Elastomeric Poly(propylene) from "Dual-side" Metallocenes: Reversible Chain Transfer and its Influence on Polymer Microstructure

The influence of the cocatalyst nature on the distribution of the stereoerrors along the polymer chain has been studied using either MAO or [(C6H5)3C+] [(C6F5)4B−] to activate a C1‐symmetric (Flu‐Ind) complex in propene polymerization experiments. The in situ activation with borate indicated the chain back‐skip as the decisive mechanism responsible for stereoerror formation. When MAO is used for activation, additionally the reversible chain transfer to aluminum occurs, which can be called into account as a second mechanism for stereoerror formation. By the combination of 13C NMR, DSC, WAXS and SFM, it was shown that the differences in polymerization mechanisms result in variations of stereoerror formation. Due to this, the isotactic block length niso as well as their distribution along the chain changes. Using MAO activation, polypropenes with crystallizable blocks consisting of 23–32 monomers in isotactic sequences were generated, which co‐crystallized in α‐ and γ‐phase lamellae. When the reversible chain transfer was occluded (in situ borate activation) the bimodal distribution of crystalline lamellae strongly referred to a homogeneous random distribution of stereoerrors. In this case, two crystalline populations were present. The prevailing one, which crystallized in the orthorhombic γ‐modification, contained 23 consecutive isotactic blocks. Additionally, small amounts of α‐phase lamellae were present consisting of longer isotactic blocks (niso > 35). The different crystalline modifications resulted in different polymer morphologies. These changes caused in turn variations in the mechanical properties, such as elasticity and mechanical strength. This clearly shows that, by using different cocatalysts for activating C1‐symmetric complexes, the properties of poly(propylenes) with statistically distributed stereoerrors can be tailored.