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Polymers with low ceiling temperatures ( T c ) are highly desirable as they can depolymerize under mild conditions, but they typically suffer from demanding synthetic conditions and poor stability. We envision that this challenge can be addressed by developing high- T c polymers that can be converted into low- T c polymers on demand. Here, we demonstrate the mechanochemical generation of a low- T c polymer, poly(2,5-dihydrofuran) (PDHF), from an unsaturated polyether that contains cyclobutane-fused THF in each repeat unit. Upon mechanically induced cycloreversion of cyclobutane, each repeat unit generates three repeat units of PDHF. The resulting PDHF completely depolymerizes into 2,5-dihydrofuran in the presence of a ruthenium catalyst. The mechanochemical generation of the otherwise difficult-to-synthesize PDHF highlights the power of polymer mechanochemistry in accessing elusive structures. The concept of mechanochemically regulating the T c of polymers can be applied to develop next-generation sustainable plastics. Polymers with low ceiling temperatures (Tc) are highly desirable as they can depolymerize under mild conditions, but they typically suffer from demanding synthetic conditions and poor stability. Here, the authors envision that this challenge can be addressed by developing high-Tc polymers that can be converted into low-Tc polymers on demand.

Helicenes are polycyclic aromatic compounds with a helicoidal architecture that derive their properties from the molecule?s extensive electron delocalization, structural deformability, and spatial chirality. Introduction of helicenes into a polymer matrix may manifest properties such as high service temperature, optical rotation, mechano-optical coupling, and electromechanical coupling when used as an additive or polymer building block. Despite the potential for helicenes to be disruptive to emergent soft material technologies, the production scale for broad utility of this molecule suffers from several major drawbacks which bottleneck the effective translation from lab to engineering scale. These include dilute reaction conditions which limit quantitative yields, difficulty in separating isomeric side products, as well as the multi-step synthesis of helicene precursors. Helicene synthesis is traditionally a statistical photochemical reaction that generates the compound as one of potentially several regio-isomers, commonly at low yield. The goal of this research is to develop a cost-effective and time-efficient approach for scalable synthesis of helicenes before subsequent utilization as polymer building blocks. A number of steps were taken to overcome or circumvent these synthetics challenges and address the limitations to scaling. Critically, a more complete understanding of the photocyclodehydrogenation reaction which forms helicenes and the unique selectivity towards helicene products afforded by helicene precursor design and reaction conditions was achieved. This resulted in high-yielding synthesis of functionalized helicenes which have great potential for application in engineering-scale endeavours. Chapter I provides an introductory review of the recent and historical advances made in helicene synthesis both photochemical and otherwise. Chapter II presents a study of the isomerization and cyclization of stilbenes upon UV irradiation and the effect that concentration and oxidizing agent have on these processes. This provided critical insight into how undesired intermolecular [2+2] cycloaddition can be avoided and lead to the development of synthetic conditions for gram-scale synthesis of functionalized phenanthrenes and [4]helicenes. Chapter III presents a systematic study of helicene precursor design and reaction conditions. The results of this study give key insights into helicene precursor behaviour, elucidate mechanistic pathways for helicene formation, and provide a better understanding of preferential helicene formation via photocyclodehrydrogenation. Chapter IV describes the synthesis of helicene-based and helicene precursor-based polyesters. These systems were developed to assess the viability of cyclization of helicene precursor moieties to form helicenes under chain-confined conditions. Chapter V summarizes this work and outlines several potential future studies which may be undertaken to expand and improve upon the results presented in this manuscript.