In Situ Generation of Cationic Methylzirconium Complexes from rac-(EBI)Zr(NMe2)2 and NMR-Scale Polymerization of Propylene

Abstract

ABSTRACT Sequential NMR-scale reactions have been carried out in order to generate cationic methylzirconium complexes by the reaction of rac-(EBI)Zr(NMe2)2 (rac- 1 , EBI = Et(indenyl)2) with methylaluminoxane (MAO) or various anionic compounds. By reacting 40 equiv. of MAO with rac- 1 in an NMR tube containing CD 2CI2 as a solvent at room temperature, rac- 1 is completely activated to give stable cationic methylzirconium complexes, [(EBI)ZrMe]+[MAO]− which polymerize propylene to isotactic polypropylene (iPP). The formation of the cationic species is achieved after rac- 1 is methylated to form rac-(EBI)ZrMe2 (rac- 2 ) by MAO and/or free Al2Me6 contained in MAO. The same sequential reaction has been performed by using rac(EBI)ZrCl2 (rac- 3 ) for the comparison. MAO cannot generate the cationic species at the same reaction conditions in the reaction of rac- 3 and MAO, mainly due to the difficulties of methylation of rac- 3. Ansa ziconocene amide rac- 1 is stoichiometrically methylated by 2 equiv. of Al2Me6 to give rac- 2. Introduction of 1 equiv. of noncoordinating to the solution mixture of rac- 1 and 2 equiv. of Al2Me6 leads to the formation of stable cationic methylzirconium species, [rac-(EBI)Zr(μ-Me)2AlMe2]+. NMR-scale polymerizations have been carried out by adding a small amount of liquid propylene to these cationic species. The meso pentad values of iPP isolated in these polymerizations are in the range of 80.2–84.7%. By changing the order of sequential reaction, i.e., by reacting rac- 1 with noncoordinating anions prior to methylation by Al2Me6, the yield to give cationic methylzirconium species is decreased. Coordinative anions such as [HNMe2Ph][BPh4] and [HNBu3][BP4] are less effective for the generation of the active zirconium cations than noncoordinating anions. The amount of MAO needed to activate rac- 1 can be decreased by the pre-methylation of rac- 1 by Al2Me6.