Syntheses, Characterization, and Ethylene (Co-)Polymerization Screening of Amidate Half-Titanocene Dichlorides----lanzhou Branch;Chinese Academy of Sciences

Polyolefins, the most popularly used synthetic polymer, have been synthesized by employing commercial catalysts of Ziegler-Natta, Phillip, and metallocene type. The constrained-geometry compounds (CGC) of half-metallocene catalysts represent single-site catalysts that are particularly considered due to the properties of the resultant polyolefins such as controlled molecular weight, specific tacticity, and narrow molecular weight distribution, as well as better comonomer incorporation. Since the 1990s, inspired by fast developments in coordination and organometallic chemistry, more and more transition metal procatalysts have been designed and investigated for olefin polymerization.

The catalytic behavior of these procatalysts and also the properties of the resultant polyolefin could be fine tuned by modifying the substituents of the complexes’ ligands. Attempts to combine the merits of the individual systems have led to the successful development of a series of nonbridged half-metallocene procatalysts, Cp'M(L)Xn (M=Ti, Zr, Hf; L=anionic ligand;X=halogen or alkyl), which exhibited promising catalytic behavior and overcame the synthetic problems associated with metallocene and/or CGC procatalysts

Therefore, Prof. SUN Wenhua, a visiting professor and his group have investigated the half-titanocene Cp'M- (L)Xn procatalysts containing a dianionic or monoanionic amidate ligands.

They have synthesized and fully characterized the amidate half-titanocene dichlorides,Cp'Ti(L)Cl2 (Cp'= Cp or Cp*; L=amidate ligand, C1-C7), including single-crystal X-ray diffraction for C2 and C4. All procatalysts show high activities toward ethylene polymerization in the presence of MAO. The procatalysts bearing substituted cyclopentadienyl groups give higher activity, while the use of stronger electron-donating groups of arylamidates enhances the catalytic activities of the corresponding procatalysts. It is likely there is a single active species above 40℃, which produces PEs with narrow molecular weights. Regarding the results with procatalyst C6, the copolymerization of ethylene with 1-hexene or 1-octene produced copolymers incorporating branching of about 15%.

The work was published in Organometallics (Organometallics 2010, 29, 2459–2464).

Abstract of the paper published in Organometallics