ECNU researchers propose new method for synthesis of cyclic polymers


A research team led by Prof. Xi Meiran of the School of Chemistry and Molecular Engineering has made significant progress in the study of metathesis polymerization.  

The study "Blocking-cyclization technique for precise synthesis of cyclic polymerswith regulated topology" was published in the Nature Communications on Dec. 14. It was completed independently by the ECNU team.

The study is published in the Nature Communications.

Prof. Xie Meiran and Ph.D student Chen Jie.

With extraordinary features such as unique topological properties, surface structures and low hydrodynamic volumes, cyclic polymers have peaked the interests of researchers and brought it to the attention of broad-based academic research.  

Despite previous research findings and extensive reports on the formation of cyclic polymers, the strict synthetic strategies were divided into only two categories: ring-closure technique and ring-expansion technique.  

The ring-closure technique is a traditional route, containing two separatesteps of linear polymer precursor synthesis and its cyclization. This process occurs at extremely low levels of concentration. Many scientists in the field find this method to be highly challenging and less effective.  

Another synthetic route is ring-expansion technique.The in-situ cyclization invariably arises from the cyclic living species, in which two types of cyclic catalysts play a crucial role - but the formidable synthesis challenge of cyclic catalyst make this approach hard to be popularized.

In this latest study, Xie and his co-reseachers proposeda strategy entitled “blocking-cyclization technique”, which is also referred to as the third route to prepare cyclic polymers with regulated ring size and ring number by ring-opening metathesis polymerization of di- and monofunctional monomers in a one-pot process.  

This strategy has broken through the limitations of the previous two cyclizing routes. Itthus opens new pathways to the various cyclic polymers. What the researchers found is thatCommercial Grubbs catalysts and conventional metathesis polymerization make the synthesis facile and popular.  

The“blocking-cyclization technique” is different from the previous two cyclization techniques in that it possesses some distinguishing features: (i) cyclization is realized by conventional ROMP, and the used Grubbs catalyst is commercially available, (ii) using ladderphane as the cyclizing unit to ensure the efficient cyclization and high purity cyclic structure, (iii) a relatively high monomer concentration is permitted, (iv) without the separation and purification treatments of linear polymer precursor, and (v) the ring size and ring number are regulated by a simple process.  

Briefly stated, the prominent feature of this technique is that the cyclization multiply happens in situ on the two ends of polymer intermediate containing living species, and the propagating ladderphane is simultaneously served as the cyclizing unit. Therefore, the developed blocking-cyclization strategy could be considered as the third route, meanwhile acting as a versatile platform to cyclic polymers by commercial Grubbs catalyst and conventional ROMP.

In recent years, Professor Xie has devoted himself to metathesis polymerization methodology and research innovation. Xie revealed the reactive characteristics of ring-opening metathesis polymerization and metathesis cyclization polymerization in imidazolium ionic liquids.

uring the research process, they used advanced technologies to effectively characterize the ring polymer. For instance, the differences in hydrodynamic volumes between annular and linear polymers were compared by gel permeation chromatography.  

Analyzing the visualization of annular polymer morphology is an important basis to judgewhether or not the synthesis was successful. However, in this study, it is was a great challenge to observe the topological structure and morphology of annular polymer monomolecules by using high-resolution transmission electron microscopy. This is because the annular polymer monolayers are entangled by pion-pi stacking between aromatic side groups, making its annular nanotopography difficult to observe.

Moreover, functional studies of cyclic polymers are rare. So in this study, the group explored the responsiveness of the dielectric loss (β-relaxation) of linear and toroidal polymers to gauge its frequency.They found that linear polymers with different molecular weights vary with dielectric loss at low frequencies in which the difference is large and the corresponding ring polymer has a dielectric loss that almost coincides with the frequency change.

The first author of the essay is credited toClass of 2016 Ph.D student Chen Jie, while the correspondent authors are associate professor Sun Ruyi and professor Xie Meiran. The research was funded by the National Natural Science Foundation.

Edited by Siyuan Zhang    Proofread by Joshua Mayfield    Reviewed by Wenjun Guo


East China Normal University