Polymer Chemistry
Polymerization
Building polymers from monomers — addition, condensation, and modern methods
Polymerization is the chemical process linking many small molecules (monomers) into long chains (polymers). Two main types: (1) Addition (chain growth) — radicals/cations/anions/coordination catalysts; monomers add one at a time to growing chain. Examples: polyethylene, polypropylene. (2) Condensation (step growth) — small molecule (often water) eliminