Synthesis and Characterization of iPP-sPP Stereoblock Produced by a Binary Metallocene System

Stereoblock polypropylenes comprising of iPP and sPP segments are synthesized by polymerization of the following binary system of metallocenes: the C_s-symmetric [2,7-t-Bu₂(Flu)₂Ph₂C(Cp)ZrCl₂] and the C₂-symmetric rac-Me₂Si(2-Me-4-Ph-Ind)₂ZrCl₂. Blends of samples made either by each catalyst individually (solution blend) with materials obtained with the mixed catalyst system (reactor blend) are compared. The simultaneous presence of MAO and DEZ, enhancing fast and reversible transfer of the growing chains between the two active centers, leads to the formation of a stereoblock microstructure. In this case, low molecular weight polymers are obtained. The junction between the blocks is qualitatively observed in ¹³C NMR. When made in toluene, the stereoblock material consists of a majority of syndiotactic sequences, whereas the ratio is more equilibrated when the polymerization was conducted in the more polar chlorobenzene. This is confirmed by the results obtained with ¹³C NMR, CRYSTAF, HT HPLC, DSC, SSA, WAXD, and optical microscopy.