Synthesis of well-defined amphiphilic poly(ε-caprolactone)-b-poly(N- vinylpyrrolidone) block copolymers via the combination of ROP and xanthate-mediated RAFT polymerization

Well-defined amphiphilic poly(ε-caprolactone)-b-poly(N- vinylpyrrolidone) (PCL-b-PNVP) block copolymers were successfully prepared via the combination of ring-opening polymerization (ROP) and xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization. Well-defined poly(ε-caprolactone) (PCL-OH) was synthesized by ROP in bulk at 110 °C using benzyl alcohol as initiator and stannous octate [Sn(Oct)₂] as catalyst. The -OH end group was then converted into its corresponding xanthate (PCL-X) via the conversion to its corresponding bromide (PCL-Br). These are verified by ¹H NMR spectroscopy. PCL-b-PNVP block copolymers were synthesized via RAFT polymerization in tetrahydrofuran (THF) at 80 °C using PCL-X as macro-chain transfer agent and characterized by ¹H NMR spectroscopy and gel permeation chromatography (GPC). The amphiphilic diblock copolymer PCL₆₃-b-PNVP₉₀ forms spherical micelles of ∼34 nm diameter in water as shown by transmission electron microscopy (TEM), supported by ¹H NMR spectroscopy, and light scattering. The critical micellar concentrations were determined by fluorescence spectroscopy using pyrene as probe. The critical micelle concentration (cmc) value of the block copolymers increases with the increase in the chain length of PNVP block. The overall hydrodynamic radius (R_h) of the micelles remains almost constant over the concentration range above the cmc value and over the angles of scattering measurement.