Combination of living anionic polymerization and ATRP via “click” chemistry as a versatile route to multiple responsive triblock terpolymers and corresponding hydrogels

Abstract

A combination of anionic polymerization, atom transfer radical polymerization (ATRP) and “click” chemistry was used to construct trishydrophilic ABC triblock terpolymers composed of a pH-sensitive A block, a water-soluble B block and two different thermo-sensitive C blocks without any block sequence limitation problems. First, an azido end-functionalized poly(2-vinylpyridine)-block-poly(ethylene oxide) (P2VP-b-PEO-N3) diblock copolymer was synthesized by anionic polymerization. In a second step, poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(oligo(ethylene glycol) methacrylate) (POEGMA) were synthesized via ATRP using an alkyne functionalized initiator. The resulting polymers were attached to the P2VP-b-PEO-N3 diblock copolymer using the 1,3-dipolar Huisgen cycloaddition (“click” chemistry). For the “click” step, P2VP-b-PEO-N3 diblock copolymers with either an azidoacetyl or a 2-azidoisobutyryl group were tested. In the latter case, however, a side reaction involving the cleavage of the formed “click” product via nucleophilic substitution occurred, preventing a permanent attachment of PDMAEMA or POEGMA to the P2VP-b-PEO-N3 diblock copolymer. Finally, P2VP-b-PEO-b-POEGMA (with POEGMA=P(MEO2MA-co-MEO8.5MA)) and P2VP-b-PEO-b-PDMAEMA triblock terpolymers were successfully synthesized and used to construct stimuli-responsive hydrogels. A concentrated solution of P2VP56-b-PEO370-b-P[(MEO2MA)89-co-(MEO8.5MA)7] showed a gel–sol–gel transition at pH 7 upon temperature increase, whereas in the case of P2VP56-b-PEO370-b-PDMAEMA70, a gel–sol or a weak gel–strong gel transition was observed, depending on the applied pH. Finally, the addition of trivalent hexacyanocobaltate(III) ions to the P2VP56-b-PEO370-b-PDMAEMA70 solution induced an upper critical solution temperature for the PDMAEMA block, which led to gel formation. This allows for the construction of light-sensitive hydrogels, utilizing the photo-aquation of hexacyanocobaltate(III) ions.