Co(III)/Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-Opening Copolymerization of CO₂ and Propylene Oxide

Abstract

The ring-opening copolymerization of carbon dioxide and propene oxide is a useful means to valorize waste into commercially attractive poly(propylene carbonate) (PPC) polyols. The reaction is limited by low catalytic activities, poor tolerance to a large excess of chain transfer agent, and tendency to form byproducts. Here, a series of new catalysts are reported that comprise heterodinuclear Co(III)/M(I) macrocyclic complexes (where M(I) = Group 1 metal). These catalysts show highly efficient production of PPC polyols, outstanding yields (turnover numbers), quantitative carbon dioxide uptake (>99%), and high selectivity for polyol formation (>95%). The most active, a Co(III)/K(I) complex, shows a turnover frequency of 800 h −1 at low catalyst loading (0.025 mol %, 70 °C, 30 bar CO 2 ). The copolymerizations are well controlled and produce hydroxyl telechelic PPC with predictable molar masses and narrow dispersity (Đ < 1.15). The polymerization kinetics show a second order rate law, first order in both propylene oxide and catalyst concentrations, and zeroth order in CO 2 pressure. An Eyring analysis, examining the effect of temperature on the propagation rate coefficient (k p ), reveals the transition state barrier for polycarbonate formation: ΔG ‡ = +92.6 ± 2.5 kJ mol −1 . The Co(III)/K(I) catalyst is also highly active and selective in copolymerizations of other epoxides with carbon dioxide.

Cooperative Effects in Multimetallic Complexes Applied in Catalysis

“…These complexes show high efficiency and outstanding yields of PPC (polypropylene carbonate) polyols in the ring-opening copolymerization reaction of CO 2 and propylene oxide. [92] The Co•••K separation measures 3.698 Å, which fall in the range to show metal-metal cooperation in ring-opening intermediate and transition state. [93] Further examples of cooperativity in transition metal/alkali metal systems have recently been reported by Garden and co-workers.…”

Heterodinuclear catalysts Zn(ii)/M and Mg(ii)/M, where M = Na(i), Ca(ii) or Cd(ii), for phthalic anhydride/cyclohexene oxide ring opening copolymerisation

“…43,44 A further advance has been to exploit synergic heterodinuclear catalysts where the mixed metal complex shows significantly better performance than either homodinuclear analogue or mixtures and has been applied in the ROCOP of CO 2 /epoxide and more recently to lactide ROP. 36,37,40,41,45,46 For example, L′ZnMgBr 2 catalyst showed 40 fold higher activity in PA/CHO ROCOP compared with an equimolar mixture of L′Zn 2 Br 2 : L′Mg 2 Br 2 (TOF = 188 h − 40 Catalytic synergy was also observed in the mechanistically similar ROCOP of CO 2 with epoxides, with metal combinations of Mg(II)Zn(II), LMg(II)Co(II) and Co(III)K(I) being most active. [35][36][37]41 Here, we investigate heterodinuclear catalysts using metals selected from groups 1, 2, 12.…”

“…36,37,40,41,45,46 For example, L′ZnMgBr 2 catalyst showed 40 fold higher activity in PA/CHO ROCOP compared with an equimolar mixture of L′Zn 2 Br 2 : L′Mg 2 Br 2 (TOF = 188 h − 40 Catalytic synergy was also observed in the mechanistically similar ROCOP of CO 2 with epoxides, with metal combinations of Mg(II)Zn(II), LMg(II)Co(II) and Co(III)K(I) being most active. [35][36][37]41 Here, we investigate heterodinuclear catalysts using metals selected from groups 1, 2, 12. The work builds upon an earlier report of a di-Zn(II) catalyst, coordinated by a dinucleating Schiff base ligand derived from ortho-vanillin, which showed an excellent activity of 198 h 47 This work targets heterodinuclear Zn(II) or Mg(II) complexes, combined with Na(I) or Ca(II) because the metals show higher ionic character and oxophilicity relative to Zn(II).…”

“…17,27 Furthermore, metals from groups 1, 2 and 12 are attractive targets due to their lack of colour, low toxicity and low cost. [35][36][37][38] Dinuclear catalysts based on both Zn(II) and Mg(II) are well known in the ROCOP of CO 2 /epoxide, [39][40][41] but are much less explored for the ROCOP of anhydride/epoxide ( Fig. S1IV and V †).…”

Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide

“…6 The most active and selective complexes contain metal centers of Al(III), 7 Fe(II) 8 In(III), 9,10 Zn(II) [11][12][13] and Y(III). 14 Cobalt based catalysts have been widely used in polymerization reactions and more recently as bio-inspired photocatalysts, [15][16][17][18][19][20][21][22] but they are far less investigated in ROP of cyclic esters. Only a few examples are known of cobalt complexes as catalysts in ROP of LA.…”