Conformational site control

Figure 3.51 Enantiomorphic site control versus conformational control in the cyclisation step during cyclopolymerisation of a, co-diolefins 1,5-hexadiene (x=l), 1,6-heptadiene (x—2) and 1,7-octadiene (x=3) in the presence of a metallocene-based catalyst of C2 symmetry...

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... 

Walden, S. E. Wheeler, R. A. "Protein Conformational Gate Controlling Binding Site Preference and Migration of Ubiquinone-B in the Photosynthetic Reaction Center of Rhodobacter sphaeroides,"J. Phys. Chem. B 2002, 106, 3001-3006. 

Therefore, assuming that conformational equilibria similar to those observed for the precursor complexes occur for the active cations as well, Erker et al. proposed that (1) C2-symmetric con-formers can produce enantiomorphic-sites-controlled isotactic polypropylene with rr stereodefects, (2) Cl-symmetric conformers are (much) less stereoselective, and (3) reversible catalyst switches between the different accessible conformations can lead to stereoblock polypropylene microstructures, including the stereoblock-isotactic one (Figure 8.1c). 

At higher temperatures, at the same time as the polydispersity decreases because of the rapid reversible 5-X interconversion, the rigid structural integrity is also more and more compromised. The concentration of enantiomorphic site control related stereo-errors (rmmr) increases, but the rate of site epimerization and concentration of rrmr pentads increase much more dramatically, parallel to the increase in the conformational interconversion rate and the resulting molecular flexibility. 

Aluminum alkyl, aluminum alkyl-alcohol and aluminum isopropoxide were also found to be catalysts forming water insoluble, optically active crystalline polymers with AA-car-boxy-c ,/-alanine anhydride. Following the Japanese authors, there are two origins to the steric control of the polymerization mechanism of the anhydride [156] the one caused by the growing polymer can be further divided into two types, control by the growing-chain, namely by a helix conformation of the chain or by the end group and an enan-tiomorphic catalyst site control Qualitative considerations led to the conclusion that the... 

The decay processes in P2VN are considered to be super-position of excimer forming processes from different conformational sites. Further, the efficiency of excimer formation at different conformational sites differs substantially. The dual decay process in P2VN, if indicative of the existence of two excimer sites, is supposed to result from ground state conformational control of excimer formation. 

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