Triethylborane-Assisted Synthesis of Random and Block Poly(ester-carbonate)s through One-Pot Terpolymerization of Epoxides, CO2, and Cyclic Anhydrides

Air-Stable Boranes and Borinanes Encapsulated in Organogels for Anionic Ring-Opening (Co)Polymerization

Organoborane reagents play a pivotal role as Lewis acids in acid-base pairs used in anionic polymerization and in other reactions; yet their high sensitivity to oxygen and moisture necessitates effective stabilization to prevent their oxidation and thus maintain their catalytic activity. In this study, we present novel encapsulation methods employing a cost-effective hexatriacontane (C36H74, C36) organogel to stabilize sensitive organoborane reagents, including triethyl borane (TEB) and a borinane-based ammonium salt (B3NBr). These organoboranes encapsulated in stable, self-standing organogel blocks enable their safe handling in open laboratory environments without the need for a glovebox. Upon heating such borane-containing organogel blocks organoboranes could be freed from the organogel and used to mediate both the homopolymerization of propylene oxide (PO) and the copolymerization of PO with CO2. Furthermore, efficient recovery of the C36 gelator from polymerization mixtures is achieved, with mass recovery ranging from 70 % to 90 %. This encapsulation method offers a practical and efficient solution for stabilizing, storing, and handling highly reactive organoborane reagents, thereby broadening their applicability and utilization in various chemical transformations.

Insights into the Distinct Behaviors between Bifunctional and Binary Organoborane Catalysts through Terpolymerization of Epoxide, CO2, and Anhydride

Alkyl borane compounds-mediated polymerizations have expanded to Lewis pair polymerization, free radical polymerization, ionic ring-opening polymerization, and polyhomologation. The bifunctional organoborane catalysts that contain the Lewis acid and ammonium or phosphonium salt in one molecule have demonstrated superior catalytic performance for ring-opening polymerization of epoxides and ring-opening copolymerization of epoxides and CO2 than their two-component analogues, i.e., the blend of organoborane and ammonium or phosphonium salt. To explore the origin of the differences of the one-component and two-component organoborane catalysts, here we conducted a systematic investigation on the catalytic performances of these two kinds of organoborane catalysts via terpolymerization of epoxide, carbon dioxide and anhydride. The resultant terpolymers produced independently by bifunctional and binary organoborane catalyst exhibited distinct microstructures, where a series of gradient polyester-polycarbonate terpolymers with varying polyester content were afforded using the bifunctional catalyst, while tapering diblock terpolymers were obtained using the binary system. The bifunctional catalyst enhances the competitiveness of CO2 insertion than anhydride, which leads to the premature incorporation of CO2 into the polymer chains and ultimately results in the formation of gradient terpolymers. DFT calculations revealed the role of electrostatic interaction and charge distribution caused by intramolecular synergistic effect for bifunctional organoborane catalyst.

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.

Poly(Propylene Carbonate)-Based Biodegradable and Environment-Friendly Materials for Biomedical Applications

Poly(propylene carbonate) (PPC) is an emerging "carbon fixation" polymer that holds the potential to become a "biomaterial of choice" in healthcare owing to its good biocompatibility, tunable biodegradability and safe degradation products. However, the commercialization and wide application of PPC as a biomedical material are still hindered by its narrow processing temperature range, poor mechanical properties and hydrophobic nature. Over recent decades, several physical, chemical and biological modifications of PPC have been achieved by introducing biocompatible polymers, inorganic ions or small molecules, which can endow PPC with better cytocompatibility and desirable biodegradability, and thus enable various applications. Indeed, a variety of PPC-based degradable materials have been used in medical applications including medical masks, surgical gowns, drug carriers, wound dressings, implants and scaffolds. In this review, the molecular structure, catalysts for synthesis, properties and modifications of PPC are discussed. Recent biomedical applications of PPC-based biomaterials are highlighted and summarized.

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.

Quasilinear polyglycidols by triethyborane-controlled anionic polymerization of unprotected glycidol

In this study, quasilinear polyglycidols (PG)s with ultralow degree of branching (DB) could be synthesized through anionic polymerization of glycidol carried out in the presence of triethylborane (TEB). PGs with DB ≤ 0.10, and molar masses up to 40 kg mol^-1 could be indeed obtained using mono- or trifunctional ammonium carboxylates as initiator and under slow monomer addition conditions. The synthesis of degradable PGs through ester linkages obtained by copolymerization of glycidol with anhydride is also described. PG-based amphiphilic di- and triblock quasilinear copolymers were also derived. The role played by TEB is discussed and a polymerization mechanism is proposed.

