Copolymerization of CO2 and

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... 

Soga K, Imai E, Hattori I (1981) Alternating copolymerization of CO2 and propylene oxide with the catalysts prepared from Zn(OH)2 and various carboxylic acids. Polym J 13(4) 407 10... 

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... 

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)... 

A number of metal-catalyzed polymerizations have utilized CO2 as both a solvent and as a reagent in the reactions. Precipitation copolymerization of either propylene oxide (83) or cyclohexene oxide (84) with CO2 in SCCO2 has been catalyzed using heterogeneous zinc catalysts. Copolymerizations of CO2 and propylene oxide formed PCs with a molecular weight of about 10 g/mol and incorporation of CO2 at greater than 90% (eq. (7)). A small percentage of propylene carbonate by-product was also observed. 

Super M, Beckman E (1998) Copolymerization of CO2 and cyclohexene oxide. J Macromol Symp 127 89-108... 

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... 

Mang S, Cooper AI, Colclough ME, Chauhan N, Hohnes A (2000) Copolymerization of CO2 and 1,2-cyclohexene oxide using a C02-soluble chromium porphyrin catalyst. Macromolecules 33 303-308... 

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... 

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.). 

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]. 

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