Synthesis of Poly(indene carbonate) from Indene Oxide and Carbon DioxideA Polycarbonate with a Rigid Backbone

The catalytic coupling of carbon dioxide with indene oxide utilizing (salen)Co(III)-2,4-dinitrophenoxide in the presence of an onium salt is presented. X-ray structural data for indene oxide monomer as well as <i>cis</i>-indene carbonate display near planarity of the fused cyclopentene and benzene rings. Low temperature (0 °C) is required to selectively afford copolymer vs cyclic carbonate from the coupling reactions of CO₂ and indene oxide. The produced poly(indene carbonate) samples have molecular weights of up to 7100 Da, with corresponding glass transition temperatures of up to 134 °C, the highest yet reported for polycarbonates produced from CO₂/epoxides coupling. Poly(indene carbonate) is thermally stable up to 249 °C. The polymerizations are well controlled, with PDI values ≤1.3

Synthesis of Poly(indene carbonate) from Indene Oxide and Carbon DioxideA Polycarbonate with a Rigid Backbone

The catalytic coupling of carbon dioxide with indene oxide utilizing (salen)Co(III)-2,4-dinitrophenoxide in the presence of an onium salt is presented. X-ray structural data for indene oxide monomer as well as <i>cis</i>-indene carbonate display near planarity of the fused cyclopentene and benzene rings. Low temperature (0 °C) is required to selectively afford copolymer vs cyclic carbonate from the coupling reactions of CO₂ and indene oxide. The produced poly(indene carbonate) samples have molecular weights of up to 7100 Da, with corresponding glass transition temperatures of up to 134 °C, the highest yet reported for polycarbonates produced from CO₂/epoxides coupling. Poly(indene carbonate) is thermally stable up to 249 °C. The polymerizations are well controlled, with PDI values ≤1.3

Synthesis of CO₂‑Derived Poly(indene carbonate) from Indene Oxide Utilizing Bifunctional Cobalt(III) Catalysts

The copolymerization of carbon dioxide and indene oxide to yield poly­(indene carbonate) has been achieved through the use of bifunctional cobalt­(III) catalysts. When compared to our earlier studies utilizing the traditional binary (salen)­Co­(III)­X/cocatalyst system, the bifunctional catalysts display large increases in activity and selectivity for polymer while maintaining good control (PDI < 1.2). The copolymerization reactions can proceed at 25 °C while maintaining >99% selectivity for poly­(indene carbonate) production. Polymer samples have been achieved with <i>M</i>_ns and <i>T</i>_gs of up to 9700 g/mol and 138 °C, respectively. This represents the highest <i>T</i>_g yet observed for polycarbonates produced from the coupling of CO₂ and epoxides. Additionally, the activation energy for the direct coupling of indene oxide and CO₂ to yield <i>cis</i>-indene carbonate employing the (salen)­CrCl/<i>n</i>-Bu₄NCl catalyst system was determined to be 114.4 ± 5.7 kJ/mol utilizing <i>in situ</i> ATR-FTIR

Carbon Dioxide Copolymerization Study with a Sterically Encumbering Naphthalene-Derived Oxide

Poly­(1,4-dihydronaphthalene carbonate) has been prepared via the catalytic coupling of carbon dioxide and 1,4-dihydronaphthalene oxide using chromium­(III) catalysts. The copolymer formation is found to be greatly dependent on the steric environment around the metal center. Traditional (salen)­Cr^IIIX/cocatalyst systems bearing bulky <i>t</i>-butyl groups hinder the approach of the large monomer, significantly diminishing polymer chain growth and providing the entropically favored cyclic byproduct in excess. In contrast, employing the sterically unencumbered azaannulene-derived catalyst, (tmtaa)­Cr^IIIX/cocatalyst system (tmtaa = tetramethyltetraazaannulene) shows polymer selectivity close to 90% with three times the activity (TOF = 20–30 h^–1). With the use of a bifunctional (salen)­Cr^III catalyst, even higher polymer selectivity (>90%) can be observed. The complete synthesis of a new bifunctional tetraazaannulene ligand for a more effective catalyst is also described herein

Thermodynamics of the Carbon Dioxide–Epoxide Copolymerization and Kinetics of the Metal-Free Degradation: A Computational Study

