Sustainable photopolymerization of benzoxazines at room temperature in aqueous media

Significance 

Polybenzoxazines, a type of phenolic resins, come from the polymerization of 1,3-benzoxazine monomers whose preparation is easy and cheap, starting from commercially available phenols and amines. These materials possess many attractive attributes such as low water absorption, near-zero volume changes during polymerization, very high chemical resistance, high char yield among other interesting properties. Nonetheless, the fact that polymerization takes place at rather high temperatures is one of their main drawbacks, not only by the energetic expenditure required, but also because monomeric 1,3-benzoxazines undergo decomposition and hydrolysis at high temperature. Recent studies have shown that catalyst addition can modestly reduce the polymerization temperature. This has in turn led to the wide spread utilization of catalysts to promote ring-opening polymerization. Regardless, so far no one has attempted room temperature synthesis of polybenzoxazines starting from monofunctional benzoxazines without the help of any catalyst or initiator. In addition, no record of any attempt to synthesize polybenzoxazines in water exists. Therefore, it would be very advantageous to find a sustainable method to polymerize mono-1,3-benzoxazines in aqueous solutions at room temperature.

Recently, Universitat Autònoma de Barcelona researchers: Dr. Jordi Salabert a Post-doctoral researcher, Associate professor Dr. Rosa M. Sebastián and Professor Dr. Jordi Marquet tested if the direct ring-opening polymerization of benzoxazines could be elicited photochemically at room temperature, taking advantage of the topologically controlled Coulombic interactions that would be produced in the benzoxazine structure upon UV irradiation. They also were the first to undertake the same using water at ambient room temperature. Their work is currently published in the research journal, Macromolecules.

Briefly, the research method employed consists in the preparation of solutions of 1,3-benzoxazines and their irradiation, using appropriate lamps and immersion reactiors. Firstly, water/organic solvent mixtures were tested, then, in a more sustainable approach, water media using a surfactant agent was performed. Benzoxazine monomers were previously prepared following two well-known methodologies from phenols, paraformaldehyde and triazine derivative in bulk or in the presence of an organic solvent, 1,4-dioxane. Monomers and obtained polymers were characterize using conventional techniques, such as nuclear magnetic resonance. Molecular weights and polydispersities of polymers were obtained by gel permeation chromatography.

The authors observed that irradiation of ‘electron-withdrawing groups’-substituted benzoxazines produced opening of the oxazine ring as expected, but no propagation of the polymerization was seen probably due to the low nucleophilicity of the intermediate phenolic species. However, with more nucleophilic electron-donor substituted benzoxazines the photopolymerization proceeded smoothly, with excellent conversions and modest degrees of polymerization. The researchers also noted that the produced materials showed a similar degree of polymerization to the one observed in the non-cross-linked soluble fraction when monobenzoxazines were polymerized under thermal conditions but having a lower dispersity.

In summary, Universitat Autònoma de Barcelona scientists demonstrated that by taking advantage of the topology of the structure of N-phenyl mono- 1,3- benzoxazines and the fact that they absorb in the UV range, it was possible to photochemically induce the O−alkyl bond cleavage and thus the ring opening through an intramolecular electron transfer process. Altogether, the model substituted monobenzoxazines could be photochemically polymerized in aqueous solutions at room temperature.

Sustainable photopolymerization of benzoxazines- Advanced Engineering

About the author

Dr. Rosa M. Sebastián. Associate professor at 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 supported 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), working also in the preparation of macrocycles and dendrimers with applications as catalyst, liquid crystals, sensors, etc.

Her research interest have 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 since 2010. For this partnership, UAB was awarded the “National Award for Public-Private Partnership Research” in 2012. She is author/co-author of more than 65 articles in high impact journals, and 10 patents.

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. Jordi Salabert. Post-doctoral researcher at the Universitat Autònoma of Barcelona (UAB, Spain). He obtained his PhD in 2013 at the UAB, under the supervision of Dr. Adelina Vallribera and Dr. Rosa M. Sebastián. He took part of some partnerships company-university as postdoctoral student, including awarded post-doctoral training program between Henkel KGaA & Co. In 2017 he became postdoctoral researcher at the Universitat Autònoma of Barcelona.

He has been involved in several aspects of organic chemistry, such as: organometallic catalysis, preparation of highly fluorinated materials by anchoring fluorinated dyes on cotton fabrics, green chemistry (reactions in water media), synthesis of polymers, mainly poly(benzoxazines), and developing new photochemical polymerization methods.

Reference

Jordi Salabert, Rosa María Sebastián, Jordi Marquet. Photochemical Polymerization of N-Phenyl Mono-1,3-benzoxazines in Aqueous Media . Macromolecules 2018, volume 51, page 3672−3679.

Go To Macromolecules

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