Research Highlight 10 – A Bronsted Acid Ionic Liquid for ‘Closed-Bi-Loop’ Recycling of a Polyester
An ideal solution for current plastic pollution is considered as chemical recycling of these plastic wastes back into its starting monomers since they can (re)produce virgin quality polymers – which thus keeps material in the infinite closed-loop-life cycles of a circular economy, and most importantly out of the environment. This also has environmental benefit, because most of the greenhouse gas (GHG) emissions associated with the polymer production chain coming from the extraction of fossil-based raw materials (~61%); and chemical recycling can lower the carbon foot-print and make tremendous contribution to the common (climate)goal of achieving global warming below 2 °C (as per UN Paris Agreement 2015). However, all existing polymers cannot be recycled back into its starting monomers – either it should contain cleavable bonds (ester, acetal, carbonate, etc.) or need highly efficient catalytic methodologies to break inert carbon-carbon bonds. Thus, researchers in the polymer field are continuously devoted to making efforts on the development of new catalytic methods that can enhance the recycling efficiency of the existing polymers or designing of new renewable and recyclable polymers with closed-loop life cycles that can compete with the existing plastics in terms of material properties and cost [1-3].
Here, Prof. Yu-Zhong Wang and his research group at the Sichuan University, China have made major progress – they have recently developed a ‘Closed-Bi-Loop Recycling Strategy’ – which integrates both recycling and upcycling (that is upgrading to similar or high value materials) of a widely used biodegradable polyester “poly(p-dioxanone) (l-PPDO)” to enhance its utility and recyclability in a circular plastic economy (see Main Figure). The work is recently published in “Green Chemistry” [4]. The Wang group strategy involves utilization of a Bronsted Acid Ionic Liquid (BAIL) such as [Et3NH+]TSO- for chemical recycling of l-PPDO – which was obtained via neutralization of p-toluene sulfonic acid (TSOH) with triethylamine at 60 °C. The BAIL acted as a dual solvent/catalyst agent and participated in alternative cyclization and ring-closed depolymerization (RCD) reaction of l-PPDO via intramolecular transesterification (by terminal hydroxyl group attack on carbonyl) [5] and produced a cyclic poly(p-dioxanone) (c-PPDO) as well as starting monomer p-dioxanone (PDO) {upcycling and chemical recycling, respectively with an overall yield of 100%}. The importance of this strategy is also that the produced cyclic product could remain in the liquid phase of the BAIL without any side reaction (meaning no reverse/repolymerization reaction) while PDO exist in the gas phase (as vapours), and no other side products were generated. This allowed recycling process to be repeated for multiple cycles, and easy separation of the recycled/upcycled products from the system – PDO monomer was continuously distilled out from the recycling mixture, whereas cyclic PDO was accumulated in the BAIL by adding fresh l-PPDO. The catalytic efficiency of the BAIL was found intact and stable even after five recycling cycles.
The recycled ‘PDO monomer’ was highly pure and can be utilized as it is for repolymerization (that is, for ring-opening polymerization (ROP)) – forming a ‘polymer–monomer–polymer’ closed-loop-life cycle. The Wang group has demonstrated two strategies for repolymerization in the article: firstly to produce l-PPDO with controlled terminal groups by using water as an initiator and TSOH as a catalyst at 60 °C, and secondly to produce high molecular weight l-PPDO with low end-group content by using stannous octoate (Sn(Oct)2) as the catalyst. Because of its excellent biodegradability, biocompatibility and mechanical properties (pliability), l-PPDO is widely used in biomedical applications – for example bone and tissue fixation devices, drug-delivery applications and it is one of the FDA (Food & Drug Administration) approved surgical suture materials used in gynecology. l-PPDO is also used for general purpose applications such as films, molded parts, coatings, adhesives and laminates [6]. The upcycled product, c-PPDO can be used just by washing with methanol (to remove the BAIL residue). c-PPDO also has unique properties such as smaller hydrodynamic volume, lower viscosity and high glass transition temperature (Tg) and is highly desirable for biomedical applications (in comparison to their linear counterpart). The distinction between c-PPDO and l-PPDO is in their solubility and thermal decomposition temperature. c-PPDO is highly soluble in organic solvents and has high decomposition temperature (appears above 300 °C), whereas l-PPDO also exhibits thermal decomposition below 300 °C due to the degradation of hydroxyl and carboxyl terminal groups, which also lowers its solubility in weak polar solvents. More importantly, c-PPDO can be depolymerized back into l-PPDO after its end-of-life via hydrolysis of ester bond using the BAIL – forming a ‘cyclic–linear polymer’ closed-loop life cycle. The resulting l-PPDO can then be depolymerized to produce pure PDO monomer and repolymerize again to produce l-PPDO – thereby achieving ‘closed-bi-loop’ life cycles. In this way, l-PPDO will circulate among ‘monomer–linear polymer–cyclic polymer’, and will never end up in landfill. The Wang group has demonstrated ‘bi-loop recycling’ in the same recycling process – after complete distillation of PDO monomer from the first cyclization–depolymerization cycle, they introduced water into the system to hydrolyse c-PPDO to l-PPDO by heating at 80 °C for 2 hours and then RCD was re-initiated by heating at 120 °C and at ~100 Pa to produce PDO monomer (conversion was ~64% of c-PPDO produced in the first cycle).
