"Oscillating" metallocene catalysts: what stops the oscillation?
"Oscillating" metallocene catalysts: what stops the oscillation?
复制标题
“振荡”茂金属催化剂：什么阻止振荡？
DOI：
--
复制
发表时间：
2003
影响因子：
15
通讯作者：
M. Vacatello
中科院分区：
文献类型：
--
作者：
V. Busico;V. van Axel Castelli;Paola Aprea;R. Cipullo;A. Segre;G. Talarico;M. Vacatello
用150 MHz（13）C NMR对聚丙烯样品进行了微观结构分析，所述聚丙烯样品是用两种具有很大空间位阻的代表性“振荡”茂金属催化剂制备的，即[（2-（3，5-二叔丁基-4-甲氧基苯基）茚基）（2）ZrP]（+）和[（2-phenylindenyl）（2）ZrP]（+）（P = polymeryl），并对立体控制的起源进行了详细讨论。原来的机制建议的“振荡”之间的外消旋样（全同立构选择性）和内消旋样（非立体选择性）的构象不能解释所观察到的聚合物的配置。用前一种催化剂获得的聚合物的全同立构-立构嵌段性质明确地证明，活性阳离子在两种对映体外消旋样构象之间以平均频率“振荡”，即使在高丙烯浓度下，该平均频率也仅略低于单体插入的频率。较少受阻的[（2-苯基茚基）（2）ZrP]（+）反而得到大部分立构不规则的聚丙烯，这是更快的配体旋转的逻辑结果;然而，取决于使用条件（特别是助催化剂的性质和溶剂的极性），聚合产物也可能含有可观量的相当全同立构的部分。这部分的特殊的微观结构，具有相同的相对配置的全同立构嵌段由非常短的无规立构的跨越，排除了后者是由于在介孔样构象的活性物种的可能性，而指向一个构象“锁定”外消旋样物种与有限的环流动性。通过对[rac-（2-phenylindenyl）（2）ZrMe（C（3）H（6））][B（C（6）F（5））（4）]离子对的计算机建模，验证了由接近抗衡阴离子诱导的立体刚性的假设，这将在外消旋-双（茚基）柄型双烯中发挥环间桥的作用，并发现是可行的。
The 150 MHz (13)C NMR microstructural analysis of polypropylene samples produced with two representative "oscillating" metallocene catalysts of largely different steric hindrance, namely [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+) (P = polymeryl), and the implications on the origin of the stereocontrol are presented and discussed in detail. The original mechanistic proposal of an "oscillation" between a rac-like (isotactic-selective) and a meso-like (nonstereoselective) conformation cannot explain the observed polymer configuration. The isotactic-stereoblock nature of the polymers obtained with the former catalyst proves unambiguously that the active cation "oscillates" between the two enantiomorphous rac-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less-hindered [(2-phenylindenyl)(2)ZrP](+) gives instead a largely stereoirregular polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by very short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and points rather to a conformationally "locked" rac-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counteranion, which would play the role of the interannular bridge in the rac-bis(indenyl) ansa-metallocenes, was tested by computer modeling on a [rac-(2-phenylindenyl)(2)ZrMe(C(3)H(6))][B(C(6)F(5))(4)] ion couple and found viable.