Synthesis of Well-Defined Macrocyclic Poly(δ-valerolactone) by “Click Cyclization”

The living ring-opening polymerization of δ-valerolactone (VL) initiated from 6-azide-1-hexanol using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1-[3,5-bis(trifluoromethyl)phenyl]-3-cyclohexylthiourea (BCT) was carried out to prepare the poly(δ-valerolactone)s (N 3 −PVL−OH) bearing azide groups at the α-chain ends with M n,NMR s (PDIs) of 2600 (1.08), 4700 (1.11), and 9900 (1.09). The acetylene functionality was introduced at the ω-end of N 3 −PVL-OH using 5-hexynoyl chloride to afford the telechelic poly(δ-valerolactone) with the azide group at the α-end and acetylene group at the ω-end (N 3 −PVL−C≡CH). The click reaction between the α-azide and the ω-acetylene of N 3 −PVL−C≡CH in DMF was carried out under the highly diluted condition as [N 3 −PVL−C≡CH] = 0.18 mM, which was monitored by IR and 1 H NMR measurements. The SEC peak of the cyclic-PVL shifted to the lower molecular weight region than that of N 3 −PVL−C≡CH, and the intrinsic viscosity of the cyclic-PVL significantly decreased. In addition, there was no change in the molecular weight of the resulting polymer through the click cyclization, which was confirmed on the basis of the MALDI-TOF MS measurement. Finally, we succeeded in the synthesis of a well-defined cyclic-PVL having a narrow polydispersity ( M w / M n = 1.09−1.15) and the predicted molecular weight ( M n,NMR = 2800−9500) in reasonable yield (60−80%) using the click cyclization.