Halogen bonding-mediated metal-free controlled cationic polymerization
Chemists at the Nagoya Institute of Technology (NITech) report a metal-free method to control cationic polymerization that provides a new framework for higher quality industrial polymers applicable to semi-conductors and bio-materials. The reaction depends on weak halogen bonding and the addition of a small amount of ammonium salt to produce long, homogeneous polymers. The study can be read in Chemistry - A European Journal.
The polymer is synthesized by cationic polymerization, as an example, through a reaction process that along with the vinyl molecule requires the combination of a catalyst and an initiator called an initiating system.
"Metal halide catalysts are well studied in cationic polymerization, but they create impurities responsible for the coloration and degradation of polymers. There is high demand for metal-free initiating systems," says Associate Professor Koji Takagi, who first-authored the study.
The challenge in cationic polymerization is to produce a long polymer that is homogeneous without adding metal-based catalysts. The role of the catalysts is to abstract an anion from the initiator, such as a halide, that reversibly interacts with the catalyst and the polymer propagating end. For this reason, weak bonding is preferred, as otherwise side effects can occur that compromise the homogeneity and size of produced polymers. In the new study, Takagi and his colleagues show that chloride makes the best halide, as it forms appropriately weak halogen bonds with a 2-iodoimidazolium catalyst.
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