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Polymerization adventure from scratch: How does living cationic polymerization change the future of polymer science?
Living cationic polymerization is a cation-based polymerization technology that can synthesize polymers with very well-defined structures and has attracted strong interest in both business and academia. The greatest advantage of living cationic polymerization is that it allows the synthesis of polymers with low molecular weight distribution as well as unusual polymer structures such as star polymers and block copolymers.
Living cationic polymerization is characterized by a clear and controllable initiation and polymerization process that minimizes side reactions and chain termination.
In this polymerization process, the main reaction steps can be divided into several stages, in which the active site is the close contact between carbene cations and anions. The process is divided into steps such as chain extension, termination and chain transfer. In an ideal living cationic polymerization system, the active cations undergoing polymerization and the dormant covalent species are in chemical equilibrium, and their exchange rate is much faster than the polymerization rate.
In addition, the monomer range of living cationic polymerization is very wide. Common monomers include vinyl ether, α-methyl vinyl ether and styrene. These monomers must have substituents that stabilize the charge of the n-carbene cation.
For example, p-methoxystyrene is more reactive than styrene. It is also worth noting that the combined effect of hydroxide and Lewis acid is crucial in this entire process.
This technology has been developed since the 1970s and 1980s, driven mainly by several important chemists. They studied different aspects of living cationic polymerization, such as the stabilization of carbene cations in the polymer and the use of effective starters. Interestingly, these studies opened the way to rapidly developing macroscopic molecular design.
Challenges of isobutylene polymerization
For the polymerization of isobutylene, it is usually carried out in mixed solvent systems, which include non-polar solvents (such as hexane) and polar solvents (such as chloroform or dichloromethane), and the reaction temperature needs to be maintained at 0°C the following. As the polar solvent increases, the solubility of polyisobutylene becomes very difficult.
In this system, starters can be alcohols, halogens and ethers, while co-starters include boron chloride and organoaluminum halides. The activity of these compounds promotes polymerization in a stable manner, which is undoubtedly instructive in today's polymer science.
The polymer of this system can reach a molecular weight of 160,000 g/mole and has a polydispersity index of only 1.02, demonstrating its superior control capabilities.
Progress in vinyl ether polymerization
Vinyl ether, a very reactive vinyl monomer, is often used as the basis for living cationic polymerization. Studies have shown that these systems rely on iodine and hydrogen iodide as well as zinc halides as catalysts to promote polymerization reactions.
Living cationic ring-opening polymerization
In living cationic ring-opening polymerization, the monomer is usually a heterocyclic ring, and epoxides, tetrahydrofuran, etc. are suitable for such polymerization. The challenge is that the ends of the active polymers are susceptible to nucleophilic attack, resulting in cyclic oligomers that halt polymerization.
The initiator for this type of polymerization needs to have strong electrophilic properties, such as trifluoroacetic acid, which can effectively start the polymerization reaction.
Future exploration
The continued development of living cationic polymerization makes the application potential of polymer science more obvious. In the context of green chemistry, this technology is expected to find further applications in the production of sustainable materials. By understanding all the details of this process, scientists have the opportunity to design more efficient and environmentally friendly polymerization reactions.
Because of this, living cationic polymerization not only leads the revolution in modern polymer science, but also paves the way for the development of new materials in the future. The advancement of science and technology is full of infinite possibilities. Can we create unprecedented materials through living cationic polymerization?
