Ring-Opening Polymerization of Diisopropyl Cyclopropane-1,1-dicarboxylate under Living Anionic Conditions: A Kinetic and Mechanistic Study

Abstract

Diisopropyl cyclopropane-1,1-dicarboxylate (1) undergoes ring-opening polymerization in the presence of thiophenolate anions acting as initiator. A carbon-chain polymer, substituted on every third carbon atom by two isopropyl ester substituents, is obtained, whose structure and molecular weight were characterized by several analytical techniques. Under typical reaction conditions, only the expected ring-opened structure with a phenylthio end group is obtained, with no evidence for side reactions during the initiation and propagation steps. A kinetic study of the polymerization, at 140 °C in the presence of sodium thiophenolate, showed that the degree of polymerization increases linearly with conversion. The final polymers have narrow molecular weight distributions (M̄w/M̄n < 1.13), and the reaction follows a first-order kinetics with respect to the monomer over the entire conversion range. These results support a living mechanism for the polymerization. The nature of the counterion and the presence of counterion complexing agents, like crown ether and cryptand, significantly increase the reaction rate. A linear Arrhenius behavior was found in the 130−190 °C range, with an activation energy of 21.3 kcal mol-1. At higher temperatures deviation from the linear Arrhenius behavior and appearance of new peaks in 1H NMR can be observed. Thermogravimetric analysis shows that the polymer is thermally stable up to 270 °C. Poly(1) is highly crystalline, with a melting point in the 168−76 °C range.