A novel breakable cross-linker and pH-responsive star-shaped and gel polymers

A bifunctional methacrylate monomer, namely, ethylene glycol di(1-methacryloyloxy)ethyl ether (1), was prepared through the addition reaction between ethylene glycol divinyl ether and methacrylic acid. 1 was used as a cross-linker in the preparation of a star-shaped poly(methyl methacrylate) [poly(MMA)], a branched soluble poly(MMA), and a polymer gel. The addition of 1 to an anionically prepared living poly(MMA) solution generated a star-shaped polymer with a central poly(1) gel core and several poly(MMA) arms. On the other hand, when MMA and 1 were simultaneously added to a tetrahydrofuran (THF) solution of an anionic initiator, a branched soluble poly(MMA) or a polymer gel was obtained, depending on the amount of 1. The cross-linking points in the above polymers could be easily broken by hydrolysis under acidic conditions, leading to linear polymers. In contrast to the common polymer gels, the present polymer gel could be broken to soluble polymers in an acidic medium. However, it was just swollen in a basic or a neutral medium. The hydrolyzed product from the star-shaped polymer was a block copolymer consisting of poly(MMA) and poly(methacrylic acid) segments, and those hydrolyzed from the branched polymers and polymer gels were random copolymers of MMA and methacrylic acid. All the hydrolyzed polymers possessed quite different solubilities than those of their precursors.