Copolymerization of CO2 and epoxide

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

Coates GW, Moore DR (2004) Discrete metal-based catalysts for the copolymerization of CO2 and epoxides discovery, reactivity, optimization, and mechanism. Annew Chem Int Ed 43 6618-6639... 

Lu X-B, Wang Y (2004) Highly active, binary catalyst systems for the alternating copolymerization of CO2 and epoxides under mild conditions. Angew Chem Int Ed 43 3574—3577... 

The second class of catalysts are zinc(II) mono- or dialkoxides obtained from polyhydric phenols and dialkylzinc with partly polymeric stmctures. This system, extensively studied by Kuran [84], is an optimization of the water/diethylzinc and polyphenol/diethylzinc systems developed by Inoue [85]. The use of soluble zinc phenoxides and their analogous cadmium complexes as catalyst for the copolymerization of CO2 and epoxide was studied extensively by the Darensbourg group [86]. This work focused on the use of mononuclear phenoxide derivatives with bulky substituents, e. g., phenyl- and fe/t-butyl groups, on the aromatic ring to a homogeneous catalytic system and thus enhance the activity of the Zn phenoxides. The catalysts developed are stabilized through ancillary neutral... 

Both catalytic systems, alkoxides and carboxylates, are often described as efficient catalysts for the copolymerization of CO2 and epoxides but some drawbacks which hamper a widespread industrial utilization need to be pointed out. The phenoxides, though displaying good selectivities, have up to now only been tested with model substrates, e. g., propylene- and cyclohexene oxides, and the carboxylates, though active, present low-to-fair selectivities. Cyclization... 

Cheng M, Moore DR, Reczek JJ, Chamberlain BM, Lobkovsky BE, Coates GW (2001) Single-site p-diiminate zinc catalysts for the alternating copolymerization of CO2 and epoxides catalyst synthesis and unprecedented polymerization activity. J Am Chem Soc 123 8738-8749... 

The carboxylation of epoxides may afford either monomers, as discussed above, or polycarbonates for example, Al-porphyrin complexes [175, 176] or Zn-compounds [177] promote the formation of polycarbonates. The pioneering studies of Inoue [178] and Kuran [179] have opened the route to the investigation of the copolymerization of CO2 and epoxides. The key issue here is to master the alternate insertion epoxide-C02. 

Scheme 6.24 Routes to polycarbonates copolymerization of CO2 and epoxides (top left), co-polymerization of diaUcylcarbonates and diols (bottom), polymerization of pure cyclic monomers (top right)...

Darensbourg DJ, Mackiewicz RM, Phelps AL, Billodeaux DR (2004) Copolymerization of CO2 and epoxides catalyzed by metal salen complexes. Acc Chem Res 37 836-844... 

Several research groups reported that various ILs could be effective cocatalysts in the copolymerization of CO2 and epoxides with metal salen or metal porphyrin complexes [126-130]. In some cases, it was shown that the activities of theses metal complexes were drastically enhanced by the co-presence of IL, although they had no or a very low activity for the coplymerization in the absence of IL. 

DMC materials have also been widely studied for the copolymerization of CO2 and epoxides (Scheme 1.3), a reaction that leads to the long-term fixation of CO2 in valuable products [16,17]. 

It is clear from the numerous accounts in literature that DMCs can efficiently catalyze the copolymerization of CO2 and epoxides. DMCs can however also be used to develop systems that selectively catalyze the CO2 cycloaddition rather than the copolymerization (Scheme 1.4) as is illustrated by the work of Dharman et al. [20]. By itself, a Zn-Co-DMC is an efficient catalyst for the copolymerization reaction. However, the addition of a quaternary ammonium salt to the reaction mixture switches the selectivity of the catalytic system toward the exclusive formation of the cyclic carbonate. The quaternary ammonium ion plays two important roles in the catalytic system it accelerates the diffusion of CO2 into the reaction mixture and it favors a backbiting mechanism. As such, it hinders the growth of the polymer chain and it enables the selective cyclic carbonate production. Although most zinc-containing catalysts for this reaction are very sensitive toward water, Wei et al. have shown that, for example, the combination of Zn-Co-DMC with CTAB (cetyltrimethylammonium bromide) could even use water-contaminated epoxides as an epoxide feed [21]. 

The high reactivity of these two classes of catalysts, carboxylate and alkoxide derivatives, has been confirmed by recent work of Coates and co-workers [87]. They reported the synthesis of two new types of Zn diimido complexes (30 and 31) as shown in Scheme 7 and successfully utilized both types of complexes in the copolymerization of CO2 with epoxides. Their high activities and selectivies in regard to the carbon dioxide insertion (up to 96% carbonate linkages) are unprecedented. 

