CAS: Collaborative Research: Processive Ring-Opening Metathesis Polymerization Through Molecularly Confined Catalysts

在化学系大分子、超分子和纳米化学（MSN）项目和化学催化（CAT）项目的支持下，波士顿学院的牛佳和爱荷华州州立大学的黄文宇正在开发由限制在分子限定笼内的有机金属钌络合物催化的开环易位聚合（ROMP）。 尽管开环易位聚合在生产大量功能聚合物中具有重要意义，但现有技术仍然不能完全防止链转移和终止，特别是当涉及低应变单体时。 结果，形成难以分离的不同尺寸的线性和环状大分子的混合物。 本研究致力于直接解决这一未满足的挑战，并介绍了一个独特的策略进行ROMP。 在这种策略中，聚合催化剂将被封装在分子限定的笼状结构中，使得理想地只有单体而不是新生聚合物可以接近催化剂。 这将由此抑制由催化剂和新生聚合物之间的相互作用引起的链转移和终止，从而产生具有受控分子量的线性大分子。 所开发的方法也将应用于可持续性导向的单体的ROMP，这些单体由于其低应变而被认为对ROMP具有挑战性。这些单体包括具有低上限温度的环烯烃和由可降解官能团组成的那些。 这项研究将为学生提供聚合物合成和可持续性的跨学科培训。 合作团队还将开发一个为期三周的混合夏季研讨会，旨在将聚合物化学原理和实践引入3D打印。该研讨会将与现有的推广计划相结合，以激发波士顿学院和爱荷华州州立大学所服务社区的大学生和高中生对聚合物科学和催化的兴趣。从商品到特种产品的合成聚合物是现代社会的中心。在许多性质中，分子量、序列和分散性在决定聚合物的材料性能方面是必不可少的。因此，对这些性质的精确控制是现代合成聚合物化学的中心目标。 本研究将致力于发展低应变环烯烃的开环易位聚合（ROMP），通过将有机金属钌催化剂限制在分子限定的笼中。 预期这种方法允许接近单体分子，但防止新生聚合物链接近这些受限的催化剂。 结果，可以实现来自ROMP的定义明确的、高的和超高分子量的、低分散性的聚合物。 第一个目标将集中在均匀和异质锆基笼的设计使用可逆的光圈开/关的方法。 然后，这些笼将用于包封基于石墨烯的Grubbs和Hoveyda-Grubbs ROMP催化剂。 第二个目标将利用分子限制的催化剂，使聚合的可持续性为导向的单体含有可降解的功能。 最后，系统研究了分子受限催化剂介导的开环易位聚合动力学行为。与该项目相关的基本指导原则是通用的，并有可能应用于各种其他催化剂介导的链增长聚合技术，其中控制终止和/或链转移事件是可取的。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

With the support of the Macromolecular, Supramolecular and Nanochemistry (MSN) program and the Chemical Catalysis (CAT) program in the Division of Chemistry, Jia Niu of Boston College and Wenyu Huang of Iowa State University are developing ring-opening metathesis polymerization (ROMP) catalyzed by organometallic ruthenium complexes confined within molecularly defined cages. Despite the significance of ROMP in producing a plethora of functional polymers, existing techniques still cannot completely prevent chain transfer and termination, in particular when low-strain monomers are involved. As a result, mixtures of linear and cyclic macromolecules of different sizes are formed that are difficult to separate. This research endeavors to directly address this unmet challenge and introduce a distinct strategy for processive ROMP. In this strategy, polymerization catalysts will be encapsulated in molecularly defined cage structures, such that, ideally only monomers, but not nascent polymers, can access the catalyst. This will thereby inhibit the chain transfer and termination caused by the interaction between the catalyst and the nascent polymer, resulting in linear macromolecules with controlled molecular weights. The developed methodology will also be applied to ROMP of sustainability-oriented monomers that are considered challenging for ROMP due to their low strain. These monomers include cyclic alkenes with low ceiling temperatures and those consisting of degradable functionalities. This research will provide interdisciplinary training for students in polymer synthesis and sustainability. The collaborative team will additionally develop a hybrid three-week summer workshop aimed to introduce polymer chemistry principles and practices into 3D-printing. The workshop will be integrated with existing outreach programs to stimulate interest in polymer science and catalysis among college and high school students in the communities served by Boston College and Iowa State University.Synthetic polymers ranging from commodity products to specialty goods are at the center of modern society. Among many properties, molecular weight, sequence, and dispersity are essential in determining the material performance of polymers. Therefore, the precise control over these properties is the central goal in modern synthetic polymer chemistry. This research will focus on developing processive ring-opening metathesis polymerization (ROMP) of low-strain cyclic alkenes through confining organometallic ruthenium catalysts into molecularly defined cages. Such an approach is expected to allow access to monomer molecules but prevent the nascent polymer chains from accessing these confined catalysts. As a result, well-defined, high, and ultra-high molecular weight, low dispersity polymers from ROMP could be realized. The first objective will focus on design of homogeneous and heterogeneous Zr-based cages using the reversable aperture opening/closing approach. These cages will then be utilized to encapsulate ruthenium-based Grubbs and Hoveyda-Grubbs ROMP catalysts. The second objective will leverage molecularly confined catalysts to enable polymerization of sustainability-oriented monomers containing degradable functionalities. Lastly, kinetic behaviors of ROMP mediated by molecularly confined catalysts will be systematically investigated. The basic guiding principles associated with this project are general and have the potential to be applied to various other catalyst-mediated chain-growth polymerization techniques in which the control of termination and/or chain-transfer events is desirable.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.