Epoxides, catalytic copolymerization with

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. 

Diiminatc zinc complexes are highly active catalysts in the copolymerization of epoxides and C02. Complexes that are catalytic are of the form ZnLX, where X is alkoxide, acetate, or bis(tri-methylsilyl)amide. Changing the ligand geometries of the complexes allows variation in the catalytic behavior and activity.941 The polymerization of lactide with diiminate zinc has also been studied.942... 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

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... 

Dendritic and nondendritic polystyrene-boimd manganese-salen complexes were described by Seebach and coworkers [30]. The supported catalysts were prepared by suspension copolymerization of styrene with the vinyl-substituted complexes and employed in the epoxidation of phenyl-substituted alkenes by m-CPBA/NMO. Activities and selectivities were similar to those obtained with the monomeric complexes. High catalyst stabilities were observed and it was demonstrated that the immobilized catalysts can be recycled up to 10 times without loss of performance. Laser ablation inductively coupled plasma mass spectrometry was used to monitor the manganese content in repeatedly used polystyrene beads and a correlation between metal leaching from the support and catalytic activity was disclosed [31]. 

Very recently, the preparation of zinc /3-diiminate derivatives became the subject of interest due to their catalytic activity in the copolymerization of CO2 with epoxides. 

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. 

Naphthaleneytterbium demonstrates a high activity not only in stoichiometric reactions but also in many catalytic processes. It catalyzes at room temperature the polymerization of styrene, methylmetacrylate, ethylacrylate, isoprene, ethylene oxide, the copolymerization of ethylene oxide with styrene, piperilene and carbon dioxide [65], the hydrogenation of alkenes, alkynes, ketones, aldehydes [78], the formation of alkylenecarbonates from epoxides and CO2 [65]. 

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]. 

Finally, the catalytic activity of DMCs in esterification reactions can be readily combined with their catalytic activity in epoxide ring-opening polymerizations, as was reported by Suh et al. [35]. This study showed that the copolymerization of propylene oxide with cyclic acid anhydrides such as succinic, maleic, or phthalic anhydride, catalyzed by a Zn-Co-DMC, afforded polyester polyols characterized by a moderate molecular weight and narrow polydispersity index. 

Detailed mechanistic studies on C02/epoxide promoted by various A1 porphyrin initiators in the presence of a nucleophile concluded on a mono-metallic pathway with the polycarbonate chain growing on one side of the (Porph)Al backbone and with the opposite side being occupied by the Al-coordinated Lewis base cocatalyst (Scheme 10) [37]. In particular, investigations on a (TPP)A1X/DMAP catalytic systems for CO2/PO copolymerization showed that coordination of the nucleophile/... 

A novel catalytic system based on aluminum porphyrins has been developed. It is particularly effective for the living ring-opening polymerization of epoxides and lactones and for the copolymerization of epoxides with carbon dioxide and with cyclic anhydrides. HNMR has demonstrated that the growing species of this initiating system is that shown in (3). 

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