N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 2: Toward dihydroxytelechelic polyethers using triethylborane

Acidity Reversal Enables Site-Specific Ring-Opening Polymerization of Epoxides from Biprotonic Compounds

Polyethers are versatile materials extensively used in advanced as well as everyday applications. The incorporation of primary amine functionality into polyethers is particularly attractive due to its well-established coupling chemistries. However, the inherent nucleophilicity of amine group poses a challenge in the anionic ring-opening polymerization (ROP) of epoxides and requires the use of robust protecting groups that can withstand the harsh conditions of ROP without triggering undesirable side reactions. In this work, we present streamlined synthesis of amino-functionalized polyethers using classic N-carbamate-protected aminoalcohols as initiators for the ROP of epoxides. A Lewis acid-excess two-component organocatalytic system is found to trigger efficient anionic ROP of epoxides while preserving the integrity of the carbamate protection. Despite the higher intrinsic acidity of the carbamate group compared to the hydroxyl group, it is noncompetitive in both the deprotonation and ring-opening steps. This is due to an intriguing acidity-reversing effect of the catalyst, which allows site-specific ethoxylation to proceed exclusively from the hydroxyl group. The resulting poly(propylene oxide) and poly(ethylene oxide) exhibit the targeted molar mass, low dispersity, and well-defined end groups. The fidelity of the amino functionalities is further corroborated and utilized in construction of polypeptoide-based hybrid block copolymers using the synthesized polyethers as macroinitiators.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.