5.4 Living polymerization and controlled radical polymerization

Living polymerization is a game-changer in polymer synthesis. It allows for precise control over molecular weight and structure, producing polymers with narrow distributions and unique architectures. This technique opens up new possibilities for creating tailored materials with specific properties.

Controlled radical polymerization takes living polymerization to the next level. It combines the benefits of living systems with the versatility of radical polymerization, enabling the creation of well-defined polymers under milder conditions. This approach has revolutionized polymer science and its applications.

Living Polymerization

Characteristics of living polymerization

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• Proceeds without chain transfer or termination reactions enabling polymerization to continue until all monomers are consumed
• Addition of more monomer after initial polymerization results in continued growth of existing chains
• Produces polymers with narrow molecular weight distribution (low dispersity) due to absence of termination and chain transfer reactions
• Exhibits linear increase in molecular weight with monomer conversion allowing for precise control over final polymer molecular weight
• Enables synthesis of block copolymers by sequential addition of different monomers to the living chain ends
• Allows for control over end-group functionality by using functional initiators or terminating agents (silyl ethers, hydroxyl groups)

Mechanisms of ionic living polymerization

• Anionic living polymerization
  • Initiated by nucleophilic species such as alkyllithium compounds (butyllithium) which add to the monomer to form a carbanion intermediate
  • Propagation occurs through sequential addition of monomers to the carbanion chain end without termination or chain transfer
  • Requires stringent reaction conditions free of moisture and oxygen to prevent termination of the highly reactive carbanion species
• Cationic living polymerization
  • Initiated by electrophilic species such as Lewis acids (titanium tetrachloride) which add to the monomer to form a carbocation intermediate
  • Propagation proceeds through sequential addition of monomers to the carbocation chain end while suppressing termination and chain transfer
  • Achieved by using non-nucleophilic counterions (hexafluorophosphate) and appropriate reaction conditions (low temperatures)
  • Less common than anionic polymerization due to the high reactivity of carbocations and difficulty in controlling the polymerization

Controlled Radical Polymerization (CRP)

Controlled radical polymerization vs conventional

• CRP is a type of living radical polymerization that enables synthesis of well-defined polymers with controlled molecular weight and architecture
• Advantages of CRP over conventional free radical polymerization:
  • Produces polymers with lower dispersity (narrower molecular weight distribution) by minimizing termination and chain transfer reactions
  • Enables synthesis of block copolymers and other complex architectures (star, brush) by sequential monomer addition or post-polymerization modification
  • Allows for control over end-group functionality by using functional initiators or chain transfer agents
  • Tolerates a wider range of functional groups (acids, amines) and reaction conditions (aqueous media, room temperature) compared to ionic living polymerization

Techniques and applications of CRP

• Atom Transfer Radical Polymerization (ATRP)
  1. Utilizes a transition metal complex (copper/ligand) as a reversible activator/deactivator to control the equilibrium between dormant and active species
  2. Applicable to a wide range of monomers (styrenes, acrylates, methacrylates) and functional groups
  3. Produces polymers with well-defined molecular weights and low dispersity ($M_w/M_n < 1.2$)
• Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization
  1. Employs a chain transfer agent (CTA) such as a dithioester or trithiocarbonate to mediate the polymerization through reversible addition-fragmentation with growing radical chains
  2. Versatile technique compatible with a wide range of monomers (vinyl esters, acrylamides) and reaction conditions (bulk, solution, emulsion)
  3. Enables synthesis of polymers with complex architectures (block, star) and end-group functionality
• Nitroxide-Mediated Polymerization (NMP)
  1. Uses stable nitroxide radicals (TEMPO) as reversible terminating agents to control the equilibrium between dormant alkoxyamine and active radical species
  2. Limited to a narrower range of monomers (styrenes, acrylates) compared to ATRP and RAFT due to the stability of the nitroxide radicals
  3. Produces polymers with controlled molecular weights and low dispersity ($M_w/M_n < 1.3$)
• Applications of CRP techniques include:
  • Synthesis of block copolymers (PS-b-PMMA), star polymers, and other complex architectures for self-assembly and phase separation studies
  • Preparation of polymers with reactive end-groups (alkyne, azide) for post-polymerization modification via click chemistry
  • Incorporation of functional monomers (glycidyl methacrylate, 4-vinylpyridine) for targeted applications in biomedical devices, catalysis, and electronics