The copolymerization reactions of carbon dioxide and epoxides to give polycarbonates were examined by density functional theory (DFT), and chemically accurate thermochemical data (benchmarked to experimental values) were obtained via composite <i>ab initio</i> methods. All of the examples studied, i.e., formation of poly­(ethylene carbonate), poly­(propylene carbonate), poly­(chloropropylene carbonate), poly­(styrene carbonate), poly­(cyclohexene carbonate), and poly­(indene carbonate), exhibited enthalpies of polymerization of 21–23 kcal/mol, with the exception of poly­(cyclopentene carbonate) (15.8 kcal/mol) which suffers both ring strain and intramolecular steric repulsion caused by the cyclopentane ring fused to the polymer chain. The metal-free carbonate backbiting reaction by a free anionic polycarbonate strand is inhibited by bulky groups at the methine carbon but is accelerated by resonance stabilization of the pentavalent transition state in the case involving poly­(styrene carbonate). Nucleophilic attack at the methylene carbon of a substituted epoxide has a lower barrier than for the corresponding reaction involving ethylene oxide due to charges being distributed onto the pendant groups. The undesired backbiting reaction to afford cyclic organic carbonates observed under polymerization conditions for many systems due to the low activation barrier (Δ<i>G</i>^‡ = 18–25 kcal/mol) was negligible for poly­(cyclohexene carbonate) because, in this instance, it must overcome an additional endergonic conformational change (Δ<i>G</i> = 4.7 kcal/mol) before traversing the activation barrier (Δ<i>G</i>^‡ = 21.1 kcal/mol) to cyclization. Backbiting from an alkoxide chain end is proposed to proceed via a tetrahedral alkoxide intermediate, where formation of this intermediate is barrierless. Further reaction of this intermediate to the cyclic carbonate has a free energy barrier 10 kcal/mol less than the carbonate chain end backbiting reaction

Mechanistic Insights into Water-Mediated Tandem Catalysis of Metal-Coordination CO₂/Epoxide Copolymerization and Organocatalytic Ring-Opening Polymerization: One-Pot, Two Steps, and Three Catalysis Cycles for Triblock Copolymers Synthesis

The addition of water as a chain transfer reagent during the copolymerization reaction of epoxides and carbon dioxide has been shown as a promising method for producing CO₂-based polycarbonate polyols. These polyols can serve as drop-in replacements for petroleum derived polyols for polyurethane production or designer block copolymers. Ironically, during the history of CO₂/epoxide coupling development, water was generally considered primarily as an aversion reagent. That is, in its presence, low catalytic activity and high polydispersity was normally observed. Recently, we reported a water-mediated tandem metal-coordination CO₂/epoxide copolymerization and organobase catalyzed ring-opening polymerization (ROP) approach for the one-pot synthesis of an ABA CO₂-based triblock copolymers. As in previous studies, water was deemed as the chain transfer reagent in this tandem strategy for producing CO₂-based polyols. Herein is presented a mechanistic study aimed at determining the intimate role water plays during the metal-catalyzed CO₂/epoxide copolymerization process. In this regard, it was observed that under the commonly employed (salen)­Co­(trifluoroacetate)/onium salt binary catalyst system, water was not the true chain-transfer reagent, but instead reacted initially with the epoxides to afford the corresponding diols which serves as the chain-transfer reagent. The further studies in resultant afforded α,ω-dihydroxyl end-capped polycarbonates were utilized in direct chain extension via ROP of the water-soluble cyclic phosphate monomer, 2-methoxy-2-oxo-1,3,2-dioxaphospholene employing an organocatalyst. These triblock copolymers displayed narrow PDI and were found to provide nanostructure materials which should be of use in biomedical applications

Depolymerization of Polycarbonates Derived from Carbon Dioxide and Epoxides to Provide Cyclic Carbonates. A Kinetic Study

The depolymerization reactions of several polycarbonates produced from the completely alternating copolymerization of epoxides and carbon dioxide have been investigated. The aliphatic polycarbonates derived from styrene oxide, epichlorohydrin, or propylene oxide and CO₂ were found to undergo quantitative conversion to the corresponding cyclic carbonate following deprotonation of their −OH end group by azide ion. The process was shown to involve the unzipping of the copolymer in a backbiting fashion leading to a steady decrease in the copolymer’s molecular weight while maintaining its narrow molecular weight distribution. This pathway for depolymerization was further supported by the observation that upon end-capping the copolymer with an acetate group, it was stabilized. Temperature-dependent kinetic studies provided energy of activation (<i>E</i>_a) barriers for cyclic carbonate formation which increased in the order: poly­(styrene carbonate) (46.7 kJ/mol) < poly­(CO₂-<i>alt</i>-epichlorohydrin) (76.2 kJ/mol) ≤ poly­(propylene carbonate) (80.5 kJ/mol). On the other hand, upon addition of the (salen)­CrCl copolymerization catalyst, the depolymerization process was greatly suppressed, e.g., the E_a determined for poly­(styrene carbonate) in this instance was 141.2 kJ/mol. By way of contrast, the copolymer produced from the alicyclic epoxide, cyclohexene oxide, was only found to undergo depolymerization to <i>trans</i>-cyclohexene carbonate in the presence of (salen)­CrCl plus <i>n</i>Bu₄NN₃, albeit extremely slowly

