Unexpected radical polymerization behavior of oligo(2-ethyl-2-oxazoline) macromonomers

A well-defined oligo(2-ethyl-2-oxazoline)acrylate (OEtOxA) macromonomer was obtained by direct end functionalization of the living cationic oxazolinium species from the cationic ring-opening polymerization of EtOx with in situ deprotonated acrylic acid. Kinetic studies during subsequent reversible addition–fragmentation chain transfer (RAFT) polymerization as well as nitroxide mediated polymerization (NMP) experiments revealed proceeding monomer consumption but no increase of the molar mass of the resulting comb polymers. The chain transfer during the radical polymerizations is proposed to result from backbiting and subsequent β-scission of the formed mid-chain radical and took place in a well-defined manner, so that POEtOxA could also be obtained by free radical polymerization with a PDI value below 1.2. A series of POEtOxA was synthesized by RAFT polymerization with varying [monomer]/[chain transfer agent] (M/CTA) ratios and analyzed in detail by means of analytical ultracentrifugation and small angle neutron scattering, indicating that the backbone DP does not exceed 25, which is in accordance with the thermal polymer properties in bulk and in aqueous solution (Tg = 32 °C, Tcp ≈ 73 °C).