Authors: Huiying Liu
Source: IDEALS (University of Illinois Urbana-Champaign) (article, 2019-04-15)
Field: Chemistry — Organic Chemistry

Dynamic remodeling and rapid manufacturing of functional materials by ring-opening metathesis polymerization Ring-opening metathesis polymerization (ROMP) is a powerful and broadly applicable method to synthesize polymeric materials with unique architectures and useful functions. Remarkable progress has been made on the molecular design of catalysts and monomers that allows precise control over ROMP. The research presented in this dissertation investigates ROMP behavior of cyclic olefins with different ring nature in the bulk state, which is much less explored in the literature compared to solution ROMP. Structure-property relationships were constructed to advance fundamental understanding and provide guidance for further monomer design. Potential applications of these monomers in dynamic remodeling and rapid manufacturing were also demonstrated.\nAs ROMP is driven by the release of ring strain, a monomer-polymer equilibrium would be established for monomers with low ring strain energy. Chapter 2 investigates the reversibility of equilibrium ROMP and explores the feasibility of depolymerization in the bulk state upon a mild thermal stimulus. We conducted a systematic study on ceiling temperatures (Tc) of substituted cyclopentenes to quantitatively describe the polymerizability of the low-strain monomers both in solution and in the bulk state. This study also identified the important role of anchor group effect in Tc. With the establishment of tunable Tcs, Chapter 3 focuses on the development of thermally reversible networks for remodeling applications by employing multifunctional cyclopentenes. These neat monomers undergo ROMP at room temperature to afford mechanically robust, cross-linked polymers; at slightly elevated temperatures, the resulting polymers readily depolymerize to a free-flowing liquid. This polymerization-depolymerization process, characterized by thermal analysis and rheological tests, is triggered solely by temperature changes and is reversible for several cycles.\nWhen cyclic olefins with large ring strain energy are polymerized, ROMP becomes irreversible and highly exothermic. The heat generated can be utilized as the energy source to trigger further polymerization; ultimately, a propagating reaction wave is produced to convert the available monomer to polymer. This process is termed as frontal ring-opening metathesis polymerization (FROMP) which has great potential in manufacturing large parts of thermosets and composites in a rapid and energy-efficient manner. Since only exo- and endo-dicyclopentadiene have been reported as FROMP monomers, Chapter 4 expands the scope of monomers and builds the structure-property relationship to guide functional materials design. We investigated 30 strained cyclic olefins and correlated FROMP reactivity with the thermodynamic, kinetic and physical properties of the monomer using linear regression analysis. Due to the complexity of FROMP, linear regression did not perform well for structurally disparate monomers. Thus, machine learning approaches were applied with structural parameters of the monomer as inputs and FROMP-related properties (heat released and frontal velocity) as outputs. Models by random forest algorithm with reasonable predictability were constructed, and important features that determine FROMP behavior were also identified. With the expansion of FROMP toolbox, Chapter 5 examines the copolymerization behavior in FROMP. An unexpected non-monotonic increase in frontal velocity was observed in copolymerization with di-norbornenyl cross-linkers, which is counterintuitive to the mixing rules. We believe that the degree of cross-linking is the main contributor to this unusual behavior, which is supported by a series of copolymerization experiments with mono-norbornenyl derivatives. The copolymerization study not only provides a strategy to systematically modify materials properties (such as interfacial shear strength and mechanical properties) but also strengthens further understanding of the FROMP process. Polymerization ; Chemistry ; Materials science ; Polymer ; Composite material Synthetic Organic Chemistry Methods ; Manufacturing Process and Optimization ; Additive Manufacturing and 3D Printing Technologies