Poly (methyl methacrylate) - PMMA - and poly (methacryloyl chloride) - PMACl - were S,synthesized by free radical solution polymerization. The thermal stabilities of these polymers were determined with the help of thermogravimetry and differential thermal anal is. The structural changes in these materials at different temperatures (90, 140, 220 and 410 degreesC) were studied by X-ray diffraction. These. Studies indicate that PMACl is less amorphous than PMMA due to the presence of bulky chloride groups.