Quantitative end-group functionalization of PNIPAM from aqueous SET-LRP: Via in situ reduction of Cu(II) with NaBH₄

Aqueous phase SET-LRP has become a useful method to produce water-soluble polymers directly in water. However, the major issue of hydrolysis of the halide chain-end has restricted the use of these water-synthesized polymers from further 'click' reactions. Here, we examined the kinetics of hydrolysis of poly(N-isoproplyacrylamide) (PNIPAM) synthesized using a new method to reduce Cu(ii) directly and quantitatively to Cu(0) with no trace of Cu(i). It was found that hydrolysis followed a pseudo first order loss rate, reaching near completion after ∼15 h. The hydrolysis rate was not influenced by either molecular weight or the amount of Cu(0) or Cu(ii) in the system. To overcome the issues of hydrolysis, we carried out an aqueous-phase in situ azidation, which usually is a very slow process (from 10-24 h) in organic solvents, to quantitatively transform the halide end-groups to azides within 30 s. This allowed the polymer to be purified and further coupled to an alkyne PNIPAM with greater than 97% coupling efficiency. Our work provides a direct and quantitative method to produce polymers made in water with stable 'click' functional end-groups, expanding the use of such polymers in the construction of more complex polymer architectures.