Microwave heating continues to attract significant attention as a promising alternative to conventional heating. It is environmentally friendly and generally suitable for both large-scale production and lab-scale research. The attractiveness of microwave heating technology can also be attributed to its capability to accelerate chemical reactions to achieve rapid transformation, increased yields and fewer byproducts. Microwave irradiation has been extensively applied in different chemistry fields, including materials, analytical and medicinal chemistry. However, its application in polymer synthesis has remained underexplored. The recent development of new microwave reactors with improved reaction parameters control has proved useful in enabling microwave-assisted polymerization to synthesize different polymers via either control radical polymerization or conventional free radical polymerization.
To date, several studies have been conducted to investigate the free-radical solution polymerization, providing a good avenue for comparing the performance of microwave and conventional heating assisted polymerizations. In most reaction conditions, microwave heating technology exhibited improved performance than conventional heating. Other studies, however, reported no significant differences between the two heating methods. For instance, the reported studies on the copolymerization of different monomers in different solvents produced no differences in the monomers studied. The controversial results have raised many questions on their reliability and validity. Despite the development of specialized microwave reactors, research shows that controlling the reaction temperature still requires double monitoring using sensory devices. These revelations provide a new understanding of the microwave effects that can be attributed to the thermal effects. Nonetheless, comparative experimental studies on the microwave and conventional heating-assisted polymerization processes are still lacking.
To this note, Dr. Bertha Pérez-Martínez, Professor José Ramon Leiza and Professor Radmila Tomovska from the University of the Basque Country UPV/EHU, in collaboration with Dr. Ali Aboudzadeh (Centro de Física de Materiales, CSIC-UPV/EHU,) and Professor Ulrich S. Schubert (Friedrich Schiller University Jena) performed both conventional heating and microwave irradiation assisted free-radical polymerization reactions in different organic solvents and in the presence of different initiators. The aim was to obtain reliable and comparable results. The original research article is found in the Chemical Engineering Journal.
In their approach, similar reaction conditions were created for both microwave and conventional heating reactors leveraging the advantages of the microwave reactor design, similar temperature profiles, and the condition of both the polymerization reactions. Copolymerization of different monomers with different dielectric properties and different polarities was studied in different organic solvent solutions using different initiators. Also, the effects of co-monomer systems, solvents, and initiators on the reaction rates and on the composition and properties of the copolymers were investigated.
The authors observed no changes in the molar masses, polymerization rates and polymer composition under similar reaction conditions for conventional heating and microwave irradiation reactors, for all studied monomer couples, contrary to some previous reports, where, mainly the enhancement in the reaction rate was observed. The exception from this happened in the presence of organometallic monomer in the monomer couple, where microwave irradiation exhibited an enhancement in the reaction rate as well as differences in the reactivity ratios and the copolymer compositions. This effect, also known as the specific microwave effect, was more pronounced in toluene since it was exposed to more microwave irradiation to maintain the same temperature profile.
In summary, the study reported a reliable comparison of microwave irradiation and conventional heating-assisted polymerization by creating similar reaction conditions. The interaction between the reaction components and the microwave irradiation was vital in determining any microwave irradiation effects on the reaction and resulting products. The microwave effects, demonstrated for the time in free-radical solution polymerization, were explained in detail via the heating properties of the microwave irradiation in the multi-component solution. From the comparisons of the energy consumption between the polymerization systems, the authors concluded that selecting appropriate solvent is crucial in saving energy without compromising the product quality. In a statement to Advances in Engineering, Professor Radmila Tomovska said the findings will advance future research on the effects of microwave irradiation in free-radical polymerization processes.
Pérez-Martínez, B., Aboudzadeh, M., Schubert, U., Leiza, J., & Tomovska, R. (2020). Microwave irradiation versus conventional heating assisted free-radical copolymerization in solution. Chemical Engineering Journal, 399, 125761.