Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Cross-linked polyurethane (PU) is extensively used as thermoset foam; however, methods to directly reprocess PU foam waste derived from commercial sources into similar value materials have not been developed. We demonstrate that introducing dibutyltin dilaurate (DBTDL) into cross-linked PU foams and films enables their reprocessing at elevated temperatures via dynamic carbamate exchange reactions. Both model and commercial cross-linked PU foams were continuously reprocessed using twin-screw extrusion to remove gaseous filler and produce PU filaments or films with elastomeric or rigid thermoset mechanical properties. The properties of microcompounded model PU foam were in excellent agreement with PU film synthesized using the same monomers, indicating that this process occurs efficiently. These findings will enable the bulk reprocessing of commercial thermoset PU waste and inspire the further development of reprocessing methods for other thermosets and the compatibilization of chemically distinct cross-linked materials.

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a–b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

Quantum dot (QD) sensitized photon upconversion follows a multi-step energy transfer process from QD to transmitter ligand to a soluble annihilator. Using a novel 10-R-anthracene-1,8diphosphoric acid (R = octyl, 2-hexyldecyl, phenyl) ligand with high binding affinity for CdSe quantum dot (QD) surfaces, we demonstrate a photon upconversion process that is limited by the transmitter to annihilator transfer efficiency. Using 1 H NMR spectroscopy we demonstrate that these bidentate diphosphate ligands rapidly and irreversibly displace two carboxylate ligands. These ligands mediate energy transfer from the photoexcited QDs to a triplet annihilator (1,10-diphenylanthracene), producing overall photon upconversion quantum efficiencies as high as 17%, the highest for QDs with no shells. Transient absorption spectroscopy shows that the ADP ligand supports a 3.4 fold longer triplet state lifetime compared to 9-ACA (299.9 ± 9.5 vs 88.2 ± 2.1 μs), increasing the probability of the energy transfer.

We report a family of substituted thiocarbonates, thiocarbamates, and thioureas and their reaction with cadmium oleate at 180-240 °C to form zincblende CdS nanocrystals (d = 2.2-5.9 nm). To monitor the kinetics of CdS formation with UV-vis spectroscopy, the size dependence of the extinction coefficient for λ max (1S e-1S 1/2h) is determined. The precursor conversion kinetics span five orders of magnitude depending on the precursor structure (2˚-thioureas > 3˚-thioureas ≥ 2˚-thiocarbamates > 2˚-thiocarbonates > 4˚-thioureas ≥ 3˚-thiocarbamates). The concentration of nanocrystals formed by the nucleation reaction increases with increasing precursor conversion reactivity, allowing the final size to be controlled by the precursor structure. 1 H NMR spectroscopy is used to monitor the reaction of dip -tolyl thiocarbonate and cadmium oleate where dip -tolyl carbonate and oleic anhydride coproducts can be identified. These coproducts further decompose into p-tolyl oleate and p-cresol. The spectral features of CdS nanocrystals produced from thiocarbonates are exceptionally narrow (95-161 meV FWHM) compared to those made from thioureas (137-174 meV FWHM) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others. Additional nanocrystal synthesis and precursor coproduct identification Figures S1-S23 and 1 H, 13 C{ 1 H}, and 19 F{ 1 H} NMR characterization of molecules. (PDF)

By varying precursor structure, core/shell and alloyed nanocrystal synthesis are performed in a single synthetic step.

The heterogeneous growth of inorganic shells on seed nanocrystals is used to synthesize heterostructured nanocrystals such as core@shell quantum dots for applications ranging from biological imaging to solid-state lighting. Control over shelling reactions can be achieved through continuous or layer-by-layer growth methods that are tedious and time-consuming, particularly for the growth of complex, multishell heterostructures. Here, we leverage high-throughput synthesis along with a library of precursors with tunable reactivity to develop a comprehensive understanding of the role of precursor reactivity, ligands, and temperature in one-step, seeded growth reactions on CdSe quantum dots. These experiments reveal a narrow range of precursor reactivity and monomer solubility that fosters the uniform, purely heterogeneous growth of shell material on the seed particles. This narrow "ideal growth" regime in experimental parameter space is sandwiched between opposing regimes that lead to secondary nucleation or ripening during growth. We also report that, at high concentrations of tri-n-octylphosphine, shell growth reactions exhibit "digestive ripening", in which size distributions focus while particles dissolve. Coupled with kinetic simulations, these experiments reveal that the precursor reaction rate and monomer solubility are highly interdependent shell growth parameters that determine the balance between secondary nucleation and ripening. In contrast, the surface energy determines the evolution of the size and polydispersity of the heterostructures over time.