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

The living ring-opening polymerization of delta-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(delta-valerolactone)s (N-3-PVL-OH) bearing azide groups at the cc-chain ends with M-n,M-NMRS (PDIs) of 2600 (1.08), 4700 (1.11), and 9900 (1.09). The acetylene functionality was introduced at the omega-end of N-3-PVL-OH using 5-hexynoyl chloride to afford the telechelic poly(delta-valerolactone) with the azide group at the alpha-end and acetylene group at the omega-end (N-3-PVL-C CH). The click reaction between the alpha-azide and the omega-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 H-1 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 M:S 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,M-NMR = 2800-9500) in reasonable yield (60-80%) using the click cyclization.