range of applications, from automotive parts to special-design engineering pieces, as well as in the pharmaceutical, food, electronics, and military industries, among others. Since the discovery of the vulcanization of natural rubber (NR) in 1838, the continuous demand for this material has intensified the quest for a synthetic substitute with similar properties. In this regard, synthetic polyisoprene rubber (IR) emerged as an attractive alternative. However, despite the efforts made, some properties of natural rubber have been difficult to match (i.e., superior mechanical properties) due not only to its high content of cis-1,4-polyisoprene but also because its structure is considered a naturally occurring nanocomposite. In this sense, cutting-edge research has proposed the synthesis of nanocomposites with synthetic rubber, obtaining the same properties as natural rubber. This review focuses on the synthesis, structure, and properties of natural and synthetic rubber, with a special interest in the synthesis of IR nanocomposites, giving the reader a comprehensive reference on how to achieve a mimic of NR.

Genetic basis for the biodegradation of a polyether-polyurethane-acrylic copolymer by a landfill microbial community inferred by metagenomic deconvolution analysis.

Plastic accumulation in the world amounts to approximately 8300 million tons. Polyurethanes (PU) account for 7.7 % of total plastics production worldwide, and their diverse chemical composition makes them highly recalcitrant to biodegradation. Several works have reported polyurethane-degrading microbial communities. However, it is still necessary to learn more about the chemical, biochemical, and genetic bases linked to the polyurethanolytic phenotype and the microbial taxonomic determinants responsible for metabolizing the PU polymer and its associated chemical additives. To shed light on this problem, we applied physical, chemical, biochemical, metagenomic, and bioinformatic analyses to explore the biodegradation capability and related biochemical and genetic determinants of the BP6 microbial community that can grow in PolyLack, a commercial coating containing a polyether polyurethane acrylate (PE-PU-A) copolymer and several additives, as sole carbon source. We observed complete additives (isopropanol, N-methyl-2-pyrrolidone, 2-butoxyethanol, alkyl glycol ethers) biodegradation and the appearance of released polymer components (toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) derivatives), and multiple degradation products since early cultivation times. The Hi-C metagenomic analysis identified a complex microbiome with 35 deconvolved Metagenome-Assembled Genomes (MAGs) - several new species - and biodegradation markers that suggest the coexistence of hydrolytic, oxidative, and reductive metabolic strategies for degrading the additives and the PU copolymer. This work also provides evidence of the metabolic capability the BP6 community has for biodegrading polyether polyurethane foams. Based on these analyses, we propose a novel metabolic pathway for 4,4'-methylenedianiline (MDA), an initial biodegradation intermediate of MDI-based PU, encoded in the complex BP6 community metagenome and suggest that this community is a potential biotechnological tool for PU bio-recycling.

Synthesis of Biobased Hydroxyl-Terminated Oligomers by Metathesis Degradation of Industrial Rubbers SBS and PB: Tailor-Made Unsaturated Diols and Polyols.

Biobased hydroxyl-terminated polybutadiene (HTPB) was successfully synthesized in a one-pot reaction via metathesis degradation of industrial rubbers. Thus, polybutadiene (PB) and poly(styrene-butadiene-styrene) (SBS) were degraded via metathesis with high yields (>94%), using the fatty alcohol 10-undecen-1-ol as a chain transfer agent (CTA) and the second-generation Grubbs−Hoveyda catalyst. The identification of the hydroxyl groups (-OH) and the formation of biobased HTPB were verified by FT-IR and NMR. Likewise, the molecular weight and properties of the HTPB were controlled by changing the molar ratio of rubber to CTA ([C=C]/CTA) from 1:1 to 100:1, considering a constant molar ratio of the catalyst ([C=C]/Ru = 500:1). The number average molecular weight (Mn) ranged between 583 and 6580 g/mol and the decomposition temperatures between 134 and 220 °C. Moreover, the catalyst optimization study showed that at catalyst loadings as low as [C=C]/Ru = 5000:1, the theoretical molecular weight is in good agreement with the experimental molecular weight and the expected diols and polyols are formed. At higher ratios than those, the difference between theoretical and experimental molecular weight is wide, and there is no control over HTPB. Therefore, the rubber/CTA molar ratio and the amount of catalyst play an important role in PB degradation and HTPB synthesis. Biobased HTPB can be used to synthesize engineering design polymers, intermediates, fine chemicals, and in the polyurethane industry, and contribute to the development of environmentally friendly raw materials.

Synthesis, characterization and biodegradation studies of polyurethanes: Effect of unsaturation on biodegradability.