Postpolymerization Functionalization of Copolymers Produced from Carbon Dioxide and 2‑Vinyloxirane: Amphiphilic/Water-Soluble CO₂‑Based Polycarbonates

Common CO₂-based polycarbonates are known to be highly hydrophobic, and this “inert” property makes them difficult for the covalent immobilization of bioactive molecules. A practical method for modifying polymers is to introduce various functional groups that permit decoration of polymer chains with bioactive substances. In this report, CO₂-based poly­(2-vinyloxirane carbonate) (PVIC) with more than 99% carbonate linkages is isolated from the CO₂/2-vinyloxirane alternating copolymerization catalyzed by the bifunctional catalyst [(1<i>R</i>,2<i>R</i>)-SalenCo­(III)­(DNP)₂] (<b>1</b>) (DNP = 2,4-dinitrophenolate) bearing a quaternary ammonium salt on the ligand framework. It was also observed that the presence of propylene oxide significantly activates 2-vinyloxirane for incorporation into the polymer chain as well as inhibits the formation of cyclic carbonate in the terpolymerization process. DSC studies demonstrate that the glass transition temperature (<i>T</i>_g) decreases with the increase in the content of vinyl groups in the polycarbonate. By way of thiol–ene coupling, showing mainly “click” characteristics and nearly quantitative yields, amphiphilic polycarbonates (PVIC-OH and PVIC-COOH) with multiple hydroxy or carboxy functionalities have been prepared, providing suitable reactivities for further modifications (ring-opening of l-aspartic acid anhydride hydrochloride salt and deprotonation by aqueous ammonium hydroxide (NH₄OH_(aq))) to successfully isolate the water-soluble CO₂-based polycarbonate PVIC-COONH₄, and the PVIC-OH-Asp polymer which shows particles dispersed in water with an average hydrodynamic diameter <i>D</i>_n = 32.2 ± 8.8 nm. It is presumed that this emerging class of amphiphilic/water-soluble polycarbonates could embody a powerful platform for bioconjugation and drug conjugation. In contrast to lower <i>T</i>_gs of PVIC, (PVIC-<i>co</i>-PC), PVIC-OH, and PVIC-COOH, the polycarbonates PVIC-OH-Asp and PVIC-COONH₄ show higher <i>T</i>_gs as a consequence of their intrinsic ionic property (ammonium salts)

An Investigation of the Pathways for Oxygen/Sulfur Scramblings during the Copolymerization of Carbon Disulfide and Oxetane

The catalytic coupling of oxetane, the symmetric isomer of propylene oxide, with carbon disulfide has been investigated utilizing (salen)­CrCl in the presence of various onium salts. Oxygen and sulfur atom exchange was observed in both the polymeric and cyclic carbonate products. The coupling of oxetane and CS₂ was selective for copolymer formation over a wide range of reaction conditions. Five different polymer linkages and two cyclic products were determined by ¹H and ¹³C NMR spectroscopy, and these results were consistent with <i>in situ</i> infrared spectroscopic monitoring of the process. The major cyclic product produced in the coupling process was trimethylene trithiocarbonate, which was isolated and characterized by single crystal X-ray crystallography. Upon increasing the CS₂/oxetane feed ratio, a decrease in the O/S scrambling occurred. The reaction temperature had the most significant effect on the O/S exchange process, increasing exchange with increasing temperature. The presence of the onium salt initiator both accelerated the coupling process and promoted O/S scrambling. COS (observed), and CO₂ intermediates are proposed in the reactions leading to various polymeric linkages

An Investigation of the Pathways for Oxygen/Sulfur Scramblings during the Copolymerization of Carbon Disulfide and Oxetane

The catalytic coupling of oxetane, the symmetric isomer of propylene oxide, with carbon disulfide has been investigated utilizing (salen)­CrCl in the presence of various onium salts. Oxygen and sulfur atom exchange was observed in both the polymeric and cyclic carbonate products. The coupling of oxetane and CS₂ was selective for copolymer formation over a wide range of reaction conditions. Five different polymer linkages and two cyclic products were determined by ¹H and ¹³C NMR spectroscopy, and these results were consistent with <i>in situ</i> infrared spectroscopic monitoring of the process. The major cyclic product produced in the coupling process was trimethylene trithiocarbonate, which was isolated and characterized by single crystal X-ray crystallography. Upon increasing the CS₂/oxetane feed ratio, a decrease in the O/S scrambling occurred. The reaction temperature had the most significant effect on the O/S exchange process, increasing exchange with increasing temperature. The presence of the onium salt initiator both accelerated the coupling process and promoted O/S scrambling. COS (observed), and CO₂ intermediates are proposed in the reactions leading to various polymeric linkages