Cobalt-catalyzed [2 + 2 + 2] cycloaddition copolymerization of diyne and internal alkyne monomers to highly branched polymers

Polymers have drawn considerable research attention owing to their remarkable properties and potential application in a wide range of disciplines. In particular, hyperbranched polymers are attractive for their unique properties arising from their surface and molecular structures. To date, numerous methods for synthesizing branched polymers have been proposed, among them polycondensation of multifunctional monomers. However, it is often difficult to control the condensation of the propagating ends during polycondensation, resulting in undesired products like oligomeric molecules. Notably, research findings have revealed that polycyclotrimerization of diyne monomers D can produce hyperbranched polymers with a high degree of branching without side reactions. Unfortunately, this reaction is highly susceptible to rapid gelation attributed to the crosslinking effects between the initial polymers with terminal alkyne moieties. And it produces insoluble materials that are difficult to process. Therefore, it is recommendable to preciously optimize reaction conditions to obtain soluble polymers to synthesize hyperbranched polymers efficiently.

Previous research findings on cyclotrimerization polymerization revealed promising results in solving the polymer solubility problem. This approach has also been identified as a promising route for effective copolymerization to form various types of branched polymers. However, it is yet to be fully explored mainly due to a lack of selective and effective catalytic systems for crosscycloaddition of internal and terminal alkynes. Motivated by these findings, a group of researchers from Kanagawa University: Ms. Nana Kikuta, Mr. Takahiro Shindo, Dr. Takeshi Yamada and led by Professor Sentaro Okamoto, together with Dr. Yu-Ki Sugiyama from the Anan National Institute of Technology developed a new synthetic method for producing hyperbranched polymers via cobalt-catalyzed [2 + 2 + 2] cycloaddition copolymerization of internal alkynes and diynes. Specifically, the authors aimed to promote crosscycloaddition of terminal and internal alkynes and cyclotrimerization of terminal alkynes. Their research is currently published in the journal, Polymer.

In brief, this was an extension of their previous work that involved developing CoC₁₂⋅6H2O/dipimp/Zn catalyst and its subsequent successful application to a quick and convergent fabrication of different polymers. In this study, a facile catalytic system was adopted to prevent the unwanted crosslinking reaction. This could be achieved by incorporating only the polymerization of internal monoalkyne and diyne D in the resulting polymer and ensuring an increase in the relative monomer concentration to that of the terminal alkyne groups. Additionally, a good choice of the alkyne monomers and monomer ratio could effectively control the degree of branching and molecular weight of the obtained polymers.

Results showed that the catalytic alkyne [2 + 2 + 2] cycloaddition polymerization of aromatic diynes with internal alkynes progressed minus gelation or formation of insoluble products, resulting in high yields of the soluble polymer. Moreover, the reproducible method enabled prediction of the polymerization behavior based on the model reaction results. This particularly allowed for effective control of the degree of branching and molecular weight of the produced polymer, through effective selection of the internal alkyne monomer and its corresponding loading amount. Furthermore, a polymer derived from a diyne containing a silyl ether linker confirmed the presence of effective end-capping, resulting in a high degree of branching above 0.5.

In summary, a facile, reproducible and highly effective polymerization method was developed for synthesizing hyperbranched polymers via catalytic alkyne [2 + 2 + 2] cycloaddition reactions based on cobalt catalyst previously developed by the authors. The approach allowed smooth polymerization without insoluble products or gelation. Besides, it was reproducible and highly effective than most of the previously reported methods. Therefore, the authors noted that the study provided an effective and facile synthesis of hyperbranched polymers ad could be potentially extended to other polymers.