Crosslinking of polymers from monofunctional acrylates via C–H bond activation

Abstract

Cross-linked acrylic polymers are highly valued for their durability and chemical resistance, making them extensively used in various applications and industries. Typically, they are synthesized by adding crosslinkers (multifunctional monomers or oligomers) or through post-treatment processes. However, our preliminary studies indicated that even monofunctional acrylic monomers can form cross-linked structures. The formation of these cross-linked structures is thought to result from a chain transfer mechanism involving hydrogen atom transfer (HAT) between monomer substituents and radicals. Despite this, the process has not been thoroughly analyzed, and few studies have applied this concept to the design of polymer networks. To clarify this phenomenon, we conducted a series of screening experiments using ten widely-used monomers and three different photoinitiation systems, discovering that 2-hydroxyethyl acrylate (HEA), 4-hydroxybutyl acrylate (HBA), and 2-[2-(2-methoxyethoxy)ethoxy]ethyl acrylate (MEEEA) exhibited superior crosslinking capabilities. Furthermore, quantum chemical calculations were employed to elucidate why these specific monomers excel in forming cross-linked networks. Assuming the propagating acryl radical acts as the hydrogen acceptor for HAT, it was found that the reactions occurring in the substituents of MEEEA and under hydrogen bonding conditions in HEA and HBA are thermodynamically stable. Consequently, the cross-linked structures formed by monofunctional acrylates were determined to be due to radicals generated at the substituents of the monomers. This enhanced understanding offers a new paradigm in the synthesis of cross-linked polymers, expanding their potential across diverse applications.