Integrating sustainability into high-performance polymers is at the forefront of research efforts to reduce the environmental adverse impact of plastics. Recent approaches have benefited from the reversible chemistry to recycle and degrade materials. Among the promising strategies, Diels–Alder (DA) cycloaddition offers exceptional features, including synthetic ease, absence of catalysts, and the thermally-activated retro-DA (rDA) reaction. The implementation of accessible DA building blocks (e.g., furan and maleimide as the diene-dienophile pair) has provided the synthetic toolbox to fabricate self-healing and degradable thermosets. However, when used as reversible step-growth polymerization, the traditional diene-dienophile or two-component systems generally suffer from the inherent mismatch in stoichiometry, solubility, and/or low rDA temperature (i.e., ~100 °C for furan-maleimide adduct). In comparison, cyclopentadiene (Cp) can serve as a dual role via self-dimerization and thus, holds potential as a solvent-free and single-component platform. Furthermore, combined with the high rDA temperature of the Cp dimer (i.e., 150–180 °C) or Cp-maleimide adduct (i.e., ~150 °C), the application of Cp analogs in polymerization is promising to fabricate advanced functional polymers with enhanced sustainability. Despite its compelling chemistry, Cp is highly reactive with the propensity to isomerize, dimerize and oligomerize, making the monomer synthesis and polymerization notoriously challenging. In fact, Cp polymerization has remained uncontrolled since the first attempt by Staudinger in 1926.
Herein, the successful stories of obtaining well-defined Cp polymer building blocks and controlling the Cp polymerization will be demonstrated, followed by the construction of complex and reversible materials based on the developed methodologies. The first part of the dissertation explores the efficient access to a well-defined tetra-functional Cp monomer under mild conditions via the optimized deprotection of a norbornadiene precursor using a tetrazine reagent. The controlled synthesis of a tough and degradable homopolymer network is subsequently showcased. The second part builds upon the deprotection platform to synthesize and study di-functional Cp derivatives with varied substituent effects. The goal is to understand and suppress uncontrolled cross-linking for the formation of reversible high-MW DA homopolymers. The utilization of a radical inhibitor, butylated hydroxytoluene, was discovered to prevent cross-linking, leading to the first general Cp homopolymerization. Notably, a Cp thermoplastic with the backbone derived from hydrogenated polybutadiene can be thermally recycled (i.e., Mn = 68 to 23 kDa and back to 68 kDa) three times without solvents and additional purification steps. The last three stories were developed by utilizing the well-defined Cp derivatives to fabricate degradable thermoplastic polyurethanes, a Cp-maleimide self-healing thermoset, and the stereo-chemical homopolymer networks. Through the development of controlled Cp polymerization, the expansion of DA chemical toolbox can be realized to fabricate a variety of promising materials with optimized mechanical performance and sustainability.
