Microstructure of Metallocene-Catalyzed Propene/1-Pentene Copolymers.

The microstructure of propene/1-pentene copolymers prepared with the metallocene catalysts rac-Et(Ind)2ZrCl2 and rac-Me2Si(2-MeBenz-[e]Ind)2ZrCl2 was studied by 13C-NMR spectroscopy. Both catalysts lead to the formation of random copolymers while rac-Me2Si(2-MeBenz-[e]Ind)2ZrCl2 favours a somewhat higher incorporation of 1-pentene than rac-Et(Ind)2ZrCl2. Surprisingly, the presence of 1-pentene has a significant influence on the stereoregularity of the copolymers formed with rac-Me2Si(2-MeBenz-[e]Ind)2ZrCl2. Propene/1-pentene copolymers produced with rac-Et(Ind)2ZrCl2 retained the isotacticity of polypropene and featured only the stereoerror pentads mmmr, mmrr and mrrm typical for enantiomorphic site control. However, propene/1-pentene copolymers obtained with rac-Me2Si(2-MeBenz-[e]Ind)2ZrCl2 are characterized by a gradual loss of tacticity concurrently with an increase in the amount of all irregular pentads. It is proposed that the presence of 1-pentene can lead to a reversible loss of stereocontrol over a whole sequence of inserted monomer units. Subsequently, more or less atactic blocks are formed besides isotactic blocks.