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Novel Synthetic Strategy for Developing an Isospecific Unbridged Metallocene System for Propylene Polymerization
Author(s) -
Kim Seong Kyun,
Kim Hwa Kyu,
Lee Min Hyung,
Yoon Seung Woong,
Do Youngkyu
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700512
Subject(s) - tacticity , cationic polymerization , polymerization , polymer chemistry , monomer , methylaluminoxane , polymer , metallocene , catalysis , coordination polymerization , lewis acids and bases , molar mass distribution , materials science , solvent , chemistry , solution polymerization , organic chemistry
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.