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Rapid and Quantitative De- tert -butylation for Poly(acrylic acid) Block Copolymers and Influence on Relaxation of Thermoassociated Transient Networks.
Macromolecules 2018 October 24
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 tert -butyl groups from poly( tert -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 tert -butyl cleavage on a ABA triblock copolymer, where poly( N -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 tert -butyl-free. Additionally, SAXS shows that the conformational properties of tert -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.
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