Soluble poly[styrene-co-(acrylic acid)] (PSA) modified by magnesium compounds was used to support TiCl4. For ethylene polymerization, four catalysts were synthesized, namely PSA/TiCl4, PSA/MgCl2/TiCl4, PSA/(n-Bu)MgCl/TiCl4, and PSA/(n-Bu)(2)Mg/TiCl4. The catalysts were characterized by a set of complementary techniques including X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and element analysis. Synthesis mechanisms of polymer-supported TiCl4 catalysts were proposed according to their chemical environments and physical structures. The binding energy of Ti 2p in PSA/TiCl4 was extremely low as TiCl4 attracted excessive electrons from -COOH groups. Furthermore, the chain structure of PSA was destroyed because of intensive reactions taking place in PSA/TiCl4. With addition of (n-Bu) MgCl or (n-Bu)(2)Mg, -COOH became -COOMg- which then reacted with TiCl4 in synthesis of PSA/(n-Bu) MgCl/TiCl4 and PSA/(n-Bu)(2)Mg/TiCl4. Although MgCl2 coordinated with -COOH first, TiCl4 would substitute MgCl2 to coordinate with -COOH in PSA/MgCl2/TiCl4. Due to the different synthesis mechanisms, the four polymer-supported catalysts correspondingly showed various particle morphologies. Furthermore, the polymer-supported catalyst activity was enhanced by magnesium compounds in the following order: MgCl2 > (n-Bu) MgCl > (n-Bu)(2)Mg > no modifier. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2517-2525, 2012