The pyrolysis of free radically prepared low molecular weight poly(n-butyl acrylate) (n-BA), (n-butyl methacrylate) (n-BMA), (t-butyl methacrylate) (t-BMA), and their copolymers with various concentrations of acrylonitrile (AN) were investigated by thermogravimetric analysis. The weight loss of copolymers was shown. Since the least stable chains have end-unsaturation, an oxy-ene reaction can explain the depolymerization of poly methacrylates. As AN% increases, n-BMA-AN copolymers resulted in a progressive stabilization. The stability of t-BMA-AN copolymers did not change since the main reaction was ester decomposition, not the depolymerization.