Structure–Property Relationships and Catalytic Mechanism of Pentacoordinate Silicon Ionic Latent Accelerators for Epoxy Resins

Thermosetting epoxy resins are indispensable materials in integrated circuit packaging, where latent accelerators are crucial for modulating the curing behavior and processability of epoxy resins. This study systematically investigates onium silicate-based latent accelerators featuring a pentacoordinate silicon (Penta-Si) anionic structure for epoxy-phenolic resin systems. Seven Penta-Si ionic accelerators were synthesized with systematic variations in chelating groups, silicon substituents, and cationic species. Results demonstrate that latency and catalytic activity are governed by these three structural factors. All Penta-Si accelerators exhibited superior latency compared to the conventional triphenylphosphine (TPP) accelerator, with epoxy resin systems containing these accelerators showing higher onset temperatures ( T 0 ) and broader processing windows, while displaying sharper exothermic peaks upon activation─indicating faster curing rates once initiated. Cured resins containing Penta-Si accelerators showed enhanced thermomechanical properties, with glass transition temperatures ( T g ) increasing by 3–12 °C relative to the TPP-cured system. A three-step initiation mechanism was proposed and validated through NMR, ESI-MS, and DFT calculations: thermally induced dissociation of the ionic pair, proton transfer from phenol to the silicon anion yielding a phenolate anion, and spontaneous association with the phosphonium or ammonium cation to form the active catalytic species that initiates epoxy ring-opening. Latency is governed by initiation energy barriers and accelerator-resin compatibility. This work provides a rational design framework for latent accelerators tailored to the processing requirements of integrated circuit packaging materials.