Today, most phosgene is used to produce isocyanates. The most important isocanates in this context are toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). Both are precursors to polyurethanes. Large amounts are also used to produce polycarbonates. Polycarbonate is produced from its reaction with bisphenol A. Polycarbonates are an important class of engineering thermoplastic. They can be found in lenses in eyeglasses.