Novel Living Cationic Polymerizations via・pi-Ligand Systems

本研究计划为期两年，主要目的为：（a）以“碳正离子π配位体稳定化”为新概念，开发新型活性阳离子聚合引发体系;（B）用NMR分析了这些活性聚合反应中的生长物种。1. π-配体体系和新引发体系的研究配体体系揭示了适合于苯乙烯和乙烯基醚的活性阳离子聚合的以下三类化合物：（1）抗衡阴离子中的π-配体体系：四苯基硼酸盐和相关的阴离子;（2）添加剂中的π-配体系统：咪唑和相关的氮-亲核试剂;（3）活化剂中的π-配体系统：具有苯氧基配体的卤化钛。初步确定，该方法在该领域首次高效地检测和分析生长中的碳阳离子。分析表明，在生物过程中，π-配体体系能显著降低阳离子浓度，这是由于π-配体与活化剂在分子间（第1类和第2类）或分子内（第3类）发生强相互作用，从而调节它们的刘易斯酸性。这种效应反过来又稳定了增长的碳正离子，从而抑制链转移和其他副反应。因此，本研究建立了“通过π-配体系统稳定碳正离子”的概念，这是实现活性阳离子聚合的一般原理。

This 2-year research project consisted of two major objectives :(a) To develop novel initiating systems for living cationic polymerizations based on the new concept, "carbocation stabilization via pi-ligand systems" ;(b) To analyze the growing species in these living polymerizations by NMR.1.Search of pi-Ligand Systems and New Initiating SystemsAn extensive search of pi-ligand systems revealed the following three classes of compounds to be suitable for living cationic polymerization of stirene and vinyl ethers :(1) pi-Ligand systems in counteranions : tetraphenylborate and related anions ;(2) pi-Ligand systems in additives : imidazole and related nitrogen-nucleophiles ;(3) pi-Ligand systems in activators : titanium halides with phenoxy ligands.2.Direct In-Situ NMR Analysis on Living Cationic PolymerizationModel reactions of these living processes were analyzed, directly and in-situ, by multinuclear NMR spectroscopy. The initial phase established that this method is highly effective to detect and analyze he growing carbocations, for the first time in this field. The following analysis revealed that in the living processes, the cation concentration is remarkably reduced by thepi-ligand systems, which is due to their strong interaction with the activators, either inter- (class 1 and 2) or intramolecularly (class 3) so as to modulate their Lewis acidity. This effect in turn stabilizes the growing carbocations and thereby suppresses chain transfer and other side-reactions.Thus, this research has established the concept "carbocation stabilization via pi-ligand systems" is a general principle to achieve living cationic polymerizations.