High-efficiency synthesis of well-defined cyclic poly(N-vinylcaprolactam) and its solution properties

- Cyclic poly(N-vinylcaprolactam) (PNVCL) is prepared at a concentration of 10 mg/mL.
- Cyclic PNVCL can be synthesized by “selective” click cyclization in micellar media.
- Cyclic PNVCL tends to form smaller aggregates compared to its linear analogue.
- Cyclic PNVCL possesses lower LCST compared to its linear analogue.
- Cyclic PNVCL possesses narrower thermal phase transition range.

Well-defined cyclic poly(N-vinylcaprolactam) (c-PNVCL) was successfully synthesized by a combination of atom transfer radical polymerization (ATRP), supramolecular self-assembly and “selective” click cyclization at a relatively high concentration (10 mg/mL). α-Alkyne-ω-chloro heterodifunctional PNVCL precursor (l-PNVCL-Cl) was synthesized by ATRP of N-vinylcaprolactam (NVCL), followed by its conversion to α-alkyne-ω-azido heterodifunctional PNVCL (l-PNVCL-N3) via nucleophilic substitution reaction with NaN3. The intramolecular cyclization of l-PNVCL-N3 by “selective” click reaction in aqueous micellar media afforded well-defined and narrow-disperse c-PNVCL (PDI = 1.11). The target polymer and intermediates were characterized by GPC/MALLS, 1H NMR, and FT-IR. The thermoresponsive property of c-PNVCL and its linear precursor (l-PNVCL-N3) was studied and compared by turbidity and dynamic light scattering (DLS) measurements, and furthermore, their self-assembly behavior was investigated by fluorescence spectroscopy and transmission electron microscopy (TEM). Cyclic poly(N-vinylcaprolactam) can self-assemble to form smaller spherical micelles in aqueous solution and possesses lower critical solution temperature (LCST) and narrower thermal phase transition range in comparison with its linear analogue.