Photoinduced Initiation of Olefin Polymerizations

在化学学部大分子、超分子和纳米化学项目的支持下，田纳西大学诺克斯维尔分校的Brian K. Long和他的团队正在开发一种新的聚合方法，即光引发烯烃聚合。烯烃是由碳和氢组成的小有机分子，其中每个碳都通过传统的化学键和另一个称为pi键的独特键与另一个碳相连。当这个键断裂时，就会产生一个活性分子，它可以与另一个烯烃结合。这个过程可以重复多次，形成长链大分子，通常被称为聚合物或塑料。塑料在许多方面使我们的社会受益。事实上，在过去的70年里，塑料已经帮助航空技术取得了巨大的进步，包括卫星、航天飞机、飞机和导弹的进步。此外，建筑、电子、包装和运输行业都从塑料的使用中受益匪浅。在这项研究中，烯烃的聚合反应将由光、酸和金属结合引发。在这些聚合中使用光是有利的，因为它以非侵入性的方式引起化学键的分裂，并且对环境无害。将进行复杂的研究，以便从机械上了解如何使用这三种组分的组合来控制烯烃的聚合。这些研究预计将提供深入了解塑料是如何生产与控制长度的聚合物链和结构。如果成功，所获得的基础知识将使许多先进应用所需的化学成为可能，例如，基于烯烃的3D可打印聚合物。研究跨越多个学科，包括高分子化学、高分子科学、有机金属化学和工程。因此，它将为参与其中的学生提供一个跨领域的教育机会。研究小组还将努力通过田纳西大学诺克斯维尔SMaRT实习计划，以及阿巴拉契亚学生促进研究与教育整合（ASPIRE）计划，从代表性不足的群体中招募本科生。本研究的重点是利用光酸发生器来激活均相茂金属和后茂金属催化剂，以控制乙烯、丙烯和更高的α-烯烃的聚合。为了实现这一重要目标，第一个目标将集中于二烷基取代锆和铪预催化剂光诱导活化的详细机理和动力学研究。设计的策略将通过用光酸发生器（PAG）取代常用的Bronsted酸活化剂来阻断预催化剂激活途径。这种PAG/预催化剂混合物在没有光线的情况下处于休眠状态，但在照射后会产生烯烃配位插入聚合所需的开放配位位点。第二个目标将应用所产生的知识和原理来控制聚烯烃的弹性和分子量分布。最后，在最后一个目标中，将发展异相光诱导烯烃聚合的引发，重点是使用乙烯或丙烯的气体进料控制表面上聚烯烃膜的形成。如果成功，与这项研究相关的基础研究将扩大基于光的3D打印聚合物类型库，包括聚烯烃。从机理研究中获得的知识将对有机金属化学和催化领域产生影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Brian K. Long and his team at the University of Tennessee-Knoxville are developing mechanistic insight and understanding of a new polymerization methodology called photoinitiated olefin polymerization. Olefins are small organic molecules composed of carbon and hydrogen in which each carbon is bonded to another carbon through a conventional chemical bond, and another unique bond called a pi-bond. When this pi-bond is broken, a reactive molecule is created that can add to another olefin. This process can be repeated many times resulting in the formation of long chain macromolecules, commonly referred to as polymers or plastics. Plastics have benefited our society in numerous ways. In fact, plastics has helped aeronautics technology take giant steps forward over the past 70 years, including advancements in satellites, shuttles, aircraft, and missiles. In addition, the building and construction, electronics, packaging, and transportation industries have all benefited greatly from the use of plastics. In this research, olefin polymerizations will be initiated by light, in combination with acid and a metal species. The use of light in these polymerizations is advantageous because it causes cleavage of chemical bonds in non-invasive ways and is environmentally benign. Sophisticated studies will be conducted in order to mechanistically understand how the combination of these three components can be used to control the polymerization of olefins. These investigations are expected to provide insight into how plastics are produced with controlled length of polymer chains and architectures. If successful, the fundamental knowledge gained will enable the chemistry required for many advanced applications such as, for example, olefin-based 3D printable polymers. The research spans multiple disciplines, including polymer chemistry, polymer science, organometallic chemistry and engineering. Consequently, it will provide a cross-cutting educational opportunity for the students involved. The research team will also strive to recruit undergraduate students from underrepresented groups through the University of Tennessee-Knoxville SMaRT internship program, as well as Appalachian Students Promoting the Integration of Research and Education (ASPIRE) initiative. This research will focus on the use of photoacid generators to activate homogeneous metallocene and post-metallocene catalysts for the controlled polymerization of ethylene, propylene and higher α-olefins. To realize this important goal, the first objective will focus on detailed mechanistic and kinetic studies of photoinduced activation of dialkyl substituted zirconium and hafnium pre-catalysts. The designed strategy will intercept the precatalyst activation pathway by replacing a commonly used Bronsted acid activator with a photoacid generator (PAG). This PAG/ pre-catalyst mixture will be dormant in the absence of light, but upon irradiation will generate the open coordination site needed for the coordination-insertion polymerization of olefins. The second objective will apply the generated knowledge and principles to enable control over polyolefin tacticity and molecular weight distribution. Finally, in the last objective, heterogenous photoinduced initiation of olefin polymerization will be developed with a strong focus on controlling polyolefin film formation on surfaces using a gaseous feed of ethylene or propylene. If successful, the fundamental studies associated with this research will expand the library of light-based, 3D printable polymer types to include polyolefins. The knowledge gained from the mechanistic studies will be impactful to organometallic chemistry and catalysis field in general.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.