Let benzoxazines feel the light: photothermal synthesis of polybenzoxazines

Significance 

Due to their unique and outstanding properties, polymers are increasingly replacing conventional materials in various practical applications. The synthesis and processing of numerous polymers require extremely higher temperatures. These processes are often energy-intensive and represent a big constraint that could compromise large-scale production and application. Therefore, developing alternative energy-efficient and environmentally friendly polymer synthesis strategies is highly desirable. 

Recently, photopolymerization has been identified as a promising alternative, owing to its ability to reduce energy demand and provide direct access to a vast range of materials under ambient conditions via advanced fabrication techniques like 3D printing. Polybenzoxazines (PBz) are examples of polymers that could benefit from these features. Generally, polybenzoxazines have drawn considerable industrial attention due to their superior properties to conventional thermosets. Unfortunately, despite the use of initiators and catalysts, the high polymerization temperature requirements are a big hindrance to large-scale and widespread production and application of PBz.

Numerous strategies have been developed to reduce the PBz polymerization temperatures. For example, the use of catalysts and initiators and photopolymerization at room temperature have been explored, though with limited results, as they reported only a limited reduction of the polymerization temperature. These and many other methods fail to ensure efficient PBz production at low temperatures mainly due to their polymerization mechanism. In other words, whereas initiators, catalysts, and light can be used to facilitate the initial ring-opening step of the monomers, high thermal energy is still required for the subsequent electrophilic aromatic substitution reactions required for polymer network growth.

In order to overcome these obstacles, Spanish scientists: Dr. Kevin Reyes-Mateo, Professor Jordi Marquet, Dr. Jordi Hernando and Professor Rosa M. Sebastián from the Autonomous University of Barcelona proposed a photothermal approach for the synthesis of polybenzoxazines at ambient conditions. In this approach, the in situ generation of local heat under light irradiation was achieved photothermally through the relaxation of light-absorbing species via electron–phonon interaction. This was realized by leveraging the intriguing optical properties of Bz-catalyst systems, which display visible light absorption. Their work is currently published in the peer-reviewed journal, Polymer Chemistry.

The research team showed that the locally generated heat was large enough to trigger Bz polymerization. Interestingly, this approach ensured compliance with the high-temperature demand for Bz polymerization without external heating. It also benefited from the inherent advantages of light-induced processes, such as precise time and space control of the reactions. As a result, a large variety of Bz monomers were converted into polymer materials. Moreover, they retained some important features like high char yields despite being more soluble with lower transition temperature values.

Besides requiring a relatively shorter processing time, the proposed photothermal approach enabled spatial control of Bz polymerization, allowing the fabrication of PBz patterns via photolithography. Compared to the conventional photopolymerization methods, which fail to render complete monomer conversion, the study findings presented a remarkable improvement. Furthermore, it was worth noting that when the resulting polymer materials were toughened by subsequent thermal treatment, they exhibited thermo-mechanical properties similar to those of conventional polybenzoxazines.

In summary, the authors developed a novel photopolymerization strategy for the low-temperature production of polybenzoxazines. By significantly reducing the high-temperature requirements for photothermal polymerization of benzoxazines and avoiding the need for extra photoabsorbers, the presented approach not only allowed the production of PBz at ambient temperature but also reduced the complexity, cost, and time required for the process. This process is not only restricted to the production of PBz but can also be extended to different mono- and bifunctional monomers. In a statement to Advances in Engineering, Professor Rosa Mª Sebastián stated that their work would pave the way for more efficient fabrication of PBz at ambient conditions.

Let benzoxazines feel the light: photothermal synthesis of polybenzoxazines - Advances in Engineering

