The present work describes the polymerization of methylmethacrylate monomer into polymethylmethacrylate (PMMA), in the presence of either coarse or ultrafine alumina powder. The flexibility of the material was modified by addition of either butylbenzyl phthalate (S-160), or butylacrylate (BA) monomer as plasticizers. It was found that BA is more compatible than S-160 in the PMMA system. A density of 97%–99% was reached with ultrafine alumina. The maximum tensile strength of the green and sintered tapes was 10 and 140 MPa, respectively. Due to improved homogeneity in the polymerized slip, the sintered tapes prepared by in situ polymerization were stronger and denser than those prepared by conventional blending of commercial PMMA.