New materials sprout from the interface of natural polymers, biocatalysis, and sustainable chemistry

The Sustainable Materials Chemistry (SUSMATCHEM) research group at the Department of Materials and Environmental Chemistry (MMK) in Stockholm University has developed an efficient nanoparticle-supported biocatalyst for circumventing oxygen sensitivity of radical polymerization reactions.

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The path leading to the first published research Article1 from the new research group headed by Assistant Professor Mika Sipponen involved many first-time encounters. Dr. Adrian Moreno was the first member to join the group. Coming from Catalonia in Spain, it was also the first time he worked in the Nordic countries. Anyone who has entered a new research group knows that there are sizeable obstacles to overcome, even more so when the challenge is to kick off experimental work in the laboratory. This was the reality that the two researchers faced.

The duo decided to explore the intersection of their expertise in lignin and colloidal chemistry, biocatalysis, and polymer chemistry. Their discussions sparked interest in the possible use of lignin nanoparticles and enzymatic catalysis to circumvent the notorious oxygen sensitivity of radical polymerization reactions. The decision to use lignin nanoparticles was motivated by the ease of their preparation and prospective use as a filler in polymer composites in large scale. Lignin belongs to natural polyphenols and is the main aromatic component of plant biomass. It is therefore abundantly available as a sideproduct from the pulp and paper industry.

Material from a preliminary Pickering emulsion polymerization experiment.

Instead of picking a low-hanging fruit presented by common water-soluble monomers, the team decided to pursue polymerization of monomers of broad utility, but which were insoluble in water that was selected as an environmentally friendly solvent. The monomers included methyl acrylate, butyl methacrylate, and styrene which are important in many commodity products such as acrylic paints and thermoplastic polymers, and also hold opportunities for the synthesis of well-defined polymers for specialty applications. The water-insolubility of the monomers represented a challenge that the researchers overcame by conducting the polymerization reactions in emulsion phase.

Emulsion polymerization has a long history, but curiously there were no earlier reports on Pickering emulsions as media for single electron transfer-living radical polymerization (SET-LRP), a versatile Cu(0)-mediated living radical polymerization.2 It was thus imperative to gain a good control of the emulsion properties as the droplet size of the monomers defined the performance of SET-LRP and the diameter of the resulting polymer microparticles. The researchers were inspired by recent reports on the use of lignin nanoparticles for the stabilization of Pickering emulsions,3,4 but did not know if the radical scavenging activity of lignin would impede the radical-mediated polymer synthesis.

Preparation of lignin nanoparticles coated with chitosan and glucose oxidase (GOx) for efficient oxygen removal during controlled radical polymerization in Pickering emulsions.1

Another challenge was to introduce glucose oxidase, and enzyme that consumes oxygen from water in the course of oxidation of glucose, onto the lignin nanoparticles to bring the enzyme at the water-oil interface where the removal of oxygen is the most critical to avoid interference of the polymerization reaction. The researchers decided to use chitosan, a natural polysaccharide derived from crustacean shells, as a coating between the lignin nanoparticles and the enzyme. The final approach was simple, green, and material-efficient, as the entire fabrication of so-called hybrid nanoparticles was carried out in a single pot and by using adsorption instead of covalent cross-linking reactions.

“It was compelling to initiate such an interdisciplinary project with several well-defined challenges, but honestly there were times that I felt unsure about the outcome of the project”, Dr. Moreno recalls. "It was immensely helpful to work in parallel on a literature survey on lignin-based smart materials that we published recently”.5 I also gained valuable insight from Dr. Sipponen on the use of lignin nanoparticles for enzyme immobilization6 and stabilization of Pickering emulsions.4 One of the interesting findings was that it is possible to use solvent-free melt processing to produce polymer composites that are reinforced by the lignin-based hybrid nanoparticles. It is easy to sense that the team is excited about their forthcoming publication that focuses on these novel composite materials.

It was compelling to initiate such an interdisciplinary project with several well-defined challenges, but honestly there were times that I felt unsure about the outcome of the project, Dr. Moreno recalls.

Dr. Adrian Moreno (front left) working along PhD student Mohammad Morsali (back left) and PhD student Jinrong Liu (right) who is instructing undergraduate student Norna Gabring (middle) at MMK in Stockholm University.

The SUSMATCHEM research group has doubled in strength since the start of the project, and will continue to do so through two more recruitments in the early 2021. “It has been a fun ride to build a new team and witness the great support and mentoring within the group and in the department”, Dr. Sipponen says. We want to be at the frontline of developing materials that are industrially feasible, and showcase the versatility of lignin as a building block for functional materials and tailored applications.

We want to be at the frontline of developing materials that are industrially feasible, and showcase the versatility of lignin as a building block for functional materials and tailored applications.

In addition to increasing emphasis on natural polymers as constituents of circular materials, MMK is starting a new Master's degree programme in Sustainable Chemistry in autumn 2021. Looking forward, we see a world of opportunities in investing in research and education in this interdisciplinary field to develop new high-performance materials and greener industrial processes.

References

  1. Moreno, A. & Sipponen, M. H. Biocatalytic nanoparticles for the stabilization of degassed single electron transfer-living radical pickering emulsion polymerizations. Nat. Commun. 11: 5599 (2020).
  2. Lligadas, G., Grama, S. & Percec, V. Single-Electron Transfer Living Radical Polymerization Platform to Practice, Develop, and Invent. Biomacromolecules 18, 2981–3008 (2017).
  3. Zou, T., Sipponen, M. H. & Österberg, M. Natural shape-retaining microcapsules with shells made of chitosan- coated colloidal lignin particles. Front. Chem. 7, 370 (2019).
  4. Sipponen, M. H., Smyth, M., Leskinen, T., Johansson, L.-S. & Österberg, M. All-lignin approach to prepare cationic colloidal lignin particles: Stabilization of durable Pickering emulsions. Green Chem. 19, 5831–5840 (2017).
  5. Moreno, A. & Sipponen, M. H. Lignin-based smart materials: a roadmap to processing and synthesis for current and future applications. Mater. Horizons 7, 2237–2257 (2020).
  6. Sipponen, M. H. et al. Spatially confined lignin nanospheres for biocatalytic ester synthesis in aqueous media. Nat. Commun. 9: 2300 (2018).

Article: Moreno, A. & Sipponen, M. H. Biocatalytic nanoparticles for the stabilization of degassed single electron transfer-living radical pickering emulsion polymerizations. Nature Communications volume 11, Article number: 5599 (2020) https://www.nature.com/articles/s41467-020-19407-3

Mika Sipponen

Assistant Professor, Stockholm University

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