Synthesis and solution properties of novel thermo- and pH-responsive poly(N-vinylcaprolactam)-based linear–dendritic block copolymers

Abstract

This study describes the synthesis and solution properties of the novel linear–dendritic block copolymers (LDBCs) based on thermoresponsive poly(N-vinylcaprolactam) (PNVCL) chains and pH-responsive poly(benzyl ether) dendrons. The peripherally methyl ester-functionalized LDBCs (CH₃OOC-G_n-b-PNVCL) were first synthesized via atom transfer radical polymerization (ATRP) of N-vinylcaprolactam (NVCL) using first- and second-generation dendritic poly(benzyl ether) chlorides with methyl ester peripheries as initiators. Their peripheral methyl ester groups were then hydrolyzed to afford the target thermoresponsive and pH-responsive poly(N-vinylcaprolactam)-based LDBCs (NaOOC-G_n-b-PNVCL). The results of ¹H NMR and GPC analysis showed that the synthesized LDBCs have controlled molecular weight and narrow polydispersity. The results of turbidity and dynamic light scattering (DLS) measurements demonstrated that the lower critical solution temperature (LCST) values of synthesized LDBCs decrease with increase in the generation of the dendritic poly(benzyl ethers) and the concentrations of the LDBCs solutions. The thermoresponsive behavior of NaOOC-G_n-b-PNVCL was also influenced by the pH value of the copolymer solutions. The self-assembly behavior of the LDBCs in an aqueous solution was investigated by fluorescence spectroscopy and transmission electron microscopy (TEM). The results indicated that the morphology of the amphiphilic CH₃OOC-G_n-b-PNVCL in an aqueous phase changed from rod-like dendritic structure to a microrectangle shape with increase in the generation of the dendritic poly(benzyl ethers). CH₃OOC-G_n-b-PNVCL form more well-defined aggregates. Second critical micelle concentration of NaOOC-G₁-b-PNVCL has been observed.