Thermal decomposition of the polydimethylsiloxane-(azobiscyano-pentanoyl ester)s (PDMS-ACP) in the presence of methyl methacrylate (MMA) monomer yielded poly(dimethylsiloxane)-poly(methyl methacrylate) block copolymers (PDMS-b-PMMA). Phosphorus functionalized poly(methyl methacrylate) (DEVP-PMMA) was prepared by copolymerization of MMA with diethyl vinyl phosphonate (DEVP) with PDMS-ACP initiator. The thermal stability and apparent activation energy for thermal degradation of these copolymers were studied by thermogravimetry and compared with unmodified poly(methyl methacrylate) (PMMA). The incorporation of PDMS segments in block polymer improved thermal stability of PMMA, whereas phosphorylated PMMA displayed a lower thermal stability. The activation energies for the degradation process in PMMA and PDMS-b-PMMA, calculated by the Ozawa method, increased approximately with increase in conversion. The values of activation energy of phosphorylated PMMA were smaller than that of PMMA and decreased with increasing phosphorus content.