Thus far, the discussion of polymerizations conducted in carbon dixiode has centered on systems where CO2 acts only as a solvent for the polymerization. However, there are also examples of polymerization systems where CO2 acts as a comonomer. Most notable among these in the context of this chapter is the coploymerization of CO2 and epoxides. The copolymerization of propylene oxide and carbon dioxide was conducted in SCCO2 using a heterogeneous zinc catalyst [142]. Additionally, Beckman and co-workers have shown that a soluble, fluorinated ZnO-based catalyst can be effectively utilized to promote the copolymerization of CO2 and cyclohexene oxide [143]. These examples indicate that supercritical carbon dioxide can be viable as both a solvent comonomer in polymerization reactions. 

Solvents that can defeat Murphy s Law of Solvents have been identified and are called switchable solvents these are liquids that have one set of properties (that presumably meet the needs of one process step) and that can be reversibly switched to another set of properties to meet the needs of the next step. As an example of the flexibility offered by switchable solvents, consider the copolymerization of CO2 and cyclohexene epoxide catalyzed by a Cr(salen)... 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

In addition, as discussed above, oxidation reactions and reactions which use CO2 as a reagent as well as a solvent are worth investigating. Examples of both are discussed below. Finally, electrophilic processes may be advantageously transferred to supercritical CO2, as demonstrated by the improved isomerization of C4-C12 paraffins catalyzed by aluminum bromide. 2,44) Below, we describe three catalytic reactions which appear promising by these criteria asymmetric catalytic hydrogenation of enamides, ruthenium-catalyzed two-phase oxidation of cyclohexene, and the catalytic copolymerization of CO2 with epoxides. 

The reaction of the carbonato complex with C02 has allowed the demonstration of a facile insertion-deinsertion equilibrium. The study of the deinsertion reaction has allowed estimation of the activation parameters as being = 130 4.0 kJ mor and AS = 121.6 11.9 J moP K . From the above values the authors have calculated an approximate value of the equiUbrium cmistant for the carboxylation reaction equal to 3 x 10 M at 195 K (or a AG value for the same reaction of < 50 kJ moP ), showing that the insertion of CO2 into the M-O-alkyl bond is both kineticaUy and thermodynamically very favored. This trend has also been confirmed for the insertion of CO2 into the Nb-OR bond in [Nb(OR)5]2 (R = methyl, ethyl, aUyl) (see Sect. 6.2.2.1), a catalyst for the synthesis of dialkyl carbonates [67]. Very recently, the facile insertion of CO2 into metal-phenoxide bonds has been reported [68] for cobalt and zinc complexes (Fig. 4.2). It should be noted that such metal systems are used as catalysts in the copolymerization of CO2 with epoxides. 

As mentioned in section 2.1., there can be some synergistic effect by the combination of IL and zinc halide for the CO2 cycloaddition to epoxide. Park et al. also investigated the influence of the co-presence of ZnBr2 on the copolymerization of CO2 and PGE with [BMIm]Cl [124]. ZnBr2 alone showed a low activity and a low Mn, but enhanced the activity of [BMImjCl. By the co-presence of ZnBr2, Mn of the copolymer produced was also increased however, its f(C02) was smaller than that of the copolymer produced with the IL alone. For the enhancement of the activity, they proposed the cooperative activation of epoxide by Zn and halide anion of the IL, which was very similar to that proposed for the CO2 cycloaddition reaction (Scheme 6 in section 2.1.2.). 

Based on the above results and previous works [3,9] on the reaction of epoxides and CO2, we tentatively propose the plausible mechanism for the copolymerization of GMA and CO2 (schane 1), Alkyhnethyl imidazolim salt (QX) and epoxide (GMA) r cted to synthesize an active spedes followed by chain propagation involving a < ncerted insertion of th e epoxide. However, more detailed mechanistic studies are needed to clairly understand the polymerization steps. 

The IL 3-butyl-l-vinylimidazolium chloride ([VBIMJCl) and the cross-linker DVB were copolymerized to prepare a highly cross-linked PSIL, in which [VBIMJCl was covalently anchored on DVB-cross-linked polymer matrix. The catalytic performance of the PSIL was investigated, and the PSIL was very active, selective, and stable for the cycloaddition of CO2 to epoxides, and could be easily separated from the products and reused [88]. 

As expected, the copolymerization of CHO and CO2 using catalyst 4 occurs rapidly under the previously reported optimized conditions (26) and has an activity similar to that of the most active catalysts reported to date (Table II, entry 1) (27). The isolated polycarbonate has >95% carbonate linkages and a narrow molecular weight distribution. Unlike the copolymerization of aliphatic epoxides, the copolymerization of alicyclic epoxides does not generate any of the cycloaddition product. 

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