Terpolymerization of Epoxides, Anhydrides, and CO ₂ Using Binary Metal-Free Catalysts

ConspectusThe development of environmentally friendly polymer materials, which align with the principles of green chemistry and sustainable development, has garnered significant attention from both academic and industrial research. The terpolymerization of carbon dioxide (CO2), epoxides, and anhydrides stands out as a promising route to synthesize CO2-based copolymers with versatile chain structures and tunable properties. The key challenge in advancing this process is the development of suitable catalysts that can bridge the gap between the two types of ring-opening copolymerization (ROCOP) involving epoxides/anhydrides and epoxides/CO2. In addition to metal-based catalysts, metal-free catalysts have become a focal point due to their ease of synthesis, low toxicity, and absence of residual metals in the final product. This has spurred efforts to develop metal-free catalytic systems that exploit the different reactivities of these monomers, enabling one-pot processes to synthesize block, tapered, or random copolymers. In this Account, we provide insights into the roles of organic base combined with Lewis acid in the terpolymerization of epoxide/anhydride/CO2. This synergistic approach has driven the development of a series of binary catalytic systems. The PPNCl (bis(triphenylphosphoranylidene)ammonium chloride)/TEB (triethyl borane) pair metal-free catalyst enables the synthesis of block copolymers with a small amount of tapering structure under a PPNCl/TEB ratio of 1/0.5. C3N3-Py-P3 (2,4,6-tris[tri(1-pyrrolidinyl)iminophosphorane]-1,3,5-triazine)/TEB can produce block, tapered, or random copolymers at C3N3-Py-P3/TEB ratios of 1/0.5, 1/1, and 1/2, respectively. Both P5Cl (tetrakis[tris(dimethylamino)phosphoranylidenamino] phosphonium chloride)/TEB and CTPB (cyclic trimeric phosphazene base)/TEB systems are superior in controlling polymer synthesis. P5Cl, with its bulky structure, enables the formation of well-defined block copolymers at a P5Cl/TEB ratio of 1/0.5 and truly random poly(ester-carbonate) copolymers at a 1/2 ratio, with a sequence distribution of ester–ester sequence (EE)/ester-carbonate sequence (EC)/carbonate-carbonate sequence (CC) = 25/47/28. CTPB allows the synthesis of well-defined block copolymers at a CTPB/TEB ratio of 1/0.5 and random copolymers at a CTPB/TEB ratio of 1/3, with a sequence distribution of EE/EC/CC = 26/50/24. Using 1,3,4-benzene tricarboxylic anhydride as the initiator, hyperbranched poly(propylene phthalate)-b-poly(propylene carbonate) has been synthesized with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)/TEB as the catalyst. The TEA (triethylamine)/TEB system is highly active and productive, even at very low TEB loadings, and produces a random PA/PO/CO2 copolymer with TEA/TEB = 1/1. The bifunctional [BBN-C5-NEt3][Br] catalyst produces a gradient-type terpolymer, whereas the corresponding binary catalyst yields a tapered version. The ratio of base to Lewis acid in binary catalytic systems critically controls alkoxide chemoselectivity toward CO2 versus cyclic anhydrides during ROCOP. At low Lewis acid ratios, strong binding between the Lewis acid and alkoxide intermediates suppresses CO2 insertion and favors cyclic anhydride incorporation, yielding blocky poly(ester-carbonate) structures. In contrast, excess Lewis acid stabilizes carbonate intermediates, lowers CO2 insertion barriers, and promotes random copolymerization. These trends are governed by subtle but distinct kinetic and thermodynamic factors, as confirmed by DFT calculations. This work has significantly advanced understanding and development of sustainable polymerization systems, demonstrating the potential of metal-free catalysts for a wide range of copolymerization applications.