ADIPRENE® LFP R375 Low Free pPDI-Terminated Polycarbonate Prepolymer. Superior to standard TDI and MDI prepolymers, the exceptional performance of ADIPRENE® Low Free pPDI prepolymers is due to pPDI (p-phenylene diisocyanate) monomer, a special building block that imparts exceptional properties to cast polyurethane elastomers.