Prof. Wang’s strategy resolves the drawbacks associated with the existing chemical recycling strategy of l-PPDO – that is pyrolysis. This process is not only energy-intensive, c-PPDO is unstable at pyrolysis temperature (210-350 °C) and thus cannot be isolated. It undergoes side reaction such as cis-elimination which results in vinyl ether end group (at one end of the chain) and hinders further RCD to PDO, instead leads to carbonization [7, 8]. This also affects the purity of the PDO monomer. The Wang group strategy is mild and highly efficient. Because of the weak acidity of [Et3NH+]TSO-, the aforementioned side reactions were not observed. The acidity of the BAIL has significant role on integrating this bi-loop recycling strategy to l-PPDO. In their preliminary studies, the Wang group observed that chemical recycling performed in neat triethylamine (Et3N) did not result in recycled (PDO monomer)/upcycled (c-PPDO) product, whereas TSOH only resulted in PDO monomer because of the strong acidity. According to the Wang group, BAIL strategy resulted in zero waste/mass loss {the green metric E factor (that is, total mass of waste/mass of final product) was zero} after multiple recycling cycles as opposed to pyrolysis (the calculated mass loss that arise from side reactions + purification of PDO was ~26% after 10 repeated pyrolysis cycles). The traditional approach for PDO monomer synthesis is by oxidative dehydrogenation of diethylene glycol using Cu(O) catalyst supported on silica particles. Thus, Prof. Wang's bi-loop recycling strategy has both environmental and economic benefits. The BAIL strategy is also highly competitive in terms of cyclic polymer production. In their multiple recycling studies, c-PPDO accumulation was 222 grams/liter after five cycles. The Wang group has stated in the article that this value is comparable to the most advanced method used for cyclic polymer synthesis such as N-heterocyclic carbene (NHC) mediated strategy and highly efficient than other traditional methods that use harsh conditions and metal catalysts such as (azide-alkyne)click reaction and polymer supported reagent strategies.
Prof. Wang’s strategy can also be modulated depending on the requirement, for example if one needs only a single-loop strategy (that is chemical recycling to PDO monomer) or recycled product – in this scenario, the recycling can be performed by introducing a hydroxyl functionalized compound such as glycerol into the recycling mixture containing l-PPDO and the BAIL, which will hinder c-PPDO formation by alcoholysis of l-PPDO – yielding only PDO monomer in high purity (~95%). Prof. Wang hopes that this BAIL strategy can be extended to other polyesters such as polylactide and polylactone since they can also produce cyclic analogs. In the highlighted article, he chose [Et3NH+]TSO- as the BAIL because it can dissolve l-PPDO at low temperature and at higher reactant concentration. Other potential BAIL candidates based on 4-dimethylaminopyridine/p-toluene sulfonic acid ([H-DMAP]TSO), imidazole/p-toluene sulfonic acid ([H-Im]TSO) can be utilized for exploring closed-bi-loop recycling of existing polyesters.
References:
[1]. Coates, G. W.; Getzler, Y. D. Y. L. Chemical recycling to monomer for an ideal, circular polymer economy. Nat. Rev. Mater. 2020, 5, 501-516.
[2]. Rosenboom, J.-G.; Langer, R.; Traverso, G. Bioplastics for a circular economy. Nat. Rev. Mater. 2022, 7, 117-137.
[3]. Global warming of 1.5 °C: Summary for policymakers (C2.3-CO2e limiting pathways for industry). Intergovernmental Panel on Climate Change (IPCC) 2018, http://www.ipcc.ch/report/sr15/.
[4]. Tian, G-.Q.; Yang, Z-.H.; Zhang, W.; Chen, S-.C.; Chen, L.; Wu, G.; Wang, Y-.Z. Integration of upcycling and closed-loop recycling through alternative cyclization–depolymerization. Green Chem. 2022, 24, 4490-4497 (Featured Article).
[5]. Libiszowski, J.; Kowalski, A.; Szymanski, R.; Duda, A.; Raquez, J-.M.; Degee, P.; Dubois, P. Monomer-linear macromolecules-cyclic oligomers equilibria in the polymerization of 1,4-dioxan-2-one. Macromolecules 2004, 37, 52-59.
[6]. Yang, K-.K.; Wang, X.-L.; Wang, Y-.Z. Poly(p-dioxanone) and its copolymers. J. Macromol. Sci.,Polym. Rev. 2002, C42, 373-398.
[7]. Li, X-.Y.; Zhou,Q.; Wen, Z-.B.; Hui, Y.; Yang, K-.K.; Wang, Y-.Z. Influence of catalysts used in synthesis of poly(p-dioxanone) on its thermal degradation behaviors. Polym. Degrad. Stab. 2015, 121, 253-260.
[8]. Nishida, H.; Yamashita, M.; Hattori, N.; Endo, T.; Tokiwa, Y. Thermal decomposition of poly(1,4-dioxan-2-one). Polym. Degrad. Stab. 2000, 70, 485-496.
[9]. All references cited in the highlighted article [3].
