Stereospecific styrene enchainment at a titanium site within a helical ligand framework:: Evidence for the formation of homochiral polystyrene
Stereospecific styrene enchainment at a titanium site within a helical ligand framework:: Evidence for the formation of homochiral polystyrene
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DOI：
10.1002/anie.200700783
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Okuda, Jun
中科院分区：
文献类型：
--
作者：
Beckerle, Klaus;Manivannan, Ramanujachary;Okuda, Jun
隐手性现象使得全同立构聚（α-烯烃）如聚丙烯是非光学活性的，因为对于由前手性烯烃形成的高分子量聚合物可以假定Cs对称性。[1，2]只有对于聚（α-烯烃）链内的高度特异性的链段序列，才能测量光学活性。[3，4]具有明确分子结构的手性（在罕见情况下为光学活性）对映体纯柄型二茂锆催化剂的出现[5]允许在低聚过程中观察到前手性单体在单个金属中心的立体化学区分。[2，6，7]据我们所知，尚未确定聚（α-烯烃）的可观察到的光学活性在何种聚合度下消失，以及“纯手性”全同立构聚-α-烯烃在何种聚合度下可观察到的光学活性是否消失。我们最近已经表明，构型刚性双衍生自线性[OSSO]型配体的（酚基）钛催化剂有效地使苯乙烯聚合得到全同立构聚苯乙烯（iPS）。[8]iPS先前使用非均相齐格勒型催化剂制备，并由Natta等人表征为第一可结晶聚（α-烯烃）。[9]我们现在已经制备了上述的光学活性变体-
The phenomenon of cryptochirality renders isotactic poly (aolefin) s such as polypropylene optically inactive, since Cs 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 above-