High Molecular‐Weight Cyclic Polyesters from Solvent‐Free Ring‐Opening Polymerization of Lactones with a Pyridyl‐Urea/MTBD

Abstract (Thio)urea/base systems have been widely used in the ring‐opening polymerization (ROP) of lactones based on their superior performance in activity and selectivity. Herein, a pyridyl‐urea ( 6C‐PU )/7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene catalytic system is used to prepare the polyesters with cyclic topology via the ROP of lactones in the absence of initiator under solvent‐free conditions. The catalytic system shows high activity in the ROP of δ‐valerolactone (δ‐VL) and cyclic polyvalerolactones (PVLs) with high molecular weights and narrow molecular weight distribution are obtained despite the existence of some linear products generated during the precipitation process. The structure of the resulting polymers is determined by proton and carbon nuclear magnetic resonance spectra, matrix assisted laser desorption/ionization time of flight mass spectrometry, and intrinsic viscosity studies. A possible mechanism is proposed on the basis of NMR analysis and characteristics of polymerization. Thermal and mechanical properties of cyclic PVLs are investigated through differential scanning calorimetry, thermogravimetric analysis, and tensile test.