Simple and Precision Approach to Polythioimidocarbonates and Hybrid Block Copolymer Derivatives

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.

Creating Remarkably Moisture- and Air-Stable Macromolecular Lewis Acid by Integrating Borane within the Polymer Chain: A Highly Active Catalyst for Homo(co)polymerization of Epoxides

Borane-based Lewis acids (LA) play an indispensable role in the Lewis pair (LP) mediated polymerization. However, most borane-based LPs are moisture- and air-sensitive. Therefore, development of moisture and air-stable borane-based LP is highly desirable. To achieve this goal, the concept of "aggregation induced enlargement effects" by chemically linking multiple borane within a nanoscopic confinement was conceived to create macromolecular LA. Accordingly, an extremely moisture and air stable macromolecular borane, namely, PVP-1B featuring poly(4-vinylphenol) backbone, was constructed. The concentration of borane active site is greatly higher than average concentration due to local confinement. Therefore, an enhanced activity was observed. Moreover, the local LA aggregation effects allow its tolerance to air and large amount of chain transfer agent. Consequently, PVP-1B showed remarkable efficiency for propylene oxide (PO) polymerization at 25 °C (TOF=27900 h-1 ). Furthermore, it enables generation of well-defined telechelic poly (CHO-alt-CO2 ) diol (0.6-15.3 kg/mol) with narrow Đs via copolymerizing cyclohexene oxide and CO2 at 80 °C. This work indicates unifying multiple borane within a polymer in a macromolecular level shows superior catalytic performance than constructing binary, bi(multi)functional systems in a molecular level. This paves a new way to make functional polyethers.

Elucidation of the Alternating Copolymerization Mechanism of Epoxides or Aziridines with Cyclic Anhydrides in the Presence of Halide Salts

Organic halide salts in combination with metal or organic compound are the most common and essential catalysts in ring-opening copolymerizations (ROCOP). However, the role of organic halide salts was neglected. Here, we have uncovered the complex behavior of organic halides in ROCOP of epoxides or aziridine with cyclic anhydride. Coordination of the chain-ends to cations, electron-withdrawing effect, leaving ability of halide atoms, chain-end basicity/nucleophilicity, and terminal steric hindrance cause three types of side reactions: single-site transesterification, substitution, and elimination. Understanding the complex functions of organic halide salts in ROCOP led us to develop highly active and selective aminocyclopropenium chlorides as catalysts/initiators. Adjustable H-bonding interactions of aminocyclopropenium with propagating anions and epoxides create chain-end coordination process that generate highly reactive carboxylate and highly selective alkoxide chain-ends.

Multidimensional Control of Repeating Unit/Sequence/Topology for One-Step Synthesis of Block Polymers from Monomer Mixtures

Synchronously and thoroughly adjusting the chemical structure difference between two blocks of the diblock copolymer is very useful for designing materials but difficult to achieve via self-switchable alternating copolymerization. Here, we report self-switchable alternating copolymerization from a mixture of two different cyclic anhydrides, epoxides, and oxetanes, where a simple alkali metal carboxylate catalyst switches between ring-opening alternating copolymerization (ROCOP) of cyclic anhydrides/epoxides and ROCOP of cyclic anhydrides/oxetanes, resulting in the formation of a perfect block tetrapolymer. By investigating the reactivity ratio of these comonomers, a reactivity gradient was established, enabling the precise synthesis of block copolymers with synchronous adjustment of each unit's chemical structure/sequence/topology. Consequently, a diblock tetrapolymer with two glass transition temperatures (T_g) can be easily produced by adjusting the difference in chemical structures between the two blocks.

Lithium achieves sequence selective ring-opening terpolymerisation (ROTERP) of ternary monomer mixtures

Heteroatom-containing degradable polymers have strong potential as sustainable replacements for petrochemically derived materials. However, to accelerate and broaden their uptake greater structural diversity and new synthetic methodologies are required. Here we report a sequence selective ring-opening terpolymerisation (ROTERP), in which three monomers (A, B, C) are selectively enchained into an (ABA'C) n sequence by a simple lithium catalyst. Degradable poly(ester-alt-ester-alt-trithiocarbonate)s are obtained in a M n range from 2.35 to 111.20 kDa which are not easily accessible via other polymerisation methodologies. The choice of alkali metal is key to achieve high activity and to control the terpolymer sequence. ROTERP is mechanistically compatible with ring-opening polymerisation (ROP) allowing switchable catalysis for blockpolymer synthesis. The ROTERP demonstrated in this study could be the first example of an entirely new family of sequence selective terpolymerisations.

Thioanhydride/isothiocyanate/epoxide ring-opening terpolymerisation: sequence selective enchainment of monomer mixtures and switchable catalysis

Lithium mediates sequence selective terpolymerisation of phtalic thioanhydride/PhNCS/butylene oxide yielding poly(ester-alt-ester-alt-dithioimidocarbonates) and enables block- and tetrapolymerisations.