Borinane Boosted Bifunctional Organocatalysts for Ultrafast Ring-Opening Polymerization of Cyclic Ethers

The design of reactive species that can either serve to initiate the ring-opening polymerization of epoxides for the synthesis of high molar mass polyethers or be alternatively used to catalyze the synthesis of polyether telechelics in the presence of chain transfer agents has long been an elusive goal. Here, we report the synthesis of a series of bifunctional borinane-based catalysts that enable the living ring-opening polymerization of epoxides with an unprecedented activity (TOF ≥ 1.8 × 105 h–1) and a molar mass up to 106 g/mol under mild conditions. When used along with chain transfer agents to generate low Mn telechelics, the same borinane-based catalysts exhibit high productivity even for loading amounts as low as 50 ppb for ethylene oxide polymerization. These newly designed catalysts also afford the polymerization of oxetane with record TOF values and molar masses

Fast and Living Ring-Opening Polymerization of α‑Amino Acid <i>N</i>‑Carboxyanhydrides Triggered by an “Alliance” of Primary and Secondary Amines at Room Temperature

A novel highly efficient strategy, based on an “alliance” of primary and secondary amine initiators, was successfully developed allowing the fast and living ring-opening polymerization (ROP) of α-amino acid <i>N</i>-carboxyanhydrides (NCAs) at room temperature

Phosphazene-Promoted Metal-Free Ring-Opening Polymerization of Ethylene Oxide Initiated by Carboxylic Acid

The effectiveness of carboxylic acid as initiator for the anionic ring-opening polymerization of ethylene oxide was investigated with a strong phosphazene base (<i>t</i>-BuP₄) used as promoter. Kinetic study showed an induction period, i.e., transformation of carboxylic acid to hydroxyl ester, followed by slow chain growth together with simultaneous and fast end-group transesterification, which led to poly­(ethylene oxide) (PEO) consisting of monoester (monohydroxyl), diester, and dihydroxyl species. An appropriate <i>t</i>-BuP₄/acid ratio was proven to be essential to achieve better control over the polymerization and low dispersity of PEO. This work provides important information and enriches the toolbox for macromolecular and biomolecular engineering with protic initiating sites

Poly(urethane–carbonate)s from Carbon Dioxide

A one-pot, two-step protocol for the direct synthesis of polyurethanes containing few carbonate linkages through polycondensation of diamines, dihalides, and CO₂ in the presence of Cs₂CO₃ and tetrabutyl­ammonium bromide is described. The conditions were optimized by studying the polycondensation of CO₂ with 1,6-hexane­diamine and 1,4-dibromo­butane as model monomers. Then, various diamines and dihalides were tested under optimal conditions. Miscellaneous samples of such carbonate-containing polyurethanes exhibiting molar masses from 6000 to 22 000 g/mol (GPC) and yields higher than 85% were obtained. The thermal properties of such polyurethanes were unveiled by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA): they were found very similar to those of traditional polyurethanes obtained by diisocyanates + diols polycondensation

Poly(urethane–carbonate)s from Carbon Dioxide

A one-pot, two-step protocol for the direct synthesis of polyurethanes containing few carbonate linkages through polycondensation of diamines, dihalides, and CO₂ in the presence of Cs₂CO₃ and tetrabutyl­ammonium bromide is described. The conditions were optimized by studying the polycondensation of CO₂ with 1,6-hexane­diamine and 1,4-dibromo­butane as model monomers. Then, various diamines and dihalides were tested under optimal conditions. Miscellaneous samples of such carbonate-containing polyurethanes exhibiting molar masses from 6000 to 22 000 g/mol (GPC) and yields higher than 85% were obtained. The thermal properties of such polyurethanes were unveiled by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA): they were found very similar to those of traditional polyurethanes obtained by diisocyanates + diols polycondensation

Well-Defined Polyethylene-Based Random, Block, and Bilayered Molecular Cobrushes

Novel well-defined polyethylene-based random, block, and bilayered molecular cobrushes were synthesized through the macromonomer strategy. Two steps were involved in this approach: (i) synthesis of norbornyl-terminated macromonomers of polyethylene (PE), polycaprolactone (PCL), poly­(ethylene oxide) (PEO), and polystyrene (PS), as well as polyethylene-<i>b</i>-polycaprolactone (PE-<i>b</i>-PCL), by esterification of the hydroxyl-terminated precursors (PE, PCL, PEO, PS, and PE-<i>b</i>-PCL) with 5-norbornene-2-carboxylic acid and (ii) ring-opening metathesis (co)­polymerization of the resulting macromonomers to afford the PE-based molecular cobrushes. The PE-macromonomers were synthesized by polyhomologation of dimethylsulfoxonium methylide, while the others by anionic polymerization. Proton nuclear magnetic resonance spectroscopy (¹H NMR) and high-temperature gel permeation chromatography (HT-GPC) were used to imprint the molecular characteristics of all macromonomers and molecular brushes and differential scanning calorimetry (DSC) for the thermal properties. The bilayered molecular cobrushes of P­(PE-<i>b</i>-PCL) adopt a wormlike morphology on silica wafer as visualized by atomic force microscopy (AFM)

