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Reversible Complexation of Iminophenylboronates with Mono- and Dihydroxy Methacrylate Monomers and Their Polymerization at Low Temperature by Photoinduced ATRP in One Pot

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journal contribution
posted on 26.02.2016 by Elkin Amado, Jörg Kressler
The unique ability of boronic acids to form reversible covalent linkages to 1,2- and 1,3-diols creating boronate ester rings and the pH-sensitiveness of their dissociation have stimulated the development of boron-containing polymers having phenyl­boronate moieties intended to be detached by competitive binding of sugars. Typically, they are synthesized by postpolymerization reactions on hydroxy-functional polymers, which cannot guarantee a quantitative functionalization, or from monomers containing the phenyl­boronate moiety synthesized through multiple reaction/separation/purification steps. A handy strategy for the synthesis of monomers containing an iminophenyl­boronate (IPB) side-chain moiety derived from glycerol monomethacrylate (GMA) or 2-hydroxy­ethyl methacrylate (HEMA) and their polymerization in one pot is presented, which solves these issues and is flexible enough to be extended to a variety of hydroxy-functional monomers. First, a family of IPB methacrylates was synthesized through complexation of the hydroxyl moieties of GMA/HEMA with formyl­phenyl­boronic acid and a series of primary aromatic amines of increasing bulkiness ((S)-(−)-α-methyl­benzylamine, (R)-(+)-1-(2-naphthyl)­ethylamine, tritylamine, and 4-tritylaniline). They were subsequently polymerized by classical or UV-photoinduced ATRP at 0 °C, in DMF or DMF/toluene mixtures and with 1,1,4,7,10,10-hexa­methyl­triethylene­tetramine (HMTETA), (+)-sparteine, or 4,4′-dinonyl-2,2′-dipyridyl (dNbpy) as ATRP ligands. Poly­(IPB methacrylate)­s fully functionalized with IPB side-chain moieties, as verified by 2D-NMR spectroscopy, were competitive decomplexed against catechol, as followed by isothermal titration calorimetry and SEC measurements, releasing the highly hydrophilic pGMA or pHEMA backbone. Poly­(IPB methacrylate)­s were highly syndiotactic (rr = 74.9–79.7% for pHEMAs and 70.7–75.5% for pGMAs). The IPB-complex pathway strategy could easily be extended to the one-pot syntheses of block or random copolymers of hydrophilic hydroxy-functionalized monomers (besides HEMA or GMA) with hydrophobic macroinitiators or comonomers, which normally require a time-consuming protection/deprotection of the hydroxyl group(s) in separated steps.

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