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