Recent Advances in Sequence-Controlled Ring-Opening Copolymerizations of Monomer Mixtures

Transforming renewable resources into functional and degradable polymers is driven by the ever-increasing demand to replace unsustainable polyolefins. However, the utility of many degradable homopolymers remains limited due to their inferior properties compared to commodity polyolefins. Therefore, the synthesis of sequence-defined copolymers from one-pot monomer mixtures is not only conceptually appealing in chemistry, but also economically attractive by maximizing materials usage and improving polymers' performances. Among many polymerization strategies, ring-opening (co)polymerization of cyclic monomers enables efficient access to degradable polymers with high control on molecular weights and molecular weight distributions. Herein, we highlight recent advances in achieving one-pot, sequence-controlled polymerizations of cyclic monomer mixtures using a single catalytic system that combines multiple catalytic cycles. The scopes of cyclic monomers, catalysts, and polymerization mechanisms are presented for this type of sequence-controlled ring-opening copolymerization.

Understanding differences in rate versus product determining steps to enhance sequence control in epoxide/cyclic anhydride copolymers

One-pot synthesis of random, gradient, and block polyesters via the ring opening copolymerization of epoxides and cyclic anhydrides is investigated using simple yttrium salt catalysts. Impact of rate versus product determining steps is discussed.

Using Triethylborane to Manipulate Reactivity Ratios in Epoxide-Anhydride Copolymerization: Application to the Synthesis of Polyethers with Degradable Ester Functions

The anionic ring-opening copolymerization (ROCOP) of epoxides, namely of ethylene oxide (EO), with anhydrides (AH) generally produces strictly alternating copolymers. With triethylborane (TEB)-assisted ROCOP of EO with AH, statistical copolymers of high molar mass including ether and ester units could be obtained. In the presence of TEB, the reactivity ratio of EO (rEO), which is normally equal to 0 in its absence, could be progressively raised to values lower than 1 or higher than 1. Conditions were even found to obtain rEO equal or close to 1. Samples of P(EO-co-ester) with minimal compositional drift could be synthesized; upon basic degradation of their ester linkages, these samples afforded poly(ethylene oxide) (PEO) diol samples of narrow molar mass distribution. In other cases where rEO were lower or higher than 1, the PEO diol samples eventually isolated after degradation exhibited a broader distribution of molar masses because of the compositional drift of initial P(EO-co-ester) samples.

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...

Orthogonally grown polycarbonate and polyvinyl block copolymers from mechanistically distinct (co)polymerizations

Mechanistically distinct polymerization systems can afford unique block copolymers that would not be accessible by mere sequential polymerization.

Indium-Catalyzed CO2/Epoxide Copolymerization: Enhancing Reactivity with a Hemilabile Phosphine Donor

Group 13 metal complexes have emerged as powerful catalysts for transforming CO₂ into added-value products. However, direct comparisons of reactivity between Al, Ga, and In catalysts are rare. We report aluminum (1), gallium (2), and indium (3) complexes supported by a half-salen H[PNNO] ligand with a pendent phosphine donor and investigate their activity as catalysts for the copolymerization of CO₂ and cyclohexene oxide. In solution, the P-donor is dissociated for the Al and Ga complexes while for the In complex it exhibits hemilabile behavior. The indium complex shows higher conversion and selectivity than the Al or Ga analogues. The mechanism of the reaction was studied by NMR and FTIR spectroscopy experiments as well as structural characterization of off-cycle catalytic intermediate indium trichloride complex [(PNNO)InCl₃][TBA] (4). This study highlights the impact of a hemilabile phosphine group on group 13 metals and provides a detailed analysis of the initiation step in CO₂/epoxide copolymerization reactions.

Diblock dialternating terpolymers by one-step/one-pot highly selective organocatalytic multimonomer polymerization

The synthesis of well-defined block copolymers from a mixture of monomers without additional actions ("one-pot/one-step") is an ideal and industrially valuable method. In addition, the presence of controlled alternating sequences in one or both blocks increases the structural diversity of polymeric materials, but, at the same time, the synthetic difficulty. Here we show that the "one-pot/one-step" ring-opening terpolymerization of a mixture of three monomers (N-sulfonyl aziridines; cyclic anhydrides and epoxides), with tert-butylimino-tris(dimethylamino)phosphorene (t-BuP1) as a catalyst, results in perfect diblock dialternating terpolymers having a sharp junction between the two blocks, with highly-controllable molecular weights and narrow molecular weight distributions (Ð < 1.08). The organocatalyst switches between two distinct polymerization cycles without any external stimulus, showing high monomer selectivity and kinetic control. The proposed mechanism is based on NMR, in-situ FTIR, SEC, MALDI-ToF, reactivity ratios, and kinetics studies.

