Ab Initio and Experimental Study on Thermally Degradable Polycarbonates:  The Effect of Substituents on the Reaction Rates

Abstract

Thermal elimination reactions on polycarbonates are investigated from both theoretical and experimental points of view, to obtain insight into the microscopic aspects that influence the reaction mechanism and rates. In particular, attention is focused on the influence of the type of substituents in the polymer chain on the reaction rates. Ab initio density functional theory calculations are performed on a series of model compound systems for the polycarbonates under study, in particular carbonates differing by the groups attached at the alpha and beta carbon atoms. Reactants, products, and transition states are optimized at the B3LYP/6-311g level of theory. The structures of the activated complex give insight into the mechanistic details of this type of E(i) elimination reactions. The C(alpha)-O bond dissociates before the C(beta)-H bond, developing some carbocation character in the transition state on the C(alpha) atom. The kinematics of the thermal decomposition reactions have been studied by means of transition state theory by construction of the microscopic partition functions. It turns out that the rates of the E(i) elimination reactions are increased by the presence of those substituents on the C(alpha) and C(beta) carbon atoms which are stabilizing the carbocation character in the transition state. In a second part, degradation temperatures have been experimentally measured for some polycarbonates through thermogravimetric analysis. It is investigated whether the relative rates of the model compound carbonate systems are representative of the behavior 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 in polycarbonates, can provide insight into the principal ingredients that govern the reaction rates.

Intramolecular π−π Stacking Interactions in 2-Substituted N,N-Dibenzylaziridinium Ions and Their Regioselectivity in Nucleophilic Ring-Opening Reactions

“…65 For this purpose, both halideaziridine carbon and nitrogen-aziridine carbon distances are tabulated in TABLE 9. Bond elongation percentages were calculated with respect to the parent aziridinium ion 12-T1.…”

Novel Hyperbranched Poly([1,2,3]‐triazole)s Derived from AB2 Monomers by a 1,3‐Dipolar Cycloaddition

“…Polymers 8a and 8b have aliphatic carbonate groups in their structure, which are known to decompose readily at temperatures of around 200 8C. [12] This makes the polymers 8a and 8b promising materials for use as porogens in low-dielectric-constant materials, for example. Experiments in which thermally labile hyperbranched poly(triazene ester)s were used as porogens were successful.…”

“…Formation of 2-substituted propene units bonded to the triazole rings, as determined by NMR analysis of the product mixture, indicates the decomposition of the carbonate group during the polymerization reaction. [12] On the other hand, polymerization of 7a and 7b in toluene at 85-90 8C leads to insoluble products. The reason for this insolubility might be a slight decomposition of the carbonate group yielding traces of olefinic units, which can react with azido groups to form dihydrotriazoles.…”

Polymer Degradation and Stabilization

“…Substituents which stabilize the transition state will also accelerate the degradation rate [17]. The optimized transition structures revealed that the C a aO bond dissociates first, followed by abstraction of the b-hydrogen atom, developing a carbocation character in the transition state on the C a atom.…”