Ca‐Mediated Styrene Polymerization: Tacticity Control by Ligand Design

Abstract

AbstractNew heteroleptic benzylcalcium complexes with modified fluorenyl ligands were prepared which include a complex with a chelating dimethylamino substituent, [9‐(2‐Me2N‐ethyl)fluorenyl][2‐Me2N‐α‐Me3Si‐benzyl]Ca (4), and a series of complexes with bulky substituents like tBu‐, Ph(Me)2C‐, Me(Ph)2C‐ and Me(4‐tBuC6H4)2C‐ in the 2‐ and 7‐positions of the fluorenyl ligand (complexes 5 through 9, respectively). Crystal structures of 4, [(2,7‐tBu‐9‐Me3Si‐fluorenyl)(2‐Me2N‐benzyl)Ca]2 (5) and [2,7‐Ph(Me)2C‐9‐Me3Si‐fluorenyl][2‐Me2N‐α‐Me3Si‐benzyl]Ca·THF (6) show that modification of the fluorenyl ligand hardly affects the coordination mode of the benzylic ligand. Also, in solution all compounds are of heteroleptic nature and it was shown that fluorenyl modifications hardly affect the barriers for inversion at the chiral benzylic carbon atom [18.7(2)–19.2(2) kcal/mol]. Complex 4 does not initiate styrene polymerization, which underscores the importance of a free coordination site at calcium. Complexes 5–9 initiate styrene polymerization, and the syndiotacticity of the obtained polymers increases with the bulkiness of the substituents on the fluorenyl ligands. Syndiotacticities up to r = 95 % (rr = 90 %) were obtained.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)