Graft copolymers of hydroxyethyl cellulose by a ‘grafting to’ method: 15N labelling as a powerful characterisation tool in ‘click’ polymer chemistry

Abstract

We demonstrate how ¹⁵N labelling can be used to probe the success of ‘grafting to’ processes, through the preparation of well-defined graft copolymers of hydroxyethyl cellulose by combining RAFT polymerisation and copper-catalysed azide-alkyne cycloaddition (CuAAC). Using synthesised alkyne-functionalised chain transfer agents, short-chain (DP = 10) poly(N-vinylpyrrolidone) (PVP) and poly(N-isopropyl acrylamide) (PNIPAAM) were prepared in high conversion in a controlled manner (Đ_M of ∼1.4 and 1.2 respectively). Separately, partially ¹⁵N-labelled N₃-HEC was synthesised and characterised using solid state ¹³C, ¹⁵N CP-MAS NMR and FTIR spectroscopies. Alkyne-terminated PVP and PNIPAAM were grafted at different graft densities onto partially ¹⁵N-labelled N₃-HEC using the click reaction. The hybrid HEC-g-polymer materials were fully characterised using solid state ¹³C and ¹⁵N CP-MAS NMR and FTIR spectroscopies. While ¹³C and FTIR spectroscopies gave indirect or weak evidence of CuAAC coupling, the cycloaddition of the alkyne-terminated polymers with N₃-HEC was proven unambiguously by ¹⁵N solid state NMR spectroscopy. This indicates the utility of ¹⁵N labelling for probing the coupling efficiency of CuAAC reactions when employed in ‘grafting to’ processes with cellulosic substrates.