Synthesis of PNVP-Based Copolymers with Tunable Thermosensitivity by Sequential Reversible Addition⁻Fragmentation...

Through the reversible addition⁻fragmentation chain transfer (RAFT) copolymerization of 3-ethyl-1-vinyl-2-pyrrolidone (C₂NVP) and N-vinylpyrrolidone (NVP), a series of well-defined P(C₂NVP-co-NVP) copolymers were synthesized (M_n = ca. 8000 to 16,000 and M_w/M_n <1.5) by using a difunctional chain transfer agent, S-(1-methyl-4-hydroxyethyl acetate) O-ethyl xanthate (MHEX). Copolymerizing kinetics and different monomer ratio in feeds were conducted to study the apparent monomer reaction rate and reactivity ratios of NVP and C₂NVP, which indicated similar reaction rates and predominantly ideal random copolymers for the two monomers. The T_gs of the obtaining P(C₂NVP-co-NVP) copolymers significantly corresponded to not only molecular weights MWs but also copolymer compositions. These copolymers presented characteristic lower critical solution temperatures (LCST) behavior. We then studied the cloud points (CPs) of the copolymers with varying MWs and compositions. With different MWs, the CPs were linearly decreased from ca. 51 to 45 °C. With different compositions, the CPs of the copolymers decreased from ca. 48 to 29 °C with C₂NVP content (i.e., from 60.8 to 89.9 mol %). Fitting the CPs by the theoretical equation, the result illustrated that the introduction of more hydrophobic units of C₂NVP suppressed the hydrophilic interaction between the polymer chain and water. We then successfully proceeded the chain extension through the ring-opening polymerization (ROP) of ε-caprolactone (CL) to the synthesis of a novel P(C₂NVP-co-NVP)-b-PCL amphiphilic block copolymer (M_n,NMR = 14,730 and M_w/M_n = 1.59). The critical micelle concentration (CMC) of the block copolymer had a value of ca. 1.46 × 10^-4 g/L. The block copolymer micelle was traced by dynamic light scattering (DLS), obtaining thermosensitive behaviors with a particle size of ca. 240 nm at 25 °C and ca. 140 nm at 55 °C, respectively.