Specifically, this invention is a process for preparing a thermoformable cast methyl methacrylate polymer sheet which comprises in sequence (1) polymerizing methyl methacrylate monomer until about 5-50 percent by weight of the methyl methacrylate monomer present has polymerized, (2) adding to the polymerizate obtained in step (1) a chain transfer agent in an amount of between about 0.05 and about