Selective polymerization of epoxides from hydroxycarboxylic esters: expediting controlled synthesis of α-carboxyl-ω-hydroxyl polyethers

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.

Polyglycidamides: from Backbone-Promoted Amidation to Degradable Polyether with Wide-Range LCST

Amide groups occur extensively in natural and synthetic polymers cultivating their vital roles in biological and industrial worlds. We report here an efficient and controlled pathway to amide-functionalized polyethers through ring-opening polymerization (ROP) of commercially available ethyl glycidate followed by amidation of the pendant ester groups. Transesterification is inhibited during the ROP by use of a two-component organocatalyst. Surprisingly, amidation can be completed even at 0 °C uncatalyzed, which is attributed to the electron-withdrawing effect of the oxygenated backbone as well as the inter-/intra-chain hydrogen bonding. Also interestingly, ethylene glycol and water are found to further accelerate the amidation while suppressing backbone degradation. The obtained polyglycidamides (PGAms) exhibit aqueous thermoresponsive properties, similar to their carbon-chain counterparts (polyacrylamides). Cloud point can be linearly modulated by co-amidation using two amines of varied ratios. Unlike polyacrylamides and regular polyethers, PGAm is degradable through retro-oxa-Michael addition under basic conditions.

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.

Chemoselectivity Streamlines the Approach to Linear and Y-Shaped Thiol-Polyethers Starting from Thiocarboxylic Acids

Thiol-functionalized polyethers, especially poly(ethylene oxide) (PEO), have extensive applications in biomedicine and materials sciences. Herein, we report a simple one-pot synthesis of α-thiol-ω-hydroxyl polyethers through ring-opening polymerization (ROP) of epoxides using thiocarboxylic acid initiators followed by in situ aminolysis. The efficient and chemoselective metal-free Lewis pair catalyst avoids transthioesterification thus achieving well-controlled molar mass, low dispersity, and high end-group fidelity. Kinetic and calculation results demonstrated a fast-initiation mode of the ROP for the strong nucleophilicity of the thiocarboxylate anion and its weak interaction with Lewis acid. The method is expanded for α-thiol-ω-dihydroxyl (Y-shaped) PEO by virtue of the stability of thioester during the ROP. The thiol functionality in linear/Y-shaped PEO is further corroborated by the intensified interaction with gold surface and the resultant protein resistance behavior.

Selective ring-opening polymerization of glycidyl ester: a versatile synthetic platform for glycerol-based (co)polyethers

Linear polyglycerol is highly valued for its excellent hydrophilicity and biocompatibility as well as its multihydroxy nature. We report here a convenient route for controlled synthesis of polyglycerol through ring-opening...

Noncovalent Protection for Direct Synthesis of α-Amino-ω-hydroxyl Poly(ethylene oxide)

The synthesis of poly(ethylene oxide) (PEO) with amino end group, a key functionality for PEGylation, is a long-standing challenge. Multistep routes based on postmodification or covalent protection have been adopted to circumvent ethoxylation of the amino group by ethylene oxide (EO). Here, we report a noncovalent protection strategy for one-step synthesis of PEO amine. An amino (di)alcohol is mixed with a small amount of mild phosphazene base and excess triethylborane (Et₃B) before addition of EO. The complexation of the amino group with Et₃B guarantees that polymerization of EO occurs selectively from the hydroxyl group through the bicomponent metal-free catalysis. Simply by precipitation in diethyl ether, the protective Et₃B as well as the catalyst can be removed to afford α-amino-ω-hydroxyl PEO with controlled molar mass, low dispersity, and complete end functionality. The effect of initiator structure and retention of Et₃B on the storage (oxidative) stability of PEO amine is also revealed.