About the author

Dr. Rosa M. Sebastián. Full professor in Organic Chemistry Section of the Department of Chemistry of the Universitat Autònoma of Barcelona (UAB, Spain). She obtained her PhD in 1997 at the UAB and then she moved for two years to Laboratorie de Chimie de Coordination (Toulouse, France) for postdoctoral stay. Then she returned to UAB with a contract financed by Ramon y Cajal program, where she became permanent Associate professor in 2006. She has been involved in several aspects of organic and organometallic chemistry (enantioselective catalysis, metal nanoparticles, immobilization of catalysts on different surfaces, modification of surfaces with hydrophobic dyes), working also in the preparation of macrocycles and dendrimers with applications as catalysts, liquid crystals, sensors, etc. Her research interest has been lately focused on the synthesis of polymers, mainly epoxy resins, poly(cyanoacrylates) and poly(benzoxazines). She has been responsible of several contracts with companies, being the most important a post-doctoral training program between Henkel KGaA & Co. and UAB (2010-2020). For this partnership, UAB was awarded the “National Award for Public-Private Partnership Research” in 2012. From 2020 she is also Vice Chancellor of Innovation, Transfer and Entrepreneurship of UAB, and Director of the “UAB Research Park”. She is author/co-author of more than 80 articles in high impact journals, and 15 patents. She has supervised 10 doctoral thesis and 5 more are ongoing.

About the author

Dr. Jordi Hernando was born and grew up in Barcelona, Spain. After studying chemistry at the Universitat of Barcelona, he joined Miguel Gonzalez’s group at the same university, where he got his PhD degree in Physical Chemistry in 2000. There he applied both laser spectroscopy techniques and theoretical calculations to investigate gas-phase reactions of relevance in atmospheric chemistry. In the early 2000s he was a post-doctoral fellow at Niek van Hulst’s group at the MESA+ Institute for Nanotechnology of the Universiteit Twente, Netherlands, where he exploited single-molecule fluorescence spectroscopy techniques to study exciton dynamics in multichromophoric aggregates. Since 2004 he is a member of the “Electrochemistry, Photochemistry and Organic Reactivity” group of the Universitat Autònoma de Barcelona, where he is currently an Associate Professor. Based on his strong background in photophysics/photochemistry and nanoscience/nanotechnology, his current research interest focuses on the preparation and characterization of optically-active molecules and materials using both bulk and micro-/nanoscopic approaches. He is co-author of about 100 publications in international journals and 5 patents, has given more than 25 talks in (inter)national conferences, co-supervised 10 graduated PhD students and co-founded a spin-off company.

About the author

Dr. Jordi Marquet received his PhD in Chemistry from the Universitat Autònoma de Barcelona (UAB) under the supervision of Dr. Marcial Moreno-Mañas. After a postdoctoral stay at the University of Florida (USA) in the group of Prof. Alan Katritzky (Heterocyclic Chemistry and Reaction Mechanisms), and another one in the University of Leiden (The Netherlands) in the group of Dr. Cyril Varma (Photochemistry and Photophysics), he became Full Professor of Organic Chemistry in the Chemistry Department of the UAB in 1993. His research interests had been focused over the years on the study of fundamental aspects of chemical reactivity, especially the activation of chemical processes by environmentally-friendly methods (Photochemistry, Electrochemistry and Catalysis). In the last few years this interest has shifted to the study of new and more efficient methods of polymer synthesis, and to the design of new polymeric materials with special properties. He is also acting as a consultant for chemical companies and in this framework he has promoted the creation of a Joint Research Centre UAB-Henkel in the UAB, operative since 2011, where he acts as co-coordinator from the UAB. He has held different senior management positions, and among them, he has been responsible of the “Technology Transfer and Innovation” policy of the UAB for 7 years as a Vicerector and Director of the “UAB Research Park”. He has more than 120 publications in international journals, he has supervised 15 doctoral thesis and holds 14 registered patents.

About the author

Dr. Kevin Reyes. He recently obtained his PhD in 2023 at the UAB, under the supervision of Prof. Rosa Mª Sebastián. He has worked in different aspects of organic, inorganic and analytical chemistry: synthesis, new polymerization processes, organometallic catalysis, nanoparticle synthesis and extended polymer analysis, among others. His research consisted in the study of the properties of benzoxazines and polybenzoxazines. His novel discoveries in this area let to use the nature of these compounds in important applications in the industrial field, such as photolithography. Currently, he is focused in the field of renewable energies as a new step in his professional career. He has published an article and a patent as a result of his work.

Reference

Reyes-Mateo, K., Marquet, J., Hernando, J., & Sebastián, R. M. (2022). Photothermal polymerization of benzoxazines. Polymer Chemistry, 13(36), 5256-5264.

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