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The fantastic journey of chain growth and aggregation: How to eliminate the termination reaction?
In the field of polymer chemistry, living polymerization is a special form of chain growth polymerization. A characteristic feature of this type of polymerization is the elimination of the ability to terminate the polymer chain. This process can be achieved in a variety of ways, ultimately allowing the polymer chains to grow at a more stable rate and maintain similar lengths, displaying a low polydispersity index. This polymerization method excels in the synthesis of block copolymers because different monomers can be used at each synthesis stage, providing predictable molecular weight and control of end groups.
Living polymerization makes it possible to achieve precision and control in macroscopic synthesis, which is critical to the properties of many new polymers.
Many novel polymer properties come from their microstructure and molecular weight, and living polymerization is an important method to achieve these properties. Although there is some degree of controversy over the definitions between living polymerization and controlled polymerization, the similarities between the two are unquestionable. Understanding the key technologies of this cycle is therefore extremely important for materials design.
The history of living polymerization
The concept of living polymerization was first demonstrated in 1956 by Michael Szwarc, who used a system of sodium naphthalene in tetrahydrofuran (THF) for the anionic polymerization of styrene. In Swartz's experiment, the flipped bond reaction was accomplished through electron transfer. The results opened a new chapter in polymer chemistry, giving scientists a deeper understanding of the ability to control polymer binding and termination.
This work has greatly expanded the application potential of polymers, and controlling the end time of polymers has expanded the application scenarios of the technology.
Living polymerization technology is now widely used in the production of many types of polymers and plastics. For example, poly(phthalate), first developed in 1967, can be synthesized through a variety of living polymerization methods, thereby controlling the chemical composition of the polymer and further affecting its structure and electronic properties.
Main characteristics of living polymerization
The key feature of living polymerization is the elimination of chain termination and transfer reactions. In other words, polymerization occurs faster than chain growth, and each polymer chain grows at the same rate, allowing precise control of the molecular weight of the polymer.
Due to the high start-up speed during the polymerization process, living polymerization can ensure a low polydispersity index (PDI), making the polymer performance more stable.
In traditional polymerization processes, chain termination is often the biggest obstacle to controlling molecular weight, while the development of living polymerization has greatly improved the flexibility of polymer design.
Main living polymerization technologies
Living polymerization technology includes anionic polymerization, cationic polymerization, ring-opening polymerization, free radical polymerization, etc. Each of these methods has its own characteristics and plays an important role in different applications.
1. Living anionic polymerization
Living anionic polymerization methods are favored, especially for the synthesis of polystyrene and butadiene. Swartz's early work utilized anions as polymerization initiators, pioneering this field.
2. Living cationic polymerization
This method works for electron-rich alkenes such as vinyl ethers and styrene. By introducing a Lewis acid coinitiator, stable cations are maintained for a long time, allowing living polymerization to occur. Although these polymers are not technically "living," they are similar in functional properties to true living polymers.
3. Living ring-opening polymerization
The ring-opening polymerization of cyclic olefins can become "living" under certain conditions. This process produces polymers with double bonds, further ensuring that the composition and properties of the polymer can be controlled.
4. Living radical polymerization
This field includes a variety of latest technologies, such as atom transfer radical polymerization (ATRP), whose principle is to create a type of dynamic equilibrium to keep free radicals "alive". This method is widely used in industry.
In short, the technological progress of living polymerization has significantly expanded the design concepts and application scope of polymers, providing new explicit and implicit properties of materials. With the deepening of research, the understanding of polymer applications and their internal structures will be further deepened. How will it affect material design in the future?
