Epoxide polymerization catalysts

The zinc cation gives by far the most active catalyst. Iron, cobalt, and nickel cations also gave salts with considerable catalytic activity. Cadmium, because of its chemical similarity to zinc, and aluminum, because of its use in other epoxide polymerization catalysts, were considered as likely candidates to give active catalysts. However, complexes of the salts of these cations were only slightly catalytic. The salts used as cation sources in catalyst preparations also affected catalytic activity. Zinc salts, especially zinc chloride and zinc bromide, were retained in considerable amounts in the finished complexes, and the use of these salts gave the most active catalysts. 

The vast majority of papers reporting stereoselective epoxide polymerization focus on isospecific propylene oxide polymerization. For clarity, this chapter is organized by the type of metal of the catalyst active center. The three most commonly used metals for discrete stereoselective epoxide polymerization catalysts are aluminum, zinc, and cobalt, and research using these metals forms the foundation of this chapter. 

SCHEME 24.3 Synthesis of AlR3/H20/acacH epoxide polymerization catalysts (R = alkyl). 

Well-defined [A(,A( -bis(2-hydroxybenzylidene)-(lR,2/ )-l,2-cyclohexane diamine] (R-salcy) aluminum complexes (e.g.. Figure 24.1, 4) have been used as stereoselective epoxide polymerization catalysts. Polymerization of racemic propylene oxide in the presence of 5 mol% 4 yields 70% conversion to poly(propylene oxide) after 62 h. The remaining unreacted monomer exhibits an optical rotation of 4-1.85°, which corresponds to an ee of 15% (Scheme 24.7). The modest r-factor... 

Although significant advances in stereoselective epoxide polymerization have been achieved over the last half-century, few known catalysts are capable of excellent levels of stereocontrol. Historically, most catalysts for epoxide polymerization have been of the heterogeneous variety and have exhibited poor selectivity. It is our opinion that the most fertile area for future catalyst exploration involves homogeneous, discrete catalysts that are capable of involving multiple metal centers in the polymerization mechanism. If the spatial environment of the active catalyst is precisely controlled, new generations of stereoselective epoxide polymerization catalysts will become available. Our current research focuses on the search for such catalysts. 

Epoxide polymerization catalysts of quite different characteristics are those constituted by the metalloporphyrins of aluminum and zinc, such as (5,10,15,20-tetraphenylporphinato)aluminum chloride ((TPP)AICI), (5,10,15,20-tetraphenylporphinato) aluminum methoxide ((TPP)AlOMe) (see Scheme 27), and (5,10,15,20-tetraphenyl-21-methylporphinato)zinc methoxide ((MTPP)ZnOMe). ° ... 

The trigonal planar zinc phenoxide complex [K(THF)6][Zn(0-2,6-tBu2C6H3)3] is formed by the reaction of a zinc amide complex, via a bis phenoxide, which is then further reacted with potassium phenoxide. TheoX-ray structure shows a nearly perfect planar arrangement of the three ligands with zinc only 0.04 A out of the least squares plane defined by the three oxygen atoms.15 Unlike the bisphenoxide complexes of zinc with coordinated THF molecules, these complexes are not cataly-tically active in the copolymerization of epoxides with C02. The bisphenoxide complex has also been structurally characterized and shown to be an effective polymerization catalyst. 43... 

Song et al. [62] reported poly-salen Co(III) complexes 18, 19 as catalyst for HKR (Figure 5) of terminal alkene epoxides. The polymeric catalysts provided product epoxides with excellent conversion (>49%) and high chiral purity (ee s, 98%) and the catalytic system could be recycled once with retention of activity and enantioselectivity. 

Week et al. [65] further reported the Co salen complex supported on norbomene polymers (23, 24) with stable phenylene-acetylene linker (Figure 8). The polymer-supported salen catalysts were investigated for HKR of the racemic terminal epoxides that showed outstanding catalytic activities and comparable selectivities to the original catalysts reported by Jacobsen. However, the polymeric catalyst was recycled only once after its precipitation with diethylether as the catalyst became less soluble and less reactive in subsequent catalytic... 

Kim et al. [67] recently reported the synthesis of heterometallic chiral polymer (salen) Co-(Al, Ga, ln)Cl3 complexes 26-32 (Figure 10) and their use in the HKR of racemic epoxides. Polymeric salen catalysts showed very high reactivity and enantioselectivity at substantially lower catalyst loadings for the asymmetric ring opening of terminal epoxide to obtain the enantio-enriched products. The performance of catalysts is retained on multiple-use and do not suffer the problems of solubility and deactivation (Scheme 5). 

In the following sections, we describe the recent development of catalyst systems for epoxide polymerization, focusing on homopolymerization, (alternating) co-polymerization with CO or GO2 reported from 1993 to 2004. Although aluminum and zinc are not classified as transition metals, polymerization catalyst systems using those metals will be discussed since they greatly contribute to the field of epoxide polymerization. 

Ghromium complexes have been found to promote the co-polymerization of epoxides with G02- Recently, Darensbourg et al. have demonstrated that the chromium-salen complexes, remarkably more stable to the air and moisture than zinc-based co-polymerization catalysts, are effective catalysts for the co-polymerization of GHO with G02. " Under the condition of 5.9MPa GO2 pressure at 80 °G, complex 35a transforms GHO to the completely alternating co-polymer with a TON of 250 mol (mol of Zn) and a TOF of 10 mol (mol of Zn) h along with a small amount of eyelie earbonate production (Table 7). 

Ishimori,M., Nakasugi,0., Takeda,N, Tsuruta,T. Studies on organometallic compounds as polymerization catalysts. II. Diethylzinc/water system for epoxide polymerization. Makromol. Chem 115,103 (1968). 

Hexacyanometalate Salt Complexes as Catalysts for Epoxide Polymerizations... 

Reasoning from the success of Price (16) and others with zinc compounds as catalysts for epoxide polymerizations, the zinc hexacyano-... 

Carbon dioxide is an attractive renewable resource being of low toxicity, abundant and the waste product of many processes. There is precedent, dating back to 1969, for the copolymerization of C02 and strained heterocycles, most notably epoxides [18]. The focus on this review will be on understanding the polymerization catalysts for the alternating copolymerization. 

Several types of copper-loaded polymer have been prepared. Hydrocarbon spacers were introduced between polystyrene and copper (23). Six synthetically useful copper-promoted reactions including a Diels-Alder reaction, an epoxide ring-opening, and an aryl iodide hydrolysis were examined by using the polymeric catalyst [34]. Use of the copper-loaded polymers often either improved the yield or reduced the reaction time compared with those for conventional copper salts. 

Song, Y., Yao, X., Chen, H., Bai, C., Hu, X., Zheng, Z. Highly enantioselective resolution of terminal epoxides using polymeric catalysts. Tetrahedron Lett. 2002, 43, 6625-6627. 

Use Bleaching textiles, paper, oils, waxes, starch polymerization catalyst bactericide and fungicide, especially in food processing epoxidation of fatty acid esters and epoxy resin precursors reagent in making caprolactam, synthetic glycerol. 

S. V. Kotov, E. Balbolov, Comparative evaluation of the activity of some homogeneous and polymeric catalysts for the epoxidation of alkenes by organic hydroperoxides, /. Mol. Catal. A 176 (2001) 41. 

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