Rapid and Quantitative De--butylation for Poly(acrylic acid) Block Copolymers and Influence on Relaxation of Thermoassociated Transient Networks.

The synthesis of charged polymers often requires the polymerization of protected monomers, followed by a polymer-analogous reaction to the polyelectrolyte product. We present a mild, facile method to cleave -butyl groups from poly(-butyl acrylate) blocks that yields poly(acrylic acid) (pAA) blocks free of traces of the ester. The reaction utilizes a slight excess of HCl in hexafluoroisopropanol (HFIP) at room temperature and runs to completion within 4 h. We compare deprotection in HFIP with the common TFA/DCM method and show that the latter does not yield clean pAA. We show the effect of complete -butyl cleavage on a ABA triblock copolymer, where poly(-isopropylacrylamide) (pNIPAM) is A and pAA is B, by means of viscosimetry, DLS, and SAXS on solutions above overlap. The pNIPAM blocks dehydrate, and their increased self-affinity above the lower critical solution temperature (LCST) results in network formation by the triblocks. This manifests itself as an increase in viscosity and a slowing down of the first-order correlation function in light scattering. However, this stickering effect manifests itself exclusively when the pAA block is -butyl-free. Additionally, SAXS shows that the conformational properties of -butyl-free pAA copolymers are markedly different from those with residual esters. Thus, we illustrate a surprising effect of hydrophobic impurities that act across blocks and assert the usefulness of HCl/HFIP in pAA synthesis.