C₁‐Symmetric Heterocyclic Zirconocenes as Catalysts for Propylene Polymerization, 1

Abstract

Summary: The synthesis of C1‐symmetric zirconocene complexes bearing the 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand (S2‐3) linked to substituted cyclopentadienes is described. Different syntheses of S2‐3, the common intermediate for the preparation of these complexes, are discussed. Many of these complexes have been found to be highly active in propylene polymerization, to require very low amounts of methylalumoxane to be activated, and to produce poly(propylene)s of low isotacticity and melting points. 13C NMR analysis shows that the poly(propylene)s are fully regioregular and that the stereoerrors are randomly distributed, as shown by the enantiomorphic‐site triad test E ≈ 1. The experimental pentad distribution was fitted using a two‐site model with different probability parameters for each site. The probability of chain back‐skip was also taken into account. The molecular weight and crystallinity of the poly(propylene)s are dependent upon the type of substituents on the cyclopentadienyl ring, and the correlation between mmmm content and melting point of the PP confirms the random distribution of stereoerrors.Correlation between % mmmm pentad and melting point.magnified imageCorrelation between % mmmm pentad and melting point.

C1‐Symmetric Heterocyclic Zirconocenes as Catalysts for Propylene Polymerization, 2

“…To prepare the bridged ligands, cyclopentadithiophene S 2 -3 and its derivatives [26,27,30] were coupled with substituted indenes [34,35] by dimethylmethylene, dimethylsilyl, and ethylene links. The ligand with dimethylmethylene bridge, 9a, was synthesized by the reaction of S 2 -3 with 6,6-dimethylbenzofulvene in THF, since no reaction with 6,6-dimethylbenzofulvene occurred in diethylether (Scheme 1).…”

“…On the other side, using the cyclopentadithiophenes having the two S atoms opposite to the methylene, such as S 2 -2 and S 2 -4 ( Figure 1), as ligands combined with substituted cyclopentadienes, led to zirconocenes of far lower catalytic performance. [26] Based on these earlier findings, we have developed several routes to the synthesis of S 2 -3 and prepared a large number of zirconocenes bearing ligands based on this moiety and other a,a 0 -disubstituted cyclopentadithiophenes such as S 2 -5 and S 2 -6 ( Figure 1). [26][27][28][29][30][31][32] A selection of liquid propylene polymerization results with the RCp-ansa-(S 2 -1) prototypes 1 and 2 and RCpansa-(S 2 -3) analogs 3-5 are reported in Table 3.…”

“…[26] Based on these earlier findings, we have developed several routes to the synthesis of S 2 -3 and prepared a large number of zirconocenes bearing ligands based on this moiety and other a,a 0 -disubstituted cyclopentadithiophenes such as S 2 -5 and S 2 -6 ( Figure 1). [26][27][28][29][30][31][32] A selection of liquid propylene polymerization results with the RCp-ansa-(S 2 -1) prototypes 1 and 2 and RCpansa-(S 2 -3) analogs 3-5 are reported in Table 3. The three complexes 3-5 mimic the three limit structures expected to produce syndiotactic, hemiisotactic, and isotactic poly-(propylene)s. [1,2] The complexes 3 and 5, analogs of their prototypes 1 and 2, did indeed yield the expected syndiotactic and isotactic poly(propylene)s, respectively.…”

“…The three complexes 3-5 mimic the three limit structures expected to produce syndiotactic, hemiisotactic, and isotactic poly-(propylene)s. [1,2] The complexes 3 and 5, analogs of their prototypes 1 and 2, did indeed yield the expected syndiotactic and isotactic poly(propylene)s, respectively. [26] However, although ligand synthesis and purity were improved with respect to the parent compounds, the catalytic performance of 3 and 5 in propylene polymerization remained uninteresting: lower molecular weight and lower stereoregularity compared to their all-carbon analogs, and compared to 1 and 2, were obtained. Complex 4 on the other side, although still producing relatively low-molecular weight PP, gave an interesting microstructure, clearly deviating from the expected hemiisotactic pentad distribution towards a partially isotactic microstructure.…”

“…Complex 4 on the other side, although still producing relatively low-molecular weight PP, gave an interesting microstructure, clearly deviating from the expected hemiisotactic pentad distribution towards a partially isotactic microstructure. [26] As part of our investigation on the properties of stereoirregular poly(propylene)s, [33] we optimized the R n Cp structures within the above family of structures in order to obtain low-melting, elastomeric poly(propylene)s. Structure optimization led us to complexes 6-8, that gave very high propylene polymerization activities at low MAO/ Zr ratios, producing low-isotacticity, low-melting, soft PP with higher molecular weights compared to 4 (Table 4). [26] None of the (R n Cp)-ansa-(S 2 -3) complexes, however, reached the minimum requirement of PP molecular weight (M v > 200 000 at T p > 60 8C) to make commercial applications foreseeable.…”