The presence of unsaturation in the main chain of the polymer promotes the biodegradation process. To elucidate this hypothesis, one unsaturated polyurethane (PUU) and another saturated polyurethane (PUS) were synthesized and then biodegraded, and evidence was found to support this hypothesis. The polyurethanes were synthesized by a polycondensation reaction with yields up to 97%. It is important to note that both polyurethanes were constituted only by the recalcitrant hard segment and showed low crystallinity and molecular weight. Spectroscopic, thermal, and chromatographic techniques were used for physical and structural characterization. Both polyurethanes were biodegraded by the BP8 microbial community and the Cladosporium tenuissimum A3.I.1 fungus during a two-month period. A postbiodegradation characterization revealed the detriment of properties in both materials, indicating successful biodegradation. As a general trend, more efficient biodegradation was observed by the Cladosporium tenuissimum fungus A3.I.1 than by the BP8 microbial community. Specifically, with the fungus, the infrared analysis showed a decrease in the characteristic bands as well as the appearance of new carboxylic acid signals (approximately 1701 cm-1), suggesting the enzymatic cleavage of the urethane group. By comparison to polyurethanes, PUU showed superior biodegradation; using the fungus, a 51% decrease in molecular weight (Mw) for PUU was achieved, in contrast with 36% achieved for PUS. Despite the low crystallinity and molecular weight, the determining factor in biodegradation was the presence of unsaturations along the main chain. Thus, a more efficient oxidative attack is carried out by microorganisms on double bonds. The novel PUU showed similar biodegradation to the known polyester-type PU with highly hydrolysable groups. Consequently, PUU represents a green alternative to conventional polyurethanes and is a key material to achieve biorecycling.

Current status on the biodegradability of acrylic polymers: microorganisms, enzymes and metabolic pathways involved.

Acrylic polymers (AP) are a diverse group of materials with broad applications, frequent use, and increasing demand. Some of the most used AP are polyacrylamide, polyacrylic acid, polymethyl methacrylates, and polyacrylonitrile. Although no information for the production of all AP types is published, data for the most used AP is around 9 MT/year, which gives an idea of the amount of waste that can be generated after products' lifecycles. After its lifecycle ends, the fate of an AP product will depend on its chemical structure, the environmental setting where it was used, and the regulations for plastic waste management existing in the different countries. Even though recycling is the best fate for plastic polymer wastes, few AP can be recycled, and most of them end up in landfills. Because of the pollution crisis the planet is immersed, setting regulations and developing technological strategies for plastic waste management are urgent. In this regard, biotechnological approaches, where microbial activity is involved, could be attractive eco-friendly strategies. This mini-review describes the broad AP diversity, their properties and uses, and the factors affecting their biodegradability, underlining the importance of standardizing biodegradation quantification techniques. We also describe the enzymes and metabolic pathways that microorganisms display to attack AP chemical structure and predict some biochemical reactions that could account for quaternary carbon-containing AP biodegradation. Finally, we analyze strategies to increase AP biodegradability and stress the need for more studies on AP biodegradation and developing stricter legislation for AP use and waste control. KEY POINTS: • Acrylic polymers (AP) are a diverse and extensively used group of compounds. • The environmental fates and health effects of AP waste are not completely known. • Microorganisms and enzymes involved in AP degradation have been identified. • More biodegradation studies are needed to develop AP biotechnological treatments.

Degradation of Recalcitrant Polyurethane and Xenobiotic Additives by a Selected Landfill Microbial Community and Its Biodegradative Potential Revealed by Proximity Ligation-Based Metagenomic Analysis.

Polyurethanes (PU) are the sixth most produced plastics with around 18-million tons in 2016, but since they are not recyclable, they are burned or landfilled, generating damage to human health and ecosystems. To elucidate the mechanisms that landfill microbial communities perform to attack recalcitrant PU plastics, we studied the degradative activity of a mixed microbial culture, selected from a municipal landfill by its capability to grow in a water PU dispersion (WPUD) as the only carbon source, as a model for the BP8 landfill microbial community. The WPUD contains a polyether-polyurethane-acrylate (PE-PU-A) copolymer and xenobiotic additives (N-methylpyrrolidone, isopropanol and glycol ethers). To identify the changes that the BP8 microbial community culture generates to the WPUD additives and copolymer, we performed chemical and physical analyses of the biodegradation process during 25 days of cultivation. These analyses included Nuclear magnetic resonance, Fourier transform infrared spectroscopy, Thermogravimetry, Differential scanning calorimetry, Gel permeation chromatography, and Gas chromatography coupled to mass spectrometry techniques. Moreover, for revealing the BP8 community structure and its genetically encoded potential biodegradative capability we also performed a proximity ligation-based metagenomic analysis. The additives present in the WPUD were consumed early whereas the copolymer was cleaved throughout the 25-days of incubation. The analysis of the biodegradation process and the identified biodegradation products showed that BP8 cleaves esters, C-C, and the recalcitrant aromatic urethanes and ether groups by hydrolytic and oxidative mechanisms, both in the soft and the hard segments of the copolymer. The proximity ligation-based metagenomic analysis allowed the reconstruction of five genomes, three of them from novel species. In the metagenome, genes encoding known enzymes, and putative enzymes and metabolic pathways accounting for the biodegradative activity of the BP8 community over the additives and PE-PU-A copolymer were identified. This is the first study revealing the genetically encoded potential biodegradative capability of a microbial community selected from a landfill, that thrives within a WPUD system and shows potential for bioremediation of polyurethane- and xenobiotic additives-contamitated sites.