🪢intro to polymer science review
1.3 Overview of polymerization mechanisms
2 min read•Last Updated on July 23, 2024
Polymerization mechanisms are the heart of polymer creation. They determine how monomers link up to form long chains. Understanding these processes is key to controlling polymer properties and designing new materials.
Step-growth and chain-growth are the two main types of polymerization. Each has unique characteristics that affect the final polymer. Free radical polymerization, living polymerization, and various copolymerization reactions offer different ways to create diverse polymer structures.
Polymerization Mechanisms
Step-growth vs chain-growth polymerization
Top images from around the web for Step-growth vs chain-growth polymerization
Polymers and plastics: a chemical introduction View original
Is this image relevant?
Hyperbranched polymers with step-growth chemistries from transfer-dominated branching radical ... View original
Is this image relevant?
Controlling molecular weight and polymer architecture during the Passerini three component step ... View original
Is this image relevant?
Polymers and plastics: a chemical introduction View original
Is this image relevant?
Hyperbranched polymers with step-growth chemistries from transfer-dominated branching radical ... View original
Is this image relevant?
1 of 3
Top images from around the web for Step-growth vs chain-growth polymerization
Polymers and plastics: a chemical introduction View original
Is this image relevant?
Hyperbranched polymers with step-growth chemistries from transfer-dominated branching radical ... View original
Is this image relevant?
Controlling molecular weight and polymer architecture during the Passerini three component step ... View original
Is this image relevant?
Polymers and plastics: a chemical introduction View original
Is this image relevant?
Hyperbranched polymers with step-growth chemistries from transfer-dominated branching radical ... View original
Is this image relevant?
1 of 3
- Step-growth polymerization involves the reaction between bifunctional or multifunctional monomers (diols, diamines) to form dimers, trimers, and longer oligomers, with high molecular weight polymers forming at high monomer conversions (polyesters, polyamides, polyurethanes)
- Chain-growth polymerization occurs with unsaturated monomers containing double or triple bonds (ethylene, styrene) initiated by an active center (free radical, cation, anion), where monomers add to the growing chain one at a time, and high molecular weight polymers form at low monomer conversions (polyethylene, polystyrene, polyvinyl chloride)
Key steps of free radical polymerization
- Initiation: decomposition of an initiator (peroxides, azo compounds) to form free radicals, followed by the addition of the first monomer to the initiator radical
- Propagation: successive addition of monomers to the growing polymer chain, with the active center (unpaired electron) transferred to the end of the growing chain
- Termination: neutralization of the active center through combination (two growing chains combine to form a single polymer molecule) or disproportionation (a hydrogen atom is transferred from one growing chain to another, resulting in two terminated chains)
Concept of living polymerization
- Living polymerization is a type of chain-growth polymerization without termination or chain transfer reactions, where the active center remains active after consuming all monomers
- Allows for the synthesis of block copolymers by sequential addition of different monomers
- Advantages of living polymerization include control over molecular weight and molecular weight distribution, and the ability to synthesize polymers with well-defined architectures (block, star, graft copolymers)
- Examples: anionic polymerization, atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization
Types of copolymerization reactions
- Random copolymerization: two or more monomers are simultaneously polymerized, with the distribution of monomers along the chain being random, and monomer reactivity ratios determining the copolymer composition
- Alternating copolymerization: two monomers alternate along the polymer chain, occurring when the reactivity ratios of both monomers are close to zero
- Block copolymerization: two or more homopolymer segments are connected by covalent bonds, achieved through living polymerization or post-polymerization reactions
- Graft copolymerization: branches of one polymer are attached to the backbone of another polymer, achieved through grafting-from, grafting-onto, or grafting-through approaches