Macromolecules are ubiquitous in modern technologies. Cyclic macromolecules have fascinated chemists, biologists and materials scientists for decades. While the properties of linear macromolecules are reasonably well understood, the properties of cyclic macromolecules remain mysterious. Constraining a large macromolecule into a ring has a significant influence its structure, dynamics and properties. Our ability to exploit these novel properties to generate new classes of materials is constrained both by our understanding and our inability to generate high molecular weight cyclic polymers with well defined structures in high purity. An innovative zwitterionic ring-expansion polymerization of lactones was discovered that provides an expedient synthesis of high molecular weight cyclic polyesters of well-defined molecular weight and narrow molecular weight distribution. This new synthetic strategy is fast, scalable and enables the generation of crystalline cyclic polyesters that span molecular weights where chain-entanglements begin to dominate their properties. Our experimental approach combines mechanistic studies of polymerization reactions with physical, rheological and spectroscopic investigations (wide- and small-angle X-ray and neutron scattering) to illuminate the role of molecular topology on macromolecular conformation, dynamics and properties. The gaps in our fundamental understanding of one of the simplest topological isomers of linear macromolecules are specific targets of the proposed studies. The experimental plan is designed to provide new knowledge to fill these gaps and to apply this knowledge for the development of new classes of high-performance thermoplastics, elastomers, and smart rheological fluids. While the focus is cyclic polyesters, the fundamental insights derived should be generalizable to any cyclic macromolecule, highlighting the broader intellectual impacts of the proposed studies
NON-TECHNICAL SUMMARY:
Plastics are ubiquitous modern materials, which impact every facet of our lives. Most plastics are derived from linear polymers, long linear chain macromolecules whose properties are reasonably well-understood. In contrast, cyclic polymers, large macromolecules closed into a ring exhibit unusual properties quite different from their linear analogs, but these differences are not well-understood. Our ability to exploit these novel properties to generate new classes of materials is limited both by our understanding and our inability to generate high molecular weight cyclic polymers with well defined structures in high purity. A new synthetic technique was developed that provides a means of generating cyclic polyesters. This new method will enable the preparation of new classes of well-defined cyclic polymers to investigate their properties and possible applications as new classes of polymeric materials. Studies will address how cyclic polymers crystallize into hard thermoplastics, how they flow when melted, and how the simple fact that their ends are connected into a ring changes their behavior. That so little is known about cyclic polymers implies that our understanding of polymers is far less sophisticated than previously imagined. That is, if connecting the ends of a large linear molecule changes its properties in ways that can't be explained, can it be said that we understand these large molecules? A major impact of the proposed studies will be new scientific understanding on the behavior of macromolecules constrained in rings and the degree to which these new insights challenge and augment our current understanding of macromolecular behavior. As new insights emerge, the novel properties of cyclic polymers can be harnessed to create new families of materials. Collaborative efforts with industrial scientists at IBM, physical scientists at NIST, and chemical engineers at Stanford will provide a unique training environment for the next generation of scientists who are able to make new classes of materials, study their properties and use these insights to generate new classes of polymeric materials.
The objective of this proposal by Professor Robert M. Waymouth of Stanford University was to develop new methods for generating high molecular weight cyclic polymers and to investigate how the properties of cyclic polymers differ from that of linear polymers. This proposal was funded by the Polymers program of NSF's Division of Materials Research (NSF-DMR 1001903). The scientific objectives were closely integrated with training and education. In the course of this project, students were able to acquire the intellectual and experimental expertise to address a broad range of research challenges in polymer chemistry and physics. This expertise included the design and preparation of new organic catalysts, the application of these catalysts to create new cyclic polymers, and the mastery of techniques to characterize the resulting materials. Students were encouraged to develop a holistic scientific approach to devise new experimental and theoretical approaches, both on their own and through collaborative interactions, to address a central question: how does the structure and bonding in macromolecules influence the behavior of materials derived from those molecules? Plastics are our most versatile modern materials; these materials derive many of their useful properties from the manner in which long linear chain molecules entangle, interact, crystallize, and deform. If the ends of a long linear chain are connected into a large cycle, the resulting materials exhibit properties very different from those derived from linear chains, but we don't fully understand why. This project focused on two interrelated intellectual activities: (1) the generation of high molecular weight cyclic molecules, and (2) investigations of the differences in behavior of large cyclic polymers and linear polymers. Generating large cyclic ring molecules is difficult - the ends of a long-chain are typically not close enough to one another to allow the ends to couple into a cycle. We developed a zwitterionic polymerization strategy that creates long chains with oppositely charged ends; the attraction of the two charged chain ends keeps the ends of the chain in close proximity to allow them to cyclize. One significant outcome of the project was the discovery that cyclic polymers crystallize more rapidly than linear polymers. Polymers which crystallize rapidly can be efficiently molded into useful articles; that cyclic polymers crystallize faster is both useful and interesting, as it illuminates how the topology of a long chain molecule (linear vs. cyclic) influences the behavior of the resulting materials. Broader impacts the project include a new technique to generate high molecular weight cyclic polymers that is being adopted worldwide. International collaborations provided students with opportunities to interact with scientists in U.S., Korea, and Greece. Two students received Ph.D.s during the grant period and were recruited and hired by Dow Chemical and LG Chem.
