Polymerization of isoprene by a single component lanthanide catalyst precursor
Polymerization of isoprene by a single component lanthanide catalyst precursor
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DOI：
10.1021/ma034385s
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发表时间：
2003-05
期刊：
影响因子：
5.5
通讯作者：
W. Evans;Dimitri Giarikos;N. T. Allen
中科院分区：
文献类型：
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作者：
W. Evans;Dimitri Giarikos;N. T. Allen
由于天然橡胶的主要成分是顺-1，4-聚异戊二烯，因此能够将异戊二烯立体定向转化为无反-1，4-聚异戊二烯或1，2-聚异戊二烯污染的聚合物的催化剂受到高度重视。虽然许多引发剂会聚合丁二烯和异戊二烯，包括元素碱金属和金属烷基，但文献中最好的生成顺式-1，4-聚异戊二烯的催化体系之一是稀土系的，使用的是钕。5-13尽管钕催化剂可以提供98%的顺-1，4-聚异戊二烯以及高顺式-1，4-聚丁二烯，但制备活性催化剂的过程复杂且不太了解。典型的钕催化剂体系的复杂性使得研究、理解和修饰这些有效的催化剂变得困难。钕催化剂通常是从一个三元体系中制备的，该体系包括一个ND(III)盐、一个乙基氯化铝(Et2AlCl2或EtAlCl2)或其他氯化物来源，以及一个异丁基铝化合物(AliBu3或AlHiBu2)。然而，文献中的方案在添加活化剂的顺序以及老化催化剂以获得最佳性能所需的时间和温度方面有所不同。使用先前制备的氯化钕和AliBu3或AlHiBu2的系统也已被报道。9，19我们在这里报道，从既不是元素金属也不是金属烷基的简单的单组分稀土引发剂开始，可以生成高顺式-1，4-聚异戊二烯。首次发现了Tm(II)、20Dy(II)、21和Nd(II)的分子络合物，使这一突破成为可能。22这些新的高度还原的氧化态最近在有机金属和氮素化学方面都取得了进展。我们现在报道它们在聚合化学中的应用。在研究TmI2、DyI2和NdI2的聚合反应活性的过程中，发现这些二价稀土二碘化合物可以在不添加任何添加剂的情况下引发异戊二烯聚合生成高顺-1，4-聚异戊二烯。这一发现引发了对LnI2络合物与异戊二烯反应性的更广泛研究。考察了使用和不使用烷基铝活化剂的聚合反应(方案1)，因为烷基铝化合物通常包括在聚合过程中以去除杂质。聚合是通过两种实验方案中的任何一种进行的。在一种情况下，将稀土引发剂添加到手套盒中的异戊二烯正己烷溶液中。聚合也可以在手套盒外进行，方法是将异戊二烯装入先前装满稀土引发剂的小瓶中。26个反应通常允许进行一夜，之后溶液用0.1%的BHT在异丙醇中淬灭。随后用5%的盐酸溶液洗涤聚合物。让残留的溶剂蒸发，然后在10-3Torr的真空下进一步干燥。用尺寸排阻色谱和~(13)C核磁共振谱对聚合物进行了分析。27聚合反应结果如表1所示。TmI2、28DyI2、21和NdI2 29均在不加任何添加剂的情况下引发异戊二烯的聚合。由于未溶解的二碘化合物似乎不溶于正己烷，这一反应似乎是多相的，分子量或多分散性都不太可能得到优化。TmI2、DyI2和NdI2的棕色、紫色和黑色分别在聚合过程中保持不变，只有在用BHT/异丙醇猝灭时才变为黄色。盐酸洗涤后，聚合物呈白色。令人惊讶的是，威力较小的还原剂Smi2，30也是…
Since the main component of natural rubber is cis-1, 4-polyisoprene, catalysts that can convert isoprene stereospecifically into this polymer without trans-1, 4-polyisoprene or 1, 2-polyisoprene contaminants are highly valued. Although many initiators will polymerize butadiene and isoprene including elemental alkali metals and metal alkyls, 1-4 one of the best catalytic systems in the literature for formation of cis-1, 4-polyisoprene is lanthanide-based and uses neodymium. 5-13 Although neodymium catalysts can provide> 98% cis-1, 4-polyisoprene as well as high cis-1, 4-polybutadienes, the procedure for preparing active catalysts is complicated and not well understood. The complexity of typical neodymium catalyst systems makes it difficult to study, understand, and modify these effective catalysts. Neodymium catalysts are typically prepared from a ternary system involving a Nd (III) salt, an ethylaluminum chloride (Et2AlCl or EtAlCl2) or other chloride sources, and an isobutylaluminum compound (AliBu3 or AlHiBu2). 6, 14-18 However the protocols in the literature vary in terms of the sequence of addition of the activators and the amount of time and temperature needed to age the catalyst for optimum performance. Systems utilizing previously prepared neodymium chloride and AliBu3 or AlHiBu2 have also been reported. 9, 19 We report here that high cis-1, 4-polyisoprene can be generated starting with a simple single component lanthanide initiator which is neither an elemental metal nor a metal alkyl. This breakthrough was made possible by the discovery of the first molecular complexes of Tm (II), 20 Dy (II), 21 and Nd (II). 22 These new highly reducing oxidation states have recently provided advances in both organometallic and dinitrogen chemistry. 23, 24 We now report their utility in polymerization chemistry. In the course of investigating the polymerization reactivity of TmI2, DyI2, and NdI2, it was discovered that these divalent lanthanide diiodides can initiate polymerization of isoprene to high cis-1, 4-polyisoprene without any additives. This discovery triggered a broader examination of the reactivity of LnI2 complexes with isoprene. Polymerizations were examined both with and without alkylaluminum activators (Scheme 1) since alkylaluminum compounds are commonly included in polymerization procedures to remove impurities. Polymerizations were performed by either of two experimental protocols. In one case, the lanthanide initiator was added to a solution of isoprene in hexane in a glovebox. 25 Polymerization could also be done outside a glovebox by syringing isoprene into a vial that had previously been charged with lanthanide initiator. 26 Reactions were typically allowed to run overnight, after which the solutions were quenched with 0.1% BHT in 2-propanol. The polymers were subsequently washed with a 5% HCl solution. Residual solvent was allowed to evaporate, and the polymers were further dried under vacuum at 10-3 Torr. The polymers were analyzed by size exclusion chromatography and 13C NMR spectroscopy. 27The results of the polymerization runs are given in Table 1. TmI2, 28 DyI2, 21 and NdI2 29 all initiate polymerization of isoprene without any additives. Since the unsolvated diiodides appear to be insoluble in hexane, this reaction appears to be heterogeneous, and it is unlikely that either the molecular weights or polydispersities are optimized. The brown, purple, and black colors of TmI2, DyI2, and NdI2, respectively, are maintained during the polymerization and only changed to yellow upon quenching with BHT/2-propanol. After the HCl wash, the polymers are white. Surprisingly, the less powerful reductant, SmI2, 30 also …