Silylcarboxonium and Silyloxonium Ion Intermediates of the Cationic Ring-Opening Polymerization of Lactones and Tetrahydrofuran Initiated by Electrophilic Trimethylsilylating Agents

Abstract

The initiation of the cationic ring-opening polymerization of lactones and tetrahydrofuran by electron-deficient organosilicon species has been investigated. The formation of the long-lived silylcarboxonium ion 1a from γ-butyrolactone and the silyloxonium ion 2a from THF has been evidenced by 1H, 13C, and 29Si NMR. The structure and NMR chemical shifts of 2a were also calculated using DFT/IGLO methods, and the results are in good agreement with experimental data. The nature of the ions is discussed in terms of the reactivity with respect to their methylated analogues. Polymerization of lactones initiated by in situ formed silylcarboxonium ions showed a much higher rate than that initiated by trimethylsilyl triflate, while polymerization of tetrahydrofuran in a similar manner failed. The mechanism of polymer initiation is discussed.