Poly(ethylene terephthalate) copolymers containing 5-nitroisophthalic units. I. Synthesis and characterization

Poly(ethylene terephthalate-co-5-nitroisophthalate) copolymers, abbreviated as PETNI, were synthesized via a two-step melt copolycondensation of bis(2-hydroxyethyl) terephthalate and bis(2-hydroxyethyl) 5-nitroisophthalate mixtures with molar ratios of these two comonomers varying from 95/5 to 50/50. Polymerization reactions were carried out at temperatures between 200 and 270 °C in the presence of tetrabutyl titanate as a catalyst. The copolyesters were characterized by solution viscosity, GPC, FTIR, and NMR spectroscopy. They were found to be random copolymers and to have a comonomer composition in accordance with that used in the corresponding feed. The copolyesters became less crystalline and showed a steady decay in the melting temperature as the content in 5-nitroisophthalic units increased. They all showed glass-transition temperatures superior to that of PET with the maximum value at 85 °C being observed for the 50/50 composition. PETNI copolyesters appeared stable up to 300 °C and thermal degradation was found to occur in two well-differentiated steps.