ReviewsScheme3.Regiofavored CÀObondcleavage of EP with different substituents.Scheme4.The idealized propagation pathway of CO 2 /EP copolymerization.Scheme2.The initiationofC O 2 /EP copolymerization. 476Reviews CHDO, whereas inferiorc atalytic activity was observed for1 ,4-CHDO/CO 2 (36.6 %s electivity of polymer formation with 57 % conversion for 1,4-CHDO), although poly(1,3-cyclohexadiene carbonate) featured as lightly lower T g (104-108 8C) than that of poly(1,4-cyclohexadiene carbonate) (T g = 123 8C).Similarly to the zinc-catalyzed synthesis of poly(lactide)-bpoly(carbonate)-b-poly(lactide) materials, [60] Co III catalysts have also found utility in the preparation of triblock CO 2 -based polymeric structures (Scheme 8). In selected studies, both PO [77] and SO [78] were copolymerized with CO 2 to form ABA-type block copolymers with ad egradable carbonate-containing Figure 3. Representative Co, Cr,and Fe-based catalysts for CO 2 /EP copolymerization.I O= indeneo xide, BO = butene oxide, CPO = cyclopentane oxide, CHDO = cyclohexadiene oxide, DNO = 1,4-dihydronaphthaleneoxide. Reviews Scheme7.Representative functional polycarbonate synthesis with cobalt-salen complexesand subsequent postpolymerization functionalization.A IBN = azobisisobutyronitrile. Scheme8.One-potsynthesis of poly(carbonate-b-ester)from CO 2 /EP/lactide terpolymerization. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, LA = lactide.Scheme9.The possible propagationr oute and deactivation pathways for Co III -catalyzed CO 2 /PO polymerization (P represents the growingpolymer chain). [86]

Designing polymeric materials for closed-loop material streams is the key to achieving a circular society. Here, a library of macrocyclic carbonates (MCs) was designed by a facile and direct one-pot, two-step synthesis approach without the use of a solvent at a 10 g scale. We demonstrate that anionic polymerization with tert-butoxide enables the ultrafast ring-opening polymerization (ROP) of MCs with high conversion (>97%) within seconds (3–10 s) at ambient temperature. The polymerization rate depends on the odd or even number of methylene groups between the carbonate linkages in the MCs, and not the overall ring size, yielding an “odd–even” effect. This polymerization rate is related to the difference in molecular conformation of the MCs, as determined by X-ray crystallography. The polymers (polypenta-, hexa-, heptamethylene carbonate) were subsequently regenerated back to their original MCs at a high selectivity (95–99 mol %) and good yields (70–85%), hence taking a step toward closing the loop on these long alkyl chain polycarbonates.

The organocatalytic coupling of oxetanes and carbon dioxide (CO 2 ) offers a sustainable route to poly(trimethylene carbonate)s and/or functional six-membered cyclic carbonate monomers. This transformation is more challenging than when using more strained epoxide comonomers and even more so when it is performed using metalfree routes. Herein, we report an organocatalytic oxetane/CO 2 coupling procedure that enables the selection of cyclic carbonate or polymer from a common intermediate. Using a novel I 2 -based binary catalyst system, cyclic carbonate with high selectivity (up to 94%) was obtained at 55 °C, whereas simply changing the temperature to 105 °C yielded the polycarbonate with M n of up to 6.4 kDa, thus showing that either trimethylene carbonate or its concomitant polycarbonate product can be selected solely by the manipulation of the reaction conditions.

The green I2/TBD catalytic complex is demonstrated competitive to metal-based systems in the preparation of PTMC through a two-step process mechanism.