Thermal decomposition of phosphonate-containing methacrylate-based copolymers

The synthesis of novel phosphonate-containing copolymers with methyl methacrylate (MMA) is reported. As phosphonate comonomers ethyl-2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate (EDHPOBA), with free phosphonic acid group, and the respective methyl ester ethyl-2-[4-(dimethoxyphosphoryl)-2-oxabutyl]acrylate (EDMPOBA) are employed. The impact of the phosphonate-containing comonomers on the thermal decomposition of P(MMA) copolymers is discussed using data obtained from thermogravimetric analysis (TGA). P(MMA) decomposes completely under nitrogen leading to a negligible residue, while the decomposition of copolymers containing the phosphonate comonomer EDHPOBA yields a significant residue (up to 32 wt.%) indicating a significant alteration of the decomposition mechanism. TGA/FTIR experiments are performed to determine the volatile decomposition products of the copolymers. It is demonstrated that incorporation of the phosphonic acid-containing monomer in P(MMA) suppresses the depolymerization of P(MMA) Alternative pyrolysis reactions take place and MMA is not detected in the gas phase. The decomposition proceeds essentially in four steps: transesterification between acrylate ester groups and phosphonic acid, anhydride formation, decarboxylation, McLafferty rearrangement and cyclization (char forming). It is found that this reaction sequence cannot occur in copolymers with the ester EDMPOBA comonomer, thus resulting in a less favored influence on the decomposition mechanism. Microscale combustion calorimetric measurements prove that incorporation of the phosphonate monomers drastically reduces the total heat release and heat release capacity.