Styrene is polymerized isospecifically by group 4 metal complexes that contain a C2-symmetrically coordinated 1,4-dithiabutane-linked bis(phenolato) ligand activated by methylaluminoxane.
The phenomenon of cryptochirality renders isotactic poly(aolefin)s such as polypropylene optically inactive, since C s symmetry can be assumed for high-molecular-weight polymers formed from prochiral olefins. [1,2] Only for highly specific sequences of segments within the poly(a-olefin) chain can optical activity be measured. [3,4] The advent of chiral, and in rare cases optically active, enantiomerically pure ansa-zirconocene catalysts with well-defined molecular structure [5] allowed the observation of the stereochemical discrimination of the prochiral monomer at a single metal center during oligomerization. [2,6, 7] To the best of our knowledge, it has not yet been determined at what degree of polymerization the observable optical activity of poly(aolefin)s disappears and whether "homochiral" isotactic poly-(a-olefin)s can be accessed.We have recently shown that configurationally rigid bis(phenolato) titanium catalysts derived from linear [OSSO]-type ligands efficiently polymerize styrene to give isotactic polystyrene (iPS).[8] iPS was previously prepared using heterogeneous Ziegler-type catalysts and characterized as the first crystallizable poly(a-olefin) by Natta et al. [9] We have now prepared optically active variants of the abovementioned catalyst precursors. By utilizing chain-transfer methodology in the presence of 1-hexene, we demonstrate that the insertion of styrene in such postmetallocene catalysts [10] occurs stereospecifically giving optically active iPS oligomers.As we reported previously, the stereorigidity of the titanium catalysts containing a [OSSO]-type ligand depend rather critically on the presence of a two-carbon backbone linking the two phenol units and on the presence of bulky ortho substituents.[11] Therefore, we introduced a trans-1,2-cyclohexanediyl backbone that connects two 4,6-di-tert-butylphenol (a) or 6-tert-butyl-4-methylphenol (b) groups by a SCCS link. We followed the reaction sequence delineated in Scheme 1 and obtained the bis(phenol) rac-1 a in three steps. Starting with cyclohexene oxide, consecutive nucleophilic Scheme 1. Synthesis of the chiral catalyst precursors 2 a.
Copolymerization of ethylene with styrene, catalyzed by 1,4-dithiabutanediyl-linked bis(phenolato) titanium complex and methylaluminoxane, produced exclusively ethylene-styrene copolymers with high activity. Copolymerization parameters were calculated to be r(E) = 1.2 for ethylene and r(S) = 0.031 for styrene, with r(E) r(S) = 0.037 indicating preference for alternating copolymerization. The copolymer microstructure can be varied by changing the ratio between the monomers in the copolymerization feed, affording copolymers with styrene content up to 68%. The copolymer microstructure was fully elucidated by C-13 NMR spectroscopy revealing, in the copolymers with styrene content higher than 50%, the presence of long styrene-styrene homosequences, occasionally interrupted by isolated ethylene units. (c) 2006 Wiley Periodicals, Inc
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