Catalytic Activity of Various β-Diketiminate Zinc Complexes toward the Ring-Opening Polymerization of Caprolactone and Derivatives

Abstract

The ring-opening polymerization (ROP) of lactones is mainly catalyzed by tin(II) octanoate Sn(Oct)2, due to its efficiency and easy handling despite the toxic issues of tin. Herein, two novel dinuclear bis(β-diketiminate) complexes with flexible alkylene bridging groups (1,2-ethylene and 1,3-propylene) and biocompatible zinc centers are introduced. The complexes are obtained by deprotonation of the protio-ligands with Zn(HMDS)2 and fully characterized. Afterwards the complexes are studied regarding their catalytic activity towards the ROP of caprolactones. Briefly, ε-caprolactone (CL), ε-phenyl-ε-caprolactone (phCL) and ε-methyl-ε-caprolactone (meCL) with and without benzyl alcohol (BnOH) as initiator in toluene are homopolymerized and compared to the known dinuclear 1,4-phenylene bridged bis(β-diketiminate) zinc catalyst and the mononuclear counterpart. Thereby, the catalysts are classified upon their efficiency by in situ IR measurements, selectivity, and polymerization control by size exclusion chromatography (SEC) of samples, which are terminated at 60% conversion. The novel zinc catalysts prevent Sn(Oct)2 typical molar mass broadening, show slower reaction rates than 1,4-phenylene bridged bis(β-diketiminate) and improve control compared to the mononuclear relative. However, a drastic loss of polymerization control resulting in high molar masses and polydispersities occurrs in the absence of BnOH. Furthermore, these complexes allow the ROP of phCL but are inactive for meCL.