A Hybrid Methacrylate-Norbornene-Thiol Photopolymerization System for One-Pot Synthesis of Adhesives with Tunable Physical and Mechanical Properties

The photopolymerization of a hybrid methacrylate-norbornene-thiol system is employed to synthesize adhesive formulations with adjustable physical and mechanical properties. The suggested one-pot synthesis offers an environmentally friendly, zero waste approach for the rapid synthesis of UV-curable solventless formulations. The combination of methacrylate and thiol-ene brings together the advantages of both free radical homopolymerization and thiol-ene step growth reactions. Thiol-ene reactions offer benefits such as delayed gel point, high conversion, low shrinkage and stress, and tolerance to oxygen. This study investigates the impact of various factors, including the thiol:ene ratio, thiol functionality, and the addition of a methacrylate as a hard monomer, on the physical properties as well as the viscoelastic and adhesive performance of these formulations. Initially, pressure-sensitive adhesives (PSAs) are produced when ethylene glycol dicyclopentenyl ether methacrylate (EGDEMA) is crosslinked with the di-functional thiol (EBM) to create crosslinked networks with high conversion (up to 100%) and low glass transition temperature (Tg=-2.5°C to -13.5°C). Various thiol:ene ratios are studied; rheological studies indicate that these formulations make PSAs exhibiting best adhesive/cohesive performance at thiol:ene ratios of 0.5-0.75. Elastomeric polymer films are then produced when EGDEMA is crosslinked with a tetra-functional thiol (PEMP) resulting in highly crosslinked films with high conversion (up to 100%), higher Tg (ranging from 9°C to 17°C), and increased gel content (up to 100%). Finally, temporary adhesives are produced by the incorporation of a hard monomer, isobornyl methacrylate (IBOMA) along with a bifunctional thiol (EBM), resulting in thermoplastic polymers with low gel content and variable Tg (-12°C to 41°C).These findings suggest the potential utilization of this hybrid system as one-step UV-curable formulations to produce polymer networks for various applications, depending on the materials used. The environmental benefits of this approach include eliminating the production of waste and by-products, as well as eliminating the need for heat, solvents, or any post polymerization processing such as purification and precipitation.