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Using an organocatalyst to stereocontrol polymerization

 

A pair of researchers at the University of North Carolina has developed a way to use an organocatalyst to stereocontrol polymerization. In their paper published in the journal Science, A. J. Teator and F. A. Leibfarth describe the process and outline some ways the results could be used


stereoselective cationic polymerization 
Designing a stereoselective cationic polymerization. (A) The chiral ligand environment in coordination-insertion polymerizations enables stereoselective monomer enchainment by directing facial addition at the propagating polymer chain end. Polar monomers are typically not compatible with this mechanism because of catalyst poisoning. Me, methyl. (B) The achiral chain end in the cationic polymerizations of vinyl ethers provides no inherent mode for stereoinduction. Monomer addition occurs at either face of the oxocarbenium ion. (C) Our catalyst-controlled approach to stereoselective cationic polymerization relies on a chiral, BINOL-based counterion to bias the stereochemistry of monomer enchainment. The resultant isotactic PVEs are semicrystalline thermoplastics with intrinsic polarity

 

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 Polymers are substances with a molecular structure that is generally made up of like units bound together—they are typically found in resins and plastics. In this new effort, the researchers endeavored to create a polymer that would have more adherence, making it more useful as a coating material. They noted that the two most popular polyolefins, polyethylene and polypropylene, do not adhere well because they are made of hydrogen and carbon—to become adhesive, they need oxygen. Prior research has shown that poly(vinyl ethers) (PVEs) can be made using free radical polymerization, but the results lack stereochemistry, which means they are not very strong or reliable. That led the researchers to devise a new approach that would allow catalyst control when making PVEs, which would give them well-defined stereochemistry using cationic polymerization. Stereochemistry refers to the three-dimensional arrangement of atoms that make up molecules and the impact that the arrangement has on chemical reactions


In their new approach, the researchers used an asymmetric phosphoric acid combined with a Lewis acid (titanium) to force the monomer into a desired orientation during ionic polymerization—the organocatalyst enabled the researchers to exert complete control over the resulting stereochemistry. The polymer they created was semi-crystalline, and testing showed it to be both strong mechanically (comparable to polyethylene) and highly adhesive

 

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Foster and O'Reilly suggest the new approach could lead to the development of both adhesive coatings and medical devices. They also note that the new approach is scalable and amenable to various types of processing such as melting, molding and coloring. They also note that the concept behind the new approach appears to be transferable to other types of polymerization systems, which suggests a whole new class of products could be forthcoming

 

Editor: Nasim Rashidi Nezhad
 

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