Preparation of Homogeneous Salen Cobalt and Heterogeneous Double Metal Cyanide Complexes for CO2/Propylene Oxide Copolymerizations

Abstract

This paper covers homo- and heterogeneous catalysts for CO2/propylene oxide copolymerizations. The catalysts we designed were aimed at developing both catalytic activity and properties of the products. Chapter 1 reviews background information of CO2/epoxide copolymerization and the history of various catalytic system for them. Chapter 2 shows that (Salen)Co(III) complex, tethered with four quaternary ammonium salts via covalent bonds, is one of the most highly active catalysts for CO2/epoxide copolymerization. In this study, we aimed to synthesize similar (Salen)Co(III) complexes which quaternary ammonium salts are linked via ionic interactions. Thus, we prepared multiple ammonium salts containing 2−5 quaternary ammonium salt units, along with (Salen)Co(III) complexes that include one or two −SO3−[PhNH3] + moieties. A binary catalytic system, compring the prepared multiple ammonium salts and (Salen)Co(III) complex containing −SO3−[(nBu)4N] + moieties, showed high activity (TOF = 1500−4500 h−1) for CO2/propylene oxide (PO) copolymerization. In contrast, a system combining multiple ammonium salts with the conventional (Salen)Co(III) complex, which lacks −SO3−[(nBu)4N] + moieties, was inactive under the polymerization conditions ([PO]/[Co] = 20,000). However, the use of a (Salen)Co(III) complex containing two −SO3−[(nBu)4N] + moieties led to the concomitant generation of a substantial amount of cyclic carbonate (25−30%). This side reaction could be reduced by ca. 50% by using a (Salen)Co(III) complex containing single −SO3−[(nBu)4N] + moiety. The formation of cyclic carbonate can be attributed to ammonium salts ([(nBu)4N] +[carbonate]−) not linked to the (Salen)Co(III) complex in the binary catalytic system. Chapter 3 indicates that double metal cyanide (DMC) complexes prepared via the salt metathesis reaction of K3Co(CN)6 with ZnCl2, are commercially utilized for propylene oxide (PO) homopolymerization. These complexes are also active in PO/CO2 copolymerization, though the insertion of CO2 is not perfect. The fraction of carbonate linkage to ether linkage (FCO2) fluctuates in a wide range (10−60 mol%), depending on the preparation method and conditions. The preparation of DMC catalysts is complicated, requiring many difficult washing processes. In this work, we propose a preparative-scale (100 g scale) synthesis of H3Co(CN)6, the structure of which was determined using X-ray crystallography. DMC catalysts were prepared using H3Co(CN)6 with Zn(EH)2 (EH = 2-ethylhexanoate) in methanol precipitated solids that were subsequently used for polymerization following solvent removal. The prepared DMCs exhibited good activity in PO homopolymerization, even when a propylene glycol (PG) starter was present. A conventional DMC synthesized with K3Co(CN)6 in water was inactive when using a simple PG starter. However, the prepared DMC was also active in PO/CO2 copolymerization, although its productivity was significantly reduced under CO2 pressure. The carbonate linkage fraction was surprisingly high (FCO2 = 0.48−0.66), depending on the CO2 pressure, although the formation of a small amount of cyclic carbonate was inevitable (~10 wt%). However, introducing a starter, such as polypropylene glycol (PPG) or adipic acid, to produce low-molecular-weight macrodiol deteriorated the catalytic performance, resulting in the concomitant generation of a substantial amount of cyclic carbonate (~30 wt%). Chapter 4 describes that despite DMC’s six decades of industrial use and significant relevance, the precise composition, structure, and working mechanism of this catalyst still remain elusive. In this study, we address these uncertainties by reevaluating the composition, identifying it as a salt composed of (NC)6Co 3− anions with 1:1 Zn2+/(X)Zn+ cations (X = Cl, RO, AcO). Utilizing a novel synthetic approach, we prepared a series of well-defined DMCs, [ClZn+][Zn2+][(NC)6Co 3−][ROH], [(RO)Zn+][Zn2+][(NC)6Co [(AcO)Zn+][Zn2+][(NC)6Co 3−], [(RO)Zn+]p[ClZn +](1−p)[Zn 2+][(NC)6Co [(AcO)Zn+]p[(tBuO)Zn +]q[Zn 2+][(NC)6Co 3−], and [(AcO)Zn+]p[(tBuO)Zn +]q[ClZn +]r[Zn 2+][(NC)6Co 3−]. The structure of [(MeOC3H6O)Zn +][Zn2+][(NC)6Co 3−] was precisely determined at the atomic level through Rietveld refinement using synchrotron X-ray powder diffraction (XRD) data. By evaluating the catalyst's performance and elucidating the chain growth mechanism, a correlation between structure and performance was established on various aspects including activity, dispersity, unsaturation level, and carbonate fraction in the resulting polyols. Ultimately, our study identified highly efficient catalysts that outperformed the state-of-the-art benchmark DMC not only in PO polymerization [DMC-(OAc/OtBu/Cl)(0.59/0.38/0.15)] but also in PO/CO2 copolymerization [DMC-(OAc/OtBu)(0.95/0.08)].