Gradient copolymers of ε-caprolactone and δ-valerolactone via solvent-free ring-opening copolymerization with a pyridyl-urea/MTBD system

The ring-opening copolymerization of epsilon-caprolactone and delta-valerolactone was carried out under solvent-free conditions with a pyridyl-urea (6C-PU)/7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene catalytic system by premixing two monomers, yielding gradient copolyesters with cyclic and linear structures in the absence or presence of an initiator, respectively. The random and block copolyesters of these two monomers were also prepared for comparison. The results of H-1 and C-13 NMR spectra determined the linear and cyclic structure of copolymers. The kinetic experiment was used to determine the consuming rates of two monomers during the polymerization. The topologies and thermal properties of all the copolymers were characterized and compared by NMR and DSC.