Development of Living Cationic Polymerization: Novel Methods for Controlled Polymer Synthesis

本研究的目标有三个：(A)建立活性阳离子聚合的一般原理；(B)开发高温活性阳离子聚合的新的引发体系；(C)将活性阳离子聚合的范围扩大到各种单体。(A)活性阳离子聚合的一般原理。活性阳离子聚合一直很困难，因为其中的碳阳离子中间体本身就不稳定。我们的理论是，不稳定增长的碳正离子的“亲核稳定”可能导致活性阳离子聚合。在这一假设的基础上，我们提出了两种稳定碳正离子生长的方法：(A)通过适当的亲核反阴离子稳定；(B)通过添加弱Lewis碱稳定。这两个原理都已被证明是有效的，并因此导致了各种新的引发体系。(B)在添加碱…存在下的高温活性阳离子聚合通常，强酸性金属卤化物(如EtAlCl2)不能诱导活性阳离子聚合，因为它们产生的反阴离子亲核性太弱，不能稳定不断增长的碳正离子。我们的原则(B)被证明对这些体系特别有效；添加弱Lewis碱，如酯、醚和吡啶衍生物，被发现通过亲核相互作用稳定增长的阳离子，从而允许乙烯基醚在高达+70ﾟC的温度下进行活性阳离子聚合。(C)苯乙烯衍生物的活性阳离子聚合。根据我们的原理(A)，利用碘化氢/碘化锌(HI/ZNi2)引发体系，不仅实现了乙烯基醚的活性阳离子聚合，而且还实现了苯乙烯衍生物的活性阳离子聚合。具体地说，即使在室温下，对叔丁氧基苯乙烯也可以得到定义明确的活性聚合物，尽管它带有对酸敏感的烷氧基。聚合物的水解导致了分子量可控的聚(对乙烯基苯酚)。合成了对叔丁氧基苯乙烯与各种乙烯基单体的嵌段共聚物。较少

This research project was directed toward three objectives: (a) to establish general principles of living cationic polymerization; (b) to develop new initiating systems for high-temperature living cationic processes; and (c) to expand the scope of living cationic polymerization to a variety of monomers.(a) General Principles of Living Cationic Polyrarization. Living cationic polymerization has been difficult because of the iherent instability of the carbocationic intermediate therein. We theorized that "nucleophilic stabilization" of the unstable growing carbocation might lead to living pationic polymerization. On the basis of this hypothesis, we have proposed two methods to stabilize the growing carbocations; (A) stabilization by a suitably nucleophilic counteranion; and (B) stabilization by a added weak Lewis base. Both principles have been shown valid and thus led to a variety of new initiating systems.(b) High-Temperature Living Cationic Polymerization in the Presence of Added Base … More s. In general, strongly acidic metal halides (like EtAlCl_2) cannot induce living cationic polymerization, because they generate counteranions too weakly nucleophilic to stabilize the growing carbocations. Our principle (B) turned out specifically effective for these systems; and addition of such weak Lewis bases as esters, ethers, and pyridine derivatives was found to stabilize the growing cation through nuclephilic interaction and thereby allows living cationic polymerization of vinyl ethers at temperatures as high as +70 ﾟC.(c) Living Cationic Polyrarization of Styrene Derivatives. With use of the hydrogen iodide/zinc iodide (HI/ZnI_2) initiating system, which is based on our principle (A), living cationic polymerization has been achieved not only for vinyl ethers but also for styrene derivatives. Specifically, well-defined living polymers were obtained from p-t-butoxystyrene even at room temperature, though it carries an acid-sensitive alkoxyl group. Hydrolysis of the polymer led to poly(p-vinylphenol) of a controlled molecular weight. Block copolymers of p-t-butoxystyrene with various vinyl monomers were also synthesized. Less