Novel synthetic strategy for developing an isospecific unbridged metallocene system for propylene polymerization.
New unbridged zirconocenes functionalized with a Lewis base, [{1-(E-C(6)H(4))-3,4-Me(2)C(5)H(2)}(2)ZrCl(2)] (E = p-NMe(2) (3); p-OMe (4); p-SMe (5)) were prepared and their propylene polymerization behavior was examined. Under methylaluminoxane (MAO) activation at atmospheric monomer pressure, these complexes afford mixtures of polymers exhibiting multimelting transition temperatures and broad molecular weight distribution, whereas they produce completely atactic polypropylenes under [Ph(3)C][B(C(6)F(5))(4)] activation. Stepwise solvent extraction of the polymer mixtures reveals that the polymers consist of amorphous, moderately isotactic, as well as, highly isotactic portions and the weight ratio of each portion is dependent upon reaction temperature. The generation of rigid rac-like cationic active species in situ by the interaction between basic sites of catalysts and acidic sites of the [Me-MAO](-) counter anion is considered to be the origin of the observed isospecificity. Further investigation of bulk polymerization in liquid propylene shows not only a considerable increase of the isotactic portion of the obtained polypropylenes but also apparent isospecificity of 4 and 5/MAO systems even at high temperature. Variation of the Lewis basic center leads to a dramatic change in stereoselectivity of the catalyst in the decreasing order of 3>4>>5, in spite of their structural similarity.
Department of Chemistry, School of Molecular Science BK-21, Center for Molecular Design and Synthesis, Polyolefin Materials Research Center, College of Natural Science, Korea Advanced Institute of Science and Technology, Daejeon, Korea.
Seong Kyun
SK
Kim
Hwa Kyu
HK
Kim
Min Hyung
MH
Lee
Seung Woong
SW
Yoon
Youngkyu
Y
Do
D000475:Alkenes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003517:Cyclopentanes / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D008970:Molecular Weight; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D011126:Polypropylenes / Q000138:chemical synthesis*; D015040:Zirconium / Q000737:chemistry*
D000475:Alkenes; D003517:Cyclopentanes; D009942:Organometallic Compounds; D011126:Polypropylenes; C013658:propylene; D015040:Zirconium