Mg(ii) heterodinuclear catalysts delivering carbon dioxide derived multi-block polymers

Carbon dioxide derived polymers are emerging as useful materials for applications spanning packaging, construction, house-hold goods and automotive components. To accelerate and broaden their uptake requires both more active and selective catalysts and greater structural diversity for the carbon dioxide derived polymers. Here, highly active catalysts show controllable selectivity for the enchainment of mixtures of epoxide, anhydride, carbon dioxide and lactone. Firstly, metal dependent selectivity differences are uncovered using a series of dinuclear catalysts, Mg(ii)Mg(ii), Zn(ii)Zn(ii), Mg(ii)Zn(ii), and Mg(ii)Co(ii), each exposed to mixtures of bio-derived tricyclic anhydride, cyclohexene oxide and carbon dioxide (1 bar). Depending upon the metal combinations, different block structures are possible with Zn(ii)Zn(ii) yielding poly(ester-b-carbonate); Mg(ii)Mg(ii) or Mg(ii)Co(ii) catalysts delivering poly(carbonate-b-ester); and Mg(ii)Zn(ii) furnishing a random copolymer. These results indicate that carbon dioxide insertion reactions follow the order Co(ii) > Mg(ii) > Zn(ii). Using the most active and selective catalyst, Mg(ii)Co(ii), and exploiting reversible on/off switches between carbon dioxide/nitrogen at 1 bar delivers precision triblock (ABA), pentablock (BABAB) and heptablock (ABABABA) polymers (where A = poly(cyclohexylene oxide-alt-tricyclic anhydride), PE; B = poly(cyclohexene carbonate), PCHC). The Mg(ii)Co(ii) catalyst also selectively polymerizes a mixture of anhydride, carbon dioxide, cyclohexene oxide and ε-caprolactone to deliver a CBABC pentablock copolymer (A = PE, B = PCHC C = poly(caprolactone), PCL). The catalysts combine high activity and selectivity to deliver new polymers featuring regularly placed carbon dioxide and biomass derived linkages.

Record Productivity and Unprecedented Molecular Weight for Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides Enabled by Organoboron Catalysts

Producing polyesters with high molecular weight (M_n ) through ring-opening copolymerization (ROCOP) of epoxides with cyclic anhydrides remains a major challenge. Herein, we communicate a metal-free, highly active, and high thermoresistance system for the ROCOP of epoxides with cyclic anhydrides to prepare polyesters (13 examples). The organoboron catalysts can endure a reaction temperature as high as 180 °C for the ROCOP of cyclohexane oxide (CHO) with phthalic anhydride (PA) without the observation of any side reactions. The average M_n of the produced poly(CHO-alt-PA) climbed to 94.5 kDa with low polydispersity (Ð=1.19). Furthermore, an unprecedented turnover number of 9900, equivalent to an efficiency of 7.4 kg of polyester/g of catalyst, was achieved at a feed ratio of CHO/PA/catalyst=20000:10000:1 at 150 °C. Kinetic studies, crystal structure analysis, ¹¹ B NMR spectra, and DFT calculations provided mechanistic justification for the effectiveness of the catalyst system.

Terpolymerization of CO2 with Epoxides and Cyclic Organic Anhydrides or Cyclic Esters

The synthesis of polymeric materials starting from CO2 as a feedstock is an active task of research. In particular, the copolymerization of CO2 with epoxides via ring-opening copolymerization (ROCOP) offers a simple, efficient route to synthesize aliphatic polycarbonates (APC). In many cases, APC display poor physical and chemical properties, limiting their range of application. The terpolymerization of CO2 with epoxides and organic anhydrides or cyclic esters offers the possibility, combining the ROCOP with ring-opening polymerization (ROP), to access a wide range of materials containing polycarbonate and polyester segments along the polymer chain, showing enhanced properties with respect to the simple APC. This review will cover the last advancements in the field, evidencing the crucial role of the catalytic system in determining the microstructural features of the final polymer.