Synthesis and characterization of double-hydrophilic model networks based on cross-linked star polymers of poly(ethylene glycol) methacrylate and methacrylic acid
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A series of model networks based on cross-linked star polymers of poly(ethylene glycol) methacrylate (PEGMA) and tetrahydropyranyl methacrylate (THPMA), a protected form of methacrylic acid (MAA), were synthesized by group transfer polymerization (GTP), covering various compositions (homopolymers and copolymers), arm architectures (homopolymer, block and statistical copolymers), and star architectures (heteroarm and star block). The synthesis took place in tetrahydrofuran (THF) using tetrabutylammonium bibenzoate (TBABB) as the catalyst and 1-methoxy-1-(trimethytsiloxy)-2-methyl-propene (MTS) es the initiator, and comprised a four- or six-step procedure: first, the formation of linear polymer chains upon the GTP of the monomer(s)next, the interconnection of the linear segments to star polymers ("arm-first" stars) by the in situ polymerization of the ethylene glycol dimethacrylate (EGDMA) cross-linker, followed by the addition of monomer(s) for a second time to grow new chains from the cores of the stars outward ("in-out" stars)finally, the cross-linking of the "in-out" stars to a network with the addition of more EGDMA. The molecular weights (MWs) and molecular weight distributions (MWDs) of all the linear and the star precursors to the networks were determined using gel permeation chromatography (GPC). The networks which contained THPMA were hydrolyzed under acidic conditions to convert the THPMA units to MAA units and thus to obtain polyelectrolytic, double-hydrophilic model networks. The aqueous degrees of swelling of all the networks were measured as a function of pH. At low pH, where MAA is protonated, low degrees of swelling were obtained, which increased with the network content in PEGMA. An increase of the degrees of swelling was observed at high pH. These high pH degrees of swelling were found to be dependent on both the composition and the structure of the network.