Ab initio and experimental study on thermally degradable polycarbonates: The effect of substituents on the reaction rates

Thermal elimination reactions on polycarbonates are investigated from both theoretical and experimental points of view, to obtain insight into the mic roscopic aspects that influence the reaction mechanism and rates. In partic ular, attention is focused on the influence of the type of substituents in the polymer chain on the reaction rates. Ab initio density functional theor y calculations are performed on a series of model compound systems for the polycarbonates under study, in particular carbonates differing by the group s attached at the alpha and beta carbon atoms. Reactants, products, and tra nsition states are optimized at the B3LYP/6-311g** level of theory. The str uctures of the activated complex give insight into the mechanistic details of this type of E-i elimination reactions. The C-alpha-O bond dissociates b efore the C-beta-H bond, developing some carbocation character in the trans ition state on the C-alpha atom. The kinematics of the thermal decompositio n reactions have been studied by means of transition state theory by constr uction of the microscopic partition functions. It turns out that the rates of the E-i elimination reactions are increased by the presence of those sub stituents on the C-alpha and C-beta carbon atoms which are stabilizing the carbocation character in the transition state. In a second part, degradatio n temperatures have been experimentally measured for some polycarbonates th rough thermogravimetric analysis. It is investigated whether the relative r ates of the model compound carbonate systems are representative, of the beh avior of the thermal degradation temperatures in polycarbonates. The study as presented here proves that ab initio calculations on small model systems , which are representative for the active area of the degradation process i n polycarbonates, can provide insight into the principal ingredients that g overn the reaction rates.