Syndiotactic Poly(propylene) from [Me 2 Si(3,6‐di‐ tert ‐butyl‐9‐fluorenyl)( N ‐ tert ‐butyl)]TiCl 2 –Based Catalysts: Chain‐End or Enantiotopic‐Sites Stereocontrol?

The origin of the syndiotactic selectivity in propene polymerization of a typical bridged fluorenyl‐amido catalyst, namely [Me 2 Si(3,6‐di‐ tert ‐butyl‐9‐fluorenyl)( N ‐ tert ‐butyl)]TiCl 2 ( 1 ), activated with methylaluminoxane or [HMe 2 N(C 6 H 5 )][B(C 6 F 5 ) 4 ]/Al( i ‐butyl) 3 , was investigated by means of 150 MHz 13 C NMR spectroscopic microstructural polymer analysis. The asymmetric induction was traced unambiguously to enantiotopic‐sites control. Compared with the better‐known cyclopentadienyl‐fluorenyl ansa ‐zirconocenes, 1 turned out to be almost identically enantioselective (in agreement with computer modeling), but less stereoselective because of a higher propensity to undergo site epimerization. This results in a chain microstructure with large predominance of m over mm stereodefects, deceptively similar to that of syndiotactic poly(propylene) produced under chain‐end control.Methyl region of the 150 MHz 13 C NMR spectrum of a poly(propylene) sample prepared with catalyst system 1 /borate/TIBAl.