Research

Regulation of polymer topological structure

The regulation of polymer topological structure is an important research direction in polymer synthesis chemistry. The common topological structures of polymers include linear structure, branched structure, hyperbranched structure, star structure and ring structure. Polymers have a variety of topological structures depending on the position of the polymer chains, for example, linear, comb, ring, hyperbranched and so on. The properties of polymers with the same chemical composition depend largely on their topological structure. By changing the topology of the polymer can be obtained different properties of polymers, so finding a simple and easy method to control the polymer topology has been a hot research topic to the polymer scientists for a long time.

Among the polymers with various structures, the linear polymer is a more transparent one, and its synthesis method is simple. Because of special structure and a spherical annular branched structure, and surface characteristics of multi functional groups, cyclic polymers and hyperbranched polymers have some unique chemical and physical properties. However, the vast majority of cyclic polymer and hyperbranched polymer synthesis methods are more complex and thus difficult to prepare, which restricts the application of cyclic polymers and hyperbranched polymers.

Recently, Professor Yuzhou Liu’s research group from the School of Chemistry of Beihang University reported their work utilizing Piers-Rubinsztajn reaction, for the first time, to form a series of cyclic polymers from simple organic silanes, as evidenced by various analysis techniques. Meanwhile,their work for the first time demonstrated the feasibility of using cyclic polymer to direct the assembly of inorganic particles, and also presented the first stable circular gold nano particle assembly which is soluble in organic solvents.

In recent years, with the continuous research and development in the field of polymer synthesis, the incorporation of constrained cyclotetrasiloxane into polymers has been proven to be key for a wide range of functional materials, including thin-film electrolyte, self-healing, thermal stable materials, high oxygen-permeable films, spreading reagents, liquid crystals and so on. However the combination between the constrained siloxane rings with cyclic polymers hasn’t been investigated due to the lack of efficient way of constructing cyclotetrasiloxane ring containing cyclic polymers, and therefore the use of cyclic polymers in these areas is limited.

Professor Yuzhou Liu’s team reported for the first time the utilization of Piers-Rubinsztajn reaction for the one-step synthesis of cyclic polysiloxanes with novel structural features. Specifically the B(C6F5)3 catalyzed coupling reaction between various organic tris(dimethylsiloxy)silane and trialkoxysilane compounds generates a series of cyclic polysiloxanes with cyclotetrasiloxane subunits. The thiolated cyclic polymer is also shown effective in directing the circular assembly gold nano particles. The presence of constrained rings on the backbone is unprecedented and may bring opportunities for novel applications of these cyclic polymers.

 

 

 

Scheme 1. (A) The cyclotetrasiloxane formation between dihydrosilane and dialkoxysilane compounds. (B) The proposed cyclotetrasiloxane subunits formation based on reaction (A) and then their intermolecular reaction to form cyclic polycyclotetrasiloxane polymers.

 

 

 

Fig 1. (A~C) AFM height images of multiple particles of 1 at different scales showing the uniform distribution of cyclic particles; insert in C: profile analysis of a single cyclic particle shown in the green line; a higher resolution of image A is provided in supporting information as Fig. S34 [M1] showing all molecules in the image as circular. (D) FESEM images of 1 on the carbon surface (samples were coated with gold before measurement).

 

 

 

Fig 2. TEM images of circular gold clusters coated on the surface of 14 (the insert shows the vial containing the purple CH2Cl2 solution of the gold nano particle assembly).

 

At the same time, Professor Yuzhou Liu and his team successfully synthesized a hyperbranched polymer with the in 6-memebered cyclotrisiloxane constrain ring (HBP-1) from simple organic silane monomer by using Piers-Rubinsztajn reaction.The preparation of hyperbranched polymer is appealing but is hindered by the lack of control of intramolecular cyclization, which usually leads to the formation of inert loops. The team succeeded in deliberately directing the intramolecular cyclization into the formation of highly constrained and thus reactive rings, which were then used for arm attachment. In this way, simple and efficient synthesis of a hyperbranched bi-block polymer (PSt-HBP-1) and tri-block polymer with incompatible arms is realized (JHBP-1) and its potential application in self-assembly is demonstrated with the formation of interesting giant toroid structures.

 

 

 

Scheme 2. The sequential polymeric chain formation and cyclization led [M2] to cyclotrisiloxane-terminated hyperbranched polymer 1 (HBP-1).

 

 

 

Fig 3. Comparison between different ways of making Janus-type branched polymers

 

 

 

Scheme 3. The preparation of JHBP-1 by ring opening reactions of the cyclotrisiloxane rings on HBP-1

Fig 4. (A-C) The AFM imaging of the self-assembled PSt-HBP-1 on the surface of silicon spin-coated with the chloroform solution of PSt-HBP-1. Image A is from the low concentration sample and images B and C are from high concentration samples. (D-F) The AFM imaging of the self-assembled JHBP-1 on the surface of silicon spin-coated with the chloroform solution of JHBP-1. Image D is from the low concentration sample and images E and F are from high concentration samples.

 

 

Yuzhou Liu, professor, school of chemistry, Beihang University, E-mail: liuyuzhou@buaa.edu.cn

 

References

[1]Jianyi Yu, Yuzhou Liu*. Angewandte Chemie International Edition. 2017, 56, 8706 –8710. DOI: 10.1002/anie.201703347.

[2]Chunyan Wua, Chunhua Hub, and Yuzhou Liu*a. Polymer Chemistry.2017, Advance Article. DOI: 10.1039/c7py01177f.