Expanding the Scope of Boron-Based Ate Complexes by Manipulating Their Reactivity: The Case of Cyclic Esters and Their (Co)Polymers

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The value and merit of triethylborane (TEB)-based ate complexes for the synthesis of various oxygenated polymers have been recently illustrated through successful examples of homopolymerization of epoxides and of their copolymerization with CO2, CS2, COS, anhydrides, isocyanates, etc. To further expand the scope of TEB-based initiating systems to a broader family of oxygenated polymers, they were used in this study to anionically polymerize cyclic esters and to copolymerize the latter monomers with propylene oxide (PO). To promote a fast and controlled ring-opening polymerization (ROP) of cyclic esters, hydrogen-bonding donors such as amines and (thio)ureas were added to TEB-based ate complexes used as initiators. Only under these conditions could the ROP of ε-caprolactone (CL), δ-valerolactone (VL), and l-lactide (LLA) proceed under controlled conditions with hardly detectable intra- or intermolecular transesterifications. The role of amines and (thio)ureas when used alone or together in association with these boron-based initiators is discussed in detail. The controlled character of the polymerization of CL, VL, and LLA is attested by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS) and other characterization techniques. As examples of the far-reaching potential of these borane-based ate complexes, random P(PO-co-VL) copolymers were derived by statistical copolymerization of VL with PO and various well-defined block copolymers─PPO-b-PVL, PVL-b-PPO, PPO-b-PCL, PPO-b-PLLA, and PPO-b-PVL-b-PPO─were grown by sequential polymerization of the corresponding monomers.