Model-Guided Design and Solvent-Free Synthesis of Polysiloxanes with High Refractive Index and Low Viscosity

High refractive index polysiloxanes have been extensively applied in the optoelectronic industry due to their excellent properties. Efficient synthetic methodologies coupled with predictive models of the structure-property relationship and effective strategies to modulate their properties are essential in developing high-performance polysiloxanes. In this study, polymethylphenylsiloxanes (PMPS) were synthesized via bulk polymerization of commercial cyclosiloxane monomers with systematic investigation into the effect of reaction conditions on refractive index (RI), viscosity, and molecular weight. Quantitative models on the molecular weight dependence of viscosity were established, revealing that PMPS with high RI inevitably exhibit high viscosity, causing challenges in processing and transportation. Guided by the models, the synthesis was optimized to successfully produce PMPS achieving both high RI (1.562 at 25 °C) and low viscosity (250 mPa·s at 25 °C). The optimized PMPS demonstrated excellent overall performance, including high optical transparency, superior surface wettability, relatively low thermo-optic coefficient, and thermal stability. This study provides a systematic methodology and design guidelines for polysiloxane development.