Eun-Hye Kang

Ultrafast Cyclopolymerization for Polyene Synthesis: Living Polymerization to Dendronized Polymers

We discovered that ultrafast cyclopolymerization of 1,6-heptadiyne derivatives reached completion in 1 min using a third-generation Grubbs catalyst. After optimization, this superior catalyst selectively produced conjugated polymers having a five-membered-ring structure with excellent molecular weight control and narrow polydispersity index (PDI). This living polymerization allowed us to prepare fully conjugated diblock copolymers with narrow PDIs. Lastly, this catalyst was active enough to polymerize macromonomers with G-3 dendrons in a living manner as well. This dendronized polymer with a highly regioregular polymer backbone and bulky dendrons was visualized by atomic force microscopy, which revealed the structure of a single molecular wire surrounded by insulating dendrons.

Strategies to Enhance Cyclopolymerization using Third-Generation Grubbs Catalyst

Cyclopolymerization (CP) of 1,6-heptadiyne derivatives using the Grubbs catalysts has been known to afford conjugated polyenes in low yields in dichloromethane (DCM), the most common solvent for olefin metathesis polymerization and a good solvent for typical conjugated polymers. Based on our previous work that showed highly efficient CP using the Grubbs catalysts in tetrahydrofuran (THF), we developed a new polymerization system using weakly coordinating additives with the third-generation Grubbs catalyst in DCM. The polymerization efficiency of various monomers and their controls dramatically increased by adding 3,5-dichloropyridine, yielding polymers with narrow polydispersity indices (PDIs) at low temperatures. These new reaction conditions not only expand the monomer scope by resolving the solubility concerns of conjugated polymers but also more effectively reduced the chain transfer. Consequently, fully conjugated diblock copolymer was successfully prepared. Additionally, kinetic analysis has revealed that low CP efficiency in DCM resulted from the rapid decomposition of the propagating carbene. This decomposition was effectively suppressed by both pyridine additives and THF, suggesting that weakly coordinating additives stabilize the living chain end. Furthermore, we observed that the turnover number of CP was higher at lower temperatures (0-10 °C) than at ambient temperatures, consistent with the understanding that the lifetime of a propagating carbene is greater at lower temperatures. Steric protection was also shown to increase the stability of the propagating carbene, as shown by a higher turnover number for the 3,3-dimethyl-substituted 1,6-heptadiyne compared to the nonfunctionalized monomer.

Controlled cyclopolymerisation of 1,7-octadiyne derivatives using Grubbs catalyst

Cyclopolymerisation of 1,6-heptadiyne derivatives by Grubbs catalysts gives the conjugated polymers containing a five-membered ring repeat unit. The first example of cyclopolymerisation of 1,7-octadiyne derivatives is demonstrated here to give a new class of conjugated polymer that contains six-memebered rings. This polymerisation was very challenging and efficient cyclopolymerisation was achieved only with monomers that showed a significant Thorpe–Ingold effect. By using a third-generation Grubbs catalyst, we obtained the polymers with excellent molecular weight control and narrow PDIs. This controlled polymerisation allowed the synthesis of diblock copolymers containing five- and six-membered rings, respectively, from 1,6-heptadiyne and the 1,7-octadiyne derivatives.

Highly β-Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z-Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts

It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote exo-cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst.

Versatile Tandem Ring-Opening/Ring-Closing Metathesis Polymerization: Strategies for Successful Polymerization of Challenging Monomers and Their Mechanistic Studies

Tandem ring-opening/ring-closing metathesis (RO/RCM) results in extremely fast living polymerization; however, according to previous reports, only monomers containing certain combinations of cycloalkenes, terminal alkynes, and nitrogen linkers successfully underwent tandem polymerization. After examining the polymerization pathways, we proposed that the relatively slow intramolecular cyclization might lead to competing side reactions such as intermolecular cross metathesis reactions to form inactive propagating species. Thus, we developed two strategies to enhance tandem polymerization efficiency. First, we modified monomer structures to accelerate tandem RO/RCM cyclization by enhancing the Thorpe-Ingold effect. This strategy increased the polymerization rate and suppressed the chain transfer reaction to achieve controlled polymerization, even for challenging syntheses of dendronized polymers. Alternatively, reducing the reaction concentration facilitated tandem polymerization, suggesting that the slow tandem RO/RCM cyclization step was the main reason for the previous failure. To broaden the monomer scope, we used monomers containing internal alkynes and observed that two different polymer units with different ring sizes were produced as a result of nonselective α-addition and β-addition on the internal alkynes. Thorough experiments with various monomers with internal alkynes suggested that steric and electronic effects of the alkyne substituents influenced alkyne addition selectivity and the polymerization reactivity. Further polymerization kinetics studies revealed that the rate-determining step of monomers containing certain internal alkynes was the six-membered cyclization step via β-addition, whereas that for other monomers was the conventional intermolecular propagation step, as observed in other chain-growth polymerizations. This conclusion agrees well with all those polymerization results and thus validates our strategies.