High Refractive Index Polymers (n > 1.7), Based on Thiol–Ene Cross-Linking of Polarizable P═S and P═Se Organic/Inorganic Monomers

A new class of high refractive index polymers was developed through the thiol–ene coupling reaction of trivinylphosphine chalcogenides, [(CH2═CH−)3P═X (X = S, Se)], with 1,2-ethane dithiol and 1,3-benzene dithiol. The polymers were obtained through a thermal initiation and polymerization process which yielded robust monolithic materials. The polymers have good transmission properties in the visible region of the spectrum with very high refractive indices ranging from 1.66 to 1.75, placing them in the top echelon of high refractive index polymers, with Abbe numbers between 22 and 31. The high refractive index is due, in large part, to the presence of the highly polarizable P═S and P═Se groups, which have molar refractivities of 13.84 and 17.33 mol/cm3, respectively. The polymers tend to have a high cross-link density that varies with the composition but results in rigid materials for most compositions. The glass-transition temperatures (Tg) for all but one composition were above 70 °C, and the storage moduli ranged from 1.5 to 3.8 GPa. The 1,2-ethane dithiol compositions had lower values of Tg than the 1,3-benzene dithiol materials. This was attributed to a lower cross-link density in the 1,2-ethane dithiol polymers, as determined from the residual unreacted vinyl groups measured by Raman spectroscopy and 31P solid-state nuclear magnetic resonance spectroscopy. The P═Se polymers were slower to react and required higher temperatures; they also produced the hardest materials with the highest refractive indices.