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

Abstract

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 CO 2 -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 CO 2 /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 CO 2 /epoxide copolymerization and organobase catalyzed ring-opening polymerization (ROP) approach for the one-pot synthesis of an ABA CO 2 -based triblock copolymers. As in previous studies, water was deemed as the chain transfer reagent in this tandem strategy for producing CO 2 -based polyols. Herein is presented a mechanistic study aimed at determining the intimate role water plays during the metal-catalyzed CO 2 /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 endcapped polycarbonates were utilized in direct chain extension via ROP of the water-soluble cyclic phosphate monomer, 2methoxy-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.

Heterodinuclear zinc and magnesium catalysts for epoxide/CO2 ring opening copolymerizations

“…the diol). 21,[54][55][56] MALDI-ToF analysis of the chain end groups substantiates this hypothesis since two series of chains are observed due to carboxylate and hydroxyl telechelic chains, respectively. When reactions were conducted with the addition of 10 equivalents of CHD or water, monomodal molar mass distributions were observed with narrow dispersity.…”

“…This induction period could be attributed to the dissociation of the bridging carboxylate ligand, providing a coordinative vacancy for epoxide binding, although other processes cannot be excluded such as rates of CO 2 dissolution 48 and/or epoxide hydrolysis to form diol species. 54 Following the initiation period, the polymerization conversion vs. time data over the range 0-20% conversion was linearly t, enabling determination of the initial rate coefficient (Table 1). Although the series showed different initiation times, nearly all the values fall within the range 2.8-3.8 Â 10 À5 s À1 .…”

“…Such bimodality has frequently been observed previously in the literature and has been extensively studied. 4,22,53,54 Bimodality commonly results because of a sidereaction between residual water and cyclohexene oxide to generate 1,2-cyclohexanediol (CHD). 55 Since all chains propagate at the same rate those initiated from mono-functional groups (i.e.…”

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation

“…Darensbourg and co-workers have established the rates of such hydrolyses typically exceed polymerization initiation and propagation rates. [50][51][52] In the case of catalyst 4 at 1 bar CO 2 pressure a trimodal molecular mass distribution was observed, with peaks being tentatively attributed to polyether and two polycarbonate series, respectively ( Table 1, entry 4). 30 At 20 bar pressure, catalyst 4 does not produce any polyether and the expected bimodal molecular mass distribution is observed (Table 1, entry 5).…”

Functional Polycarbonates from Carbon Dioxide and Tailored Epoxide Monomers: Degradable Materials and Their Application Potential

“…Furthermore, Darensbourg and Wu published the one‐pot synthesis of a triblock copolymer from PO/CO 2 and lactide . In another work, the Darensbourg group prepared ABA triblock copolymers with a central polycarbonate block and two poly(phosphoester) blocks without additional initiators in a one‐pot approach . In an elegant approach Williams and co‐workers terpolymerized cyclohexene oxide (CHO), CO 2 , and ε‐caprolactone from monomer mixtures to obtain block copolymers, using chemoselective polymerization control .…”