A “Catalyst Switch” Strategy for the Sequential Metal-Free Polymerization of Epoxides and Cyclic Esters/Carbonate

A “catalyst switch” strategy was used to synthesize well-defined polyether–polyester/polycarbonate block copolymers. Epoxides (ethylene oxide and/or 1,2-butylene oxide) were first polymerized from a monoalcohol in the presence of a strong phosphazene base promoter (<i>t</i>-BuP₄). Then an excess of diphenyl phosphate (DPP) was introduced, followed by the addition and polymerization of a cyclic ester (ε-caprolactone or δ-valerolactone) or a cyclic carbonate (trimethylene carbonate), where DPP acted as both the neutralizer of phosphazenium alkoxide (polyether chain end) and the activator of cyclic ester/carbonate. This work has provided a one-pot sequential polymerization method for the metal-free synthesis of block copolymers from monomers which are suited for different types of organic catalysts

Phosphazene-Promoted Metal-Free Ring-Opening Polymerization of Ethylene Oxide Initiated by Carboxylic Acid

The effectiveness of carboxylic acid as initiator for the anionic ring-opening polymerization of ethylene oxide was investigated with a strong phosphazene base (<i>t</i>-BuP₄) used as promoter. Kinetic study showed an induction period, i.e., transformation of carboxylic acid to hydroxyl ester, followed by slow chain growth together with simultaneous and fast end-group transesterification, which led to poly­(ethylene oxide) (PEO) consisting of monoester (monohydroxyl), diester, and dihydroxyl species. An appropriate <i>t</i>-BuP₄/acid ratio was proven to be essential to achieve better control over the polymerization and low dispersity of PEO. This work provides important information and enriches the toolbox for macromolecular and biomolecular engineering with protic initiating sites

Theoretical Mechanistic Investigation into Metal-Free Alternating Copolymerization of CO₂ and Epoxides: The Key Role of Triethylborane

The copolymerization of carbon dioxide (CO₂) and epoxides has received much attention during the past decades for the production of aliphatic polycarbonates. Remarkably, the green synthesis of polycarbonates was recently demonstrated by copolymerization of CO₂ with epoxides under metal-free conditions. In this work, the reaction mechanism of this highly selective polymerization was further investigated using DFT calculations. Four steps were studied: step I describes the epoxide ring-opening by the chloride anion in the presence of the Lewis acid triethylborane (TEB); step II is related to the subsequent insertion of CO₂; step III corresponds to the alternating insertion of an epoxide facilitated by TEB; step IV is characterized by the leaving of TEB followed by a new round of polymerization. The high selectivity to form alternating polycarbonates and the suppression of backbiting and homopolymerization that respectively generate cyclic carbonates and polyethers were confirmed by the difference of energy barriers. The key role of TEB at every step was also elucidated. Our theoretical results support the proposed experimental outcomes and provide the fundamental mechanistic insights

Lithium-Assisted Copolymerization of CO₂/Cyclohexene Oxide: A Novel and Straightforward Route to Polycarbonates and Related Block Copolymers

A facile route toward alternating polycarbonates by anionic copolymerization of carbon dioxide (CO₂) and cyclohexene oxide (CHO), using lithium halide or alkoxide as initiators and triisobutyl­aluminum (TiBA) as activator, is reported. α,ω-Heterobifunctional and α,ω-dihydroxy­poly­(cyclohexene carbonate)­s (PCHC) as well as poly­(CHC-<i>co</i>-CHO) copolymers with different carbonate composition could also be easily synthesized by adjusting the amount of TiBA or by adding inert lithium salts. The value of this initiating system also resides in the easy access to PSt-<i>b</i>-PCHC (PSt: polystyrene) and PI-<i>b</i>-PCHC (PI: polyisoprene) block copolymers which can be derived by mere one-pot sequential addition of styrene or dienes first and then of CO₂ and CHO under the same experimental conditions