Cyclic poly(vinylcarbazole) via ring-expansion polymerization-RAFT (REP-RAFT)

Construction of ring-based architectures via ring-expansion cationic polymerization and post-polymerization modification: design of cyclic initiators from divinyl ether and dicarboxylic acid

Topologically unique polymers such as tadpole and figure-eight polymers were synthesized via ring-expansion cationic polymerization (RECP) of vinyl ether with a functionalized cyclic initiator, followed by post-polymerization modification (PPM) reactions.

Evaluation of Ring Expansion-Controlled Radical Polymerization System by AFM Observation

We here present a direct link between the reaction mechanisms for the ring-expansion "vinyl" polymerization system and atomic force microscopy (AFM) observations. The brush-modification clearly discriminates the desired cyclic species with the contour lengths (L_c) of 28-132 nm and molar masses (M_AFM) of 60.2-283 kg mol^-1 from the other linear ones. The 293 polymer blushes observed in a 1.0 μm × 1.0 μm AFM image are individually characterized, eventually providing clear answers about the mechanisms of this rare polymerization system, which include ring-expansion vinyl polymerizations to generate cyclic polymers, fusions of the generated cycles to form multimers, and their scission to form linear or ring-opened species. The relationship between the molecular chain lengths and the cyclic versus linear morphologies is highlighted.

Ring-Expansion/Contraction Radical Crossover Reactions of Cyclic Alkoxyamines: A Mechanism for Ring Expansion-Controlled Radical Polymerization

Macrocyclic polymers present an important class of macromolecules, displaying the reduced radius of gyration or impossibility to entangle. A rare approach for their synthesis is the ring expansion-controlled radical "vinyl" polymerization, starting from a cyclic alkoxyamine. We here describe ring-expansion radical crossover reactions of cyclic alkoxyamines which run in parallel to chain-propagation reactions in the polymerization system. The radical crossover reactions extensively occurred at 105⁻125 °C, eventually producing high molecular weight polymers with multiple inherent dynamic covalent bonds (NOC bonds). A subsequent ring-contraction radical crossover reaction and the second ring-expansion radical crossover reaction are also described. The major products for the respective three stages were shown to possess cyclic morphologies by the molecular weight profiles and the residual ratios for the NOC bonds (φ in %). In particular, the high φ values ranging from ca. 80% to 98% were achieved for this cyclic alkoxyamine system. This result verifies the high availability of this system as a tool demonstrating the ring-expansion "vinyl" polymerization that allows them to produce macrocyclic polymers via a one-step vinyl polymerization.

Advanced Developments in Cyclic Polymers: Synthesis, Applications, and Perspectives

Due to the topological effect, cyclic polymers demonstrate different and unique physical and biological properties in comparison with linear counterparts having the same molecular-weight range. With advanced synthetic and analytic technologies, cyclic polymers with different topologies, e.g. multicyclic polymers, have been reported and well characterized. For example, various cyclic DNA and related structures, such as cyclic duplexes, have been prepared conveniently by click chemistry. These types of DNA have increased resistance to enzymatic degradation and have high thermodynamic stability, and thus, have potential therapeutic applications. In addition, cyclic polymers have also been used to prepare organic-inorganic hybrids for applications in catalysis, e.g. catalyst supports. Due to developments in synthetic technology, highly pure cyclic polymers could now be produced in large scale. Therefore, we anticipate discovering more applications in the near future. Despite their promise, cyclic polymers are still less explored than linear polymers like polyolefins and polycarbonates, which are widely used in daily life. Some critical issues, including controlling the molecular weight and finding suitable applications, remain big challenges in the cyclic-polymer field. This review briefly summarizes the commonly used synthetic methodologies and focuses more on the attractive functional materials and their biological properties and potential applications.

A trefoil knotted polymer produced through ring expansion

A synthetic strategy is reported for the production of a trefoil knotted polymer from a copper(I)-templated helical knot precursor through ring expansion. The expected changes in the properties of the knotted polymer compared to a linear analogue, for example, reduced hydrodynamic radius and lower intrinsic viscosity, together with an atomic force microscopy (AFM) image of individual molecular knots, confirmed the formation of the resulting trefoil knotted polymer. The strategies employed here could be utilized to enrich the variety of available polymers with new architectures.