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This manuscript is dedicated to Professor Mitsuo Sawamoto's outstanding achievements in polymer chemistry and recognizes his recent retirement from 40 years of exceptional service to Kyoto University.ABSTRACT: Cyclic polymers have drawn considerable interest for their peculiar physical properties in comparison to linear polymers, despite their equivalent compositions. Synthetically, cyclic polymers can be accessed through either macrocyclic ringclosure or by ring-expansion polymerization, but the main challenge with either method is the production of highly pure cyclic polymer samples. This highlight describes advances in the area of cyclic polymer synthesis, with a particular focus on ring-expansion metathesis polymerization. Methods for characterizing cyclic polymers and assessing their purity are also discussed in order to emphasize the need for additional robust and reliable methods for synthesizing and studying topologically complex macromolecules.

The development of nanostructured polymeric systems containing directionally continuous poly(ionic liquid) (poly(IL)) domains has considerable implications toward a range of transport-dependent, energy-based technology applications. The controlled, synthetic integration of poly(IL)s into block copolymer (BCP) architectures provides a promising means to this end, based on their inherent ability to self-assemble into a range of defined, periodic morphologies. In this work, we report the melt-state phase behavior of an imidazolium-containing alkyl−ionic BCP system, derived from the sequential ring-opening metathesis polymerization (ROMP) of imidazolium-and alkyl-substituted norbornene monomer derivatives. A series of 16 BCP samples were synthesized, varying both the relative volume fraction of the poly(norbornene dodecyl ester) block (f DOD = 0.42−0.96) and the overall molecular weights of the block copolymers (M n values from 5000−20 100 g mol −1 ). Through a combination of small-angle X-ray scattering (SAXS) and dynamic rheology, we were able to delineate clear compositional phase boundaries for each of the classic BCP phases, including lamellae (Lam), hexagonally packed cylinders (Hex), and spheres on a body-centered-cubic lattice (S BCC ). Additionally, a liquid-like packing (LLP) of spheres was found for samples located in the extreme asymmetric region of the phase diagram, and a persistent coexistence of Lam and Hex domains was found in lieu of the bicontinuous cubic gyroid phase for samples located at the intersection of Hex and Lam regions. Thermal disordering was opposed even in very low molecular weight samples, detected only when the composition was highly asymmetric (f DOD = 0.96). Annealing experiments on samples exhibiting Lam and Hex coexistence revealed the presence of extremely slow transition kinetics, ultimately selective for one or the other but not the more complex gyroid phase. In fact, no evidence of the bicontinuous network was detected over a 2 month annealing period. The ramifications of these results for transport-dependent applications targeting the use of highly segregated poly(IL)-containing BCP systems are carefully considered.

Cyclic polymers are topologically interesting and envisioned as a lubricant material.However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recycle a used catalyst for the synthesis of cyclic polymers. Unfortunately, it is demanding because of the catalyst's vulnerability and inseparability from polymers, which depreciates the practicality of the process. Here, we develop a continuous process streamlined in a circular way that polymerization, polymer separation, and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. It is enabled by introducing silica-supported ruthenium catalysts and a newlydesigned glassware. Also, different depolymerization kinetics of the cyclic polymer from its linear analogue is discussed. This process minimizes manual labor, maximizes security of vulnerable catalysts, and guarantees purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased reusability of precious catalysts (≥415,000 turnovers).The economy of lubrication is encumbered by a high replacement cost of lubricants in many applications 1,2 . One of its countermeasures is to increase the lifespan of lubricants, thereby decreasing the replacement frequency. The most common synthetic lubricant by far is polyalphaolefin 3-5 , which gradually loses its viscosity due to permanent chain scissions over time 6 . Cyclic hydrocarbon polymers similar to polyalphaolefins or mineral oils (e.g. polyethylene, polypropylene, polybutadiene, etc.) 7,8 are tribologically interesting because the initial chain scission of cyclic topology increases its viscosity by producing an opened linear topology with a higher chain volume [9][10][11] . This feature of cyclic polymers is envisioned as a viscosity modifier to prolong the lubricant lifetime. Since our discovery of the ring expansion route to cyclic polymers in 2002 11 , we [12][13][14][15][16] and other groups [17][18][19][20][21][22][23][24][25][26] have done exciting research on more functionalized and purer cyclic polymers.One of the most important needs is the development of a practical synthetic process to produce pure cyclic polymers on a larger scale for testing in many applications. To date, all the reported synthetic protocols were operated on a milligram scale in solution by homogeneous catalysis, which was accompanied by rigorous ex situ processes for polymer purification without actual catalyst recovery (Fig. 1a) [24][25][26][27] . Nonetheless, the residual metal catalysts remained at a few hundred ppm level 12 ; in this conventional process, it is difficult not only to obtain an uncontaminated white polymer but also to recycle the precious metal catalysts.

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