Synthesis of well-defined alkyne terminated poly(N-vinyl caprolactam) with stringent control over the LCST by RAFT

Abstract

The reversible addition-fragmentation chain transfer (RAFT) of <i>N</i>-vinyl caprolactam (NVCL) using two new xanthates with alkyne functionalities is reported. The kinetic data obtained for polymerization of this non-activated monomer using a protected alkyne-terminated RAFT agent (PAT-X₁) revealed a linear increase of the polymer molecular weight with the monomer conversion as well as low dispersity (<i>Đ</i>) during the entire course of the polymerization. The system reported here allowed us to enhance the final conversion, diminish <i>Đ</i> and reduce the polymerization temperature compared to the typical values reported in the scarce literature available for the RAFT polymerization of NVCL. The resulting PNVCL was fully characterized using ¹H nuclear magnetic resonance (¹H NMR), matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) techniques. The temperature-responsive features of PNVCL in aqueous solutions were fully investigated under different conditions using turbidimetry. The presented strategy allows the synthesis of well-defined PNVCL with sharp and reversible phase transition temperatures around 37 °C. By manipulating the polymer molecular weight, or the solution properties, it is possible to tune the PNVCL phase transition. As a proof-of concept, the alkyne functionalized PNVCL was used to afford new linear block copolymers, by reacting with an azide-terminated poly(ethylene glycol) (N₃-PEG) through the copper catalyzed azide-alkyne [3+2] dipolar cycloaddition (CuAAC) reaction. The results presented establish a robust system to afford the synthesis of PNCVL with fine tuned characteristics that will enable more efficient exploration of the remarkable potential of this polymer in biomedical applications.