Dual-side catalysts

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

Fig. 3 Dual-side catalyst for variable material properties...

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

Rieger B, Troll C, Hild S, Cobzaru C (2006) Dual-side catalysts for high and ultrahigh molecular weight homopolypropylene elastomers and plastomers. Coord Chem Rev 250 189-211... 

Rieger B, Cobzaru C (2006) Control of ultrahigh molecular weight polypropene microstructures via asymmetric dual-side catalysts. Macromol Symp 236 151-155... 

Hackmann M. Rieger, B. Metallocene catalysts in stereoselective synthesis. CaTTech 1997,2,19-91. 1. Cobzaru, C. Hild, S. Boger, A. Troll, C. Rieger, B. Dual-side catalysts for high and ullrahigh... 

Rieger B, Troll C, Preuschen J (2002) Ultrahigh molecular weight polypropylene elastomers by high activity dual-side hafnocene catalysts. Macromolecules 35 5742-5743... 

All side-chain isomers are formed in acid-catalyzed isomerization. Carbonium ions are the intermediates here. Over dual-function catalysts, such as platinum-on-alumina and platinum-on-silica-alumina, platinum increases the rate of isomerization by dehydrogenating alkanes to olefins. This facilitates the formation of carbonium ions. 

Dietrich, U. Hackmann, M. Rieger, B. Klinga, M. Leskela, M. Control of stereoerror formation with high-activity dual-side zirconocene catalysts A novel strategy to design the properties of thermoplastic elastic polypropenes. J. Am. Chem. Soc. 1999,121, 4348 355. 

To get a closer insight into the polymerization mechanism responsible for the strong dependence of stereoselectivity on monomer concentration observed for this type of asymmetric catalyst, the pentad distributions of the polypropylenes prepared with 9a were investigated using nuclear magnetic resonance (NMR) spectroscopy. The mmmm pentad content was observed to decline with increasing monomer concentration this relationship was attributed to the existence of two coordination sites in these dual-side complexes, which show different stereoselectivities for monomer coordination and insertion. Guerra et al. have supported this hypothesis in a theoretical study. 

Alkylations. Highly enantioselective alkylation of t-butyl 4,4-bis (p-dimethyl-aminophenyl)-3-butenoate and t-butyl A -diphenylmethyleneglycine in the presence of a quatemized cinchona alkaloid results. The salt plays a dual role in asymmetric induction and as a phase-transfer catalyst. The products from the former reaction can be cleaved at the double bond to furnish chiral malonaldehydic esters which have many obvious synthetic applications. A combination of PTC, LiCl, and an organic base (e.g., DBU) favors the enantioselective alkylation of a chiral A-acylimidazolidinone in which the acyl side chain is derived from glycine. ... 

It is concluded that both the aldol-type condensation and side reactions are promoted by acid-base dual function of catalyst. However, the required balances of acid base are not the same and, moreover, the balances are also different depending on the difference in the reactant (R-CH2-X). This is the reason why a good performance can be achieved by controlling the balance of acid-base so as to best fit for promoting the condensation reaction and for minimizing the side reaction, and why the best catalysts are different depending on the difference in the reactant. 

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