Synthesis and Thermoresponsive Property of Linear, Cyclic, and Star-Shaped Poly(N,N-diethylacrylamide)s Using B(C6F5)3-Catalyzed Group Transfer Polymerization as Facile End-Functionalization Method

The syntheses of linear, cyclic, and star-shaped poly(N,N-diethylacrylamide)s (PDEAAms) have been studied in order to clarify the topological effect on their thermoresponsive properties. For the group transfer polymerization of N,N-diethylacrylamide using tris(pentafluorophenyl)borane (B(C6F5)(3)) as the organocatalyst, the alpha-, omega-, and alpha,omega-end-functionalizations of the PDEAAms with well-controlled molecular weights and narrow polydispersities were quantitaively produced using the silyl ketene aminals with hydroxyl, ethynyl, and vinyl groups as functional initiators and 2-phenyl acrylate derivatives with hydroxyl, ethynyl, and bromo groups as functional terminators. The omega-end-functionalized PDEAAm with the azido group and the alpha,omega-end-functionalized PDEAAm with the ethynyl and azido groups were used as the starting materials for the inter- and intramolecular copper(I)-catalyzed click reactions leading to the 3-armed star-shaped and cyclic PDEAAms (s(3)-PDEAAm and c-PDEAAm, respectively). In order to eliminate the unit effect of the triazole (taz) group on the thermoresponsive behavior, the linear PDEAAm with the taz group at the center of the polymer chain (1-taz-PDEAAm) was prepared by the click reaction between the end-functionalized PDEAAm with the ethynyl group and that with the azido group. The thermoresponsive property of these PDEAAms with the DPs of 26-29, 50-52, and 78-80 was evaluated by the cloud point (T-c) determined by the turbidity measurements and the enthalpy changes (OH) of water molecules per molar monomer unit by highly sensitive differential scanning calorimetry (micro-DSC) measurements. The phase transition behavior of s(3)-PDEAArn on the transmittance curve was similar to that of 1-taz-PDEAAm, rather than c-PDEAAm. The Tc values decreased in the order of 1-taz-PDEAAm > c-PDEAAm > s(3)-PDEAAm. The OH values for s(3)-PDEAAm were almost the same as those for c-PDEAAm, which were lower than those for 1-taz-PDEAAm.