Cr(II) and Cr(III) complexes with pyridine-imine ligands: synthesis and application in the polymerization of olefins

Chromium complexes have been widely studied in the field of olefin polymerization in both academia and industry, in particular as regards the heterogeneous catalysts for ethylene polymerization {e.g., heterogeneous Phillips catalyst for the production of high-density poly(ethylene)}, and the homogeneous catalysts for the production of linear ?-olefins],[1] and syndiotactic 1,2 poly(butadiene).[2,3] Conversely, few reports appeared in the literature concerning the homo- and co-polymerization of cycloolefins with chromium-based catalysts.[4,5] In this communication, the synthesis of Cr(II) and Cr(III) complexes with pyridine-imine ligands having different substituents on the arylic ring is reported (Fig.1). All the synthesized chromium complexes were fully characterized and then tested, in combination with different Al-alkyls (i.e., MAO, MMAO, Et2AlCl), in the polymerization of various olefins. In the polymerization of 1,3-butadiene the complexes showed from modest to good activities, giving amorphous polymers with essentially mixed cis-1,4/1,2 structure (Tg = -85°C) or trans-1,4/1,2 (Tg = -58°C) in 1:1 ratio, depending on the ligand employed. In contrast, interestingly enough, only the complex Cr-1, in combination with MAO, proved to be active for the ethylene polymerization, giving semicrystalline poly(ethylene) (Tm = 113, 123°C, ?Hm = 210 J/g) even at low Al/Cr ratio. Finally, all the complexes exhibited a rather good activity in the polymerization of norbornene and dicyclopentadiene, giving semicrystalline oligomers through a vinyl-type addition polymerization. In analogy with that previously reported in the literature,[4] the low molecular weight polymers obtained were found to have a stereoregular 2,3-exo-diheterotactic structure. In particular, the 2,3-exo-diheterotactic dicyclopentadiene oligomer is a novel material which was never reported before. References 1.Wenlong L., Antai L., Shaofeng L., Hongqi Y., Zhibo L. Organometallics 2016, 35, 3045. 2.Ricci G., Forni A., Boglia A., Sonzogni M. Organometallics 2004, 23, 3727. 3.Ricci G., Boglia A., Boccia A. C., Zetta L., Famulari A., Meille S. V. Macromolecules 2008, 41, 3109. 4.Woodman T. J., Sarazin Y., Garrat S., Fink G., Bochmann M. J. Mol. Cat A: Chemical 2005, 235, 88. 5.Ricci G., Boglia A., Boccia A. C., Zetta L., Famulari A., Meille S. V. Macromolecules 2008, 41, 3109.