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 CO₂). 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 CO₂ 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.

Figure 1

(a) PO/CO₂ ROCOP to make poly(propylene carbonate) (PPC) polyols. (b) Illustration of one of the heterodinuclear catalysts used in this investigation (2). (c) Catalytic cycle for PO/CO₂ ROCOP illustrating both propagation and chain transfer reactions (epoxide ring-opening k₁, CO₂-insertion k₂ and chain-transfer k₃).

Figure 2

(a) Synthesis of the heterodinuclear complexes 1–4. Reagents and conditions: (i) M(OAc) [M = Na, K, Rb, or Cs], Co(OAc)₂, MeCN, 25 °C, 30 min, N₂. (ii) Ethylenediamine, MeCN, 25 °C, 16 h, N₂. (iii) AcOH (2 equiv), MeCN, air, 72 h, >50% yield. (b) ORTEP representation of the molecular structures of complexes 1–3, with hydrogen atoms and residual solvents omitted for clarity, and thermal ellipsoids are represented at 50% probability (see SI for experimental details).

Figure 3

Polymerization data using catalyst 2 (Co(III)/K(I) for PO/CO₂ ROCOP (Table S1). (a) Plot of PPC molar mass (M_n: ■) and dispersity (Đ: ▲) versus turnover number (TON). (b) Evolution of the PPC molar masses showing an increase in molar mass (g mol^–1) with turnover number (TON) (note the low molar mass shoulder present in some cases arises from chains initiated from catalyst acetate groups). (c) MALDI-ToF spectrum (1000–6000 m/z) of poly(propylene carbonate) initiated from cyclohexanediol (●) and cyclohexane diol + one ether linkage (■). (d) Expanded region of the MALDI-ToF spectrum (4000–5000 m/z) showing both polymer distributions having a repeat unit of 102 g mol^–1 consistent with the value expected for poly(propylene carbonate).

Figure 4

Kinetic data and pathway for catalyst 2 in PO/CO₂ ROCOP. (a) Semilogarithmic plot of ln[PO]_t/[PO]₀ versus time (Table 1, Entry 4). (b) Plot of ln[k_obs] vs ln[2], where [2] = 1.56–7.13 mM (Table S1). (c) Plot of k_obs vs P_CO2 from 5 to 30 bar. (d) Illustration of polymerization pathway and rate-determining step. All errors are calculated from duplicate runs and there is an average error of ±5% on all data.

Figure 5

Eyring plot, ln(k_p/T) versus 1/T, for complex 2 over the temperature range 40–70 °C, 3.57 mM catalyst, neat PO (6 mL), 1,2-cyclohexene diol (71 mmol), under 20 bar CO₂ (Table S3).