Alternating copolymerization of epoxides and carbon dioxide using single-site beta-diiminate zinc catalysts: Subtle modifications resulting in superior activities and evidence for a bimetallic epoxide enchainment

Billions of tons of polycarbonates are produced industrially every year. However, the raw materials used in their synthesis originate from nonrenewable, petroleum-based resources. Consequently, there is a growing effort to develop polycarbonates and other polymers from biorenewable resources, such as CO ₂. Recently, β-diiminate (BDI) zinc complexes have been employed in the synthesis of biodegradable polycarbonates from CO₂ and epoxides. The research described herein addresses the design and reactivity of β-diiminate zinc complexes. In addition, mechanistic studies show that the alternating copolymerization of cyclohexene oxide (CHO) and CO₂ proceeds via a bimetallic enchainment of epoxide.; New methodologies for the production of unique, unsymmetrical β-diimines and their corresponding BDI zinc complexes are reported. For example, BDI zinc acetates are now synthesized in yields over 90%, resulting in a variety of potent catalysts. X-ray crystallography studies reveal that BDI zinc alkoxides and acetates crystallize as dimers, yet 1H NMR spectroscopy experiments show that a catalyst's structure in solution is highly dependent on sterics. Bulky ortho N-aryl isopropyl substituents drive complexes into a monomer/dimer equilibrium. On the other hand, sterically unhindered complexes exist as dimers in solution and in the solid state.; By varying the ligand geometries of the β-diimines we are capable of tailoring the zinc complexes, and thereby effecting drastic changes in copolymerization activities. An unsymmetrical ligand geometry containing bulky ortho substituents accelerates the activity of the alternating copolymerization of CHO and CO₂. Furthermore, electron-withdrawing cyano substituents on the BDI backbone further enhance catalytic activity. [(BDICNiPr ₂,Me₂)Zn(μ-OMe)]₂ (3.59) exhibits the highest reported activities for CHO/CO₂ alternating copolymerization (TOF = 2290 h−1).; Mechanistic studies on CHO/CO₂ copolymerization reveal that BDI zinc alkoxides insert CO₂, while BDI zinc acetates react with epoxides. In addition, the rate-determining step in the copolymerization is epoxide ring-opening. Appropriately, rate studies illustrate a first-order dependence in epoxide and a zeroth-order dependence in CO₂. Finally, based on BDI zinc solution studies, stoichiometric initiation events, and rate studies, a bimetallic mechanism is proposed. Sterically encumbered BDI zinc complexes that are predominantly monomers in the ground state, including (BDIiPr)ZnOAc (2.26) and (BDIiPr₂,Et ₂)ZnOAc (3.10), ring-open CHO in a bimetallic transition state. Conversely, sterically unhindered BDI zinc complexes, like (BDIEt)ZnOAc (2.24), insert CHO in a bimetallic transition state but are dimers in the ground state.