Stereoselective polymerization activation energy

The chain-end stereocontrol for olefin polymerizations leads generally to lower stereoselectivities (differences in activation energy for insertion of the two enantiofaces generally lower than 2 kcal/mol) than the chiral site stereo-control.18131132 For this reason, the corresponding catalytic systems have not reached industrial relevance for propene homopolymerization. However, some of them are widely used for propene copolymerization with ethene. 

Despite the catalysts different stereoselectivity in a-olefin polymerization, all three favored the formation of the meio-hydrodimer. In case of the achiral catalyst Cp2ZrCl2 (1, Cp = cyclopentadienyl), this has to be attributed to chain-end control since the Cp ligands do not induce stereopreference. From the meso/rac ratio, a difference in the free energies of activation (AG g o - AGg g) of 1.5 kJ/mol at 30 °C has been calculated. 

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