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Cyclic polymers possess different properties compared to their linear analogues of the same molecular weight, such as smaller hydrodynamic volumes and higher glass transition temperatures (T g ). Cyclic poly(4-ethynylanisole) (cPEA) was synthesized via a catalytic ring-expansion of 4-ethynylanisole. The catalyst employed was a tungsten complex supported by a tetraanionic pincer ligand. Evidence of the cyclic topology comes from gel permeation chromatography, dynamic light scattering, static light scattering, and solution viscometry. Demethylation of cPEA with boron tribromide affords cyclic poly(4-ethynylphenol) (cPEP-OH). cPEP-OH exhibits pH-responsive water solubility, being soluble in aqueous solutions at elevated pH and becoming insoluble under acidic conditions. The linear equivalent of cPEP-OH was also synthesized, and it exhibits similar pH responsiveness.

Cyclic polymers possess properties that are significantly different from their linear analogs, such as higher densities, smaller hydrodynamic volumes, and higher glass transition temperatures. Poly­(4-methyl-1-pentene) (PMP), a linear polyolefin, is a commercial transparent thermoplastic and has applications in packaging materials and release membranes. Polymerizing 4-methyl-1-pentyne with a tungsten alkylidyne catalyst and subsequent hydrogenation (>99%) provided cyclic poly­(4-methyl-1-pentene) ( c-PMP). Evidence of a cyclic topology comes from rheology/viscosity studies, light scattering measurements, and size-exclusion chromatography. Importantly, atactic c-PMP exhibits a T g (39 °C) 10 °C higher than the linear analog. A 15 g-scale cyclic polymerization was also achieved with 1-pentyne. Subsequent hydrogenation yielded 10 g of cyclic poly­(1-pentene). Measurements of initial rates during the polymerization of 1-pentyne reveal a catalyst activity of 180,000,000 g/molcat/h.

Polymer bottlebrushes are complex macromolecular nanostructures with polymeric side chains densely grafted to a polymer backbone. In this work, a synthetic strategy for the synthesis of cyclic bottlebrush polymers was exhibited by combining ring-expansion polymerization (REP) and atom transfer radical polymerization (ATRP) by a grafting-from approach. A variety of ultra-high-molecular-weight (on the order of MDa) macrocyclic bottlebrushes were generated by employing this method. Direct visualization of the macrocyclic bottlebrushes was achieved by atomic force microscopy. Furthermore, a linear bottlebrush polymer was synthesized independently by a similar synthetic route to investigate topological differences between cyclic and linear architectures.

Cyclic polyacetylene (c-PA) is the cyclic derivative of the semiconducting linear polyacetylene. As with the linear derivative, cyclic polyacetylene is insoluble, making its characterization and processing challenging. Herein, we report the synthesis of c-PA via an indirect approach, employing ring-expansion metathesis polymerization of cyclic alkenes to form soluble polymer precursors. Subsequent retro-Diels−Alder elimination through heating provides c-PA. Dilute solution characterizations of the polymer precursors including 1 H nuclear magnetic resonance spectroscopy, gel permeation chromatography, and infrared and Raman spectroscopy confirm their cyclic structure and, by inference, the cyclic topology of the resulting c-PA. Solid-state thermal analyses via thermogravimetric analysis and differential scanning calorimetry reveal the chemical and physical transformations occurring during the retro-Diels−Alder elimination step and concurrent isomerization. Freestanding films are attainable via the soluble precursors, and when doped with I 2 , the films are semiconducting.

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Cyclic bottlebrush polymers are macrocycles that contain pendent side-chain polymers on nearly every repeat unit. We leveraged a method to generate cyclic bottlebrush polymers utilizing ring-expansion polymerization (REP) followed by subsequent grafting-from via atom-transfer radical polymerization (ATRP) to produce core−shell cyclic bottlebrush polymers and investigated their self-assembly behavior in water. The bottlebrush polymers were comprised of a cyclic backbone with side chains composed of a hydrophobic polystyrene block and a hydrophilic poly(acrylic acid) block. A two-step morphological transformation from spheres to porous spheres to nanobowls (spheres with a single large pore) was observed with solvent exchange from tetrahydrofuran to water. In contrast, under identical experimental conditions, an analogous linear bottlebrush polymer with a similar backbone and side chain degrees of polymerizations did not aggregate into nanobowls but rather exhibited a porous sphere morphology. These unusual morphologies and their propensities to transform can be attributed to changes in the internal viscosity of the aggregates during the solvent exchange process. Additionally, the rate of solvent exchange was found to influence the propensity for shape transformation to occur.

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