Abstract
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 (Mn = ca. 8000 to 16,000 and Mw/Mn <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 Tgs 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 (Mn,NMR = 14,730 and Mw/Mn = 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.
Highlights
Thermoresponsive polymers that exhibit lower critical solution temperatures (LCST) in aqueous solution are among the most important water-soluble polymers
Poly(N-isopropyl acrylamide) (PNIPAM) performs a sharp and reversible phase transition at a temperature that is close to our body temperature
The cloud point temperatures (CPs) of the copolymers were determined by transmittance measurements in a quartz cell charged with a polymer concentration of 1 mg/mL in a PerkinElmer Lambda 25 UV-vis spectrometer (PerkinElmer, Waltham, MA, USA)
Summary
Thermoresponsive polymers that exhibit lower critical solution temperatures (LCST) in aqueous solution are among the most important water-soluble polymers. A key feature of these polymers is a reversible phase transition in an aqueous solution, resulting from a reversible change of polymer conformation at a certain temperature (i.e., cloud point (CP)) This phenomenon is based on the diminishing entropy with increasing temperature caused by the immobilization of the solvent molecules on the surface of the dispersed polymer chain. (PNAPP), and poly[N-(3-methacryloyloxypropyl)-pyrrolidone] (PNMPP) [15,16] They interestingly demonstrated that the fully hydrated state of the polymers in D2 O had a phase transition occur at the cloud point, and that further heating leads to dehydration and separation from the D2 O. Scheme 1.vinylpyrrolidone (a) reversible transfer of and (b) copolymerization ring-opening (NVP) addition–fragmentation and 3-ethyl-1-vinyl-2-pyrrolidone (C2NVP)(RAFT). N-vinylpyrrolidone (NVP) and 3-ethyl-1-vinyl-2-pyrrolidone (C2 NVP) and (b) ring-opening polymerization (ROP) of ε-caprolactone (CL)
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