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 (M̄ w/M̄ n < 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.

Metal‐Free Activation in the Anionic Ring‐Opening Polymerization of Cyclopropane Derivatives

“…The livingness of the process was observed under appropriate experimental conditions, up to temperatures as high as 200 8C. [2] This remarkable selectivity results from the high stabilization of the propagating carbanion R-C(COOŔ) À 2 by two electron-withdrawing ester groups (malonate substitution). From that perspective, dialkyl cyclopropane-1,1-dicarboxylate monomers resemble an epoxide more than a classical cyclopropane.…”

“…[1][2][3] A few studies are available on the physicochemical properties exhibited by this class of unusual polymers. [4,5] The anionic ring-opening polymerization (ROP) of these monomers was performed in dimethyl sulfoxide (DMSO) using sodium or potassium thiophenolates as initiators.…”

Design of Optimized Reaction Conditions for the Efficient Living Anionic Polymerization of Cyclopropane‐1,1‐Dicarboxylates

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] The ringopening polymeri zation of cyclopropanes was not a new concept at the time we started investigating the issue, but previous attempts had always led to oligomers, with broad to very broad molecular weight dis tributions. [14] For the first time, with the advent of our activated monomer approach, efficient procedures to polymerize livingly threemembered ring carbon monomers became available, yielding carbonchain polymers substituted on every third atom alongside the backbone (see Scheme 1A for an example).…”

“…In addition, high temperatures, up to 120-140 °C, can be used and further modi fications of the obtained macrocarbanion with electrophiles are possible due to the excellent thermal stability and wellknown nucleophilicity of malonate carbanions, the propagating species involved in these polymerizations. [3,7,14] The prototypical monomers that have been efficiently polymerized thus far in this series of activated cyclopropanes are based on cyclopropyl rings 1 geminally substituted by two electronwithdrawing groups, typically esters (Scheme 1) but also nitriles. Monomers with two propyl ester groups (either as n or ipropyl) have been particularly investigated, as both the monomers and their homopolymers are soluble in a wide variety of solvents, in contrast to methyl or ethyl ester polymers whose solubility in organic solvents com patible with mandatory anionic polymerization conditions is low, due to their high susceptibility to crystallize.…”

“…One set involves simple alkali ions, [1][2][3][4] while the other one implicates the large tBuP 4 H + phosphazenium cation derived from the protonation of tBuP 4 superbase (see Scheme 3 for a mechanism). [6,7] Both systems have their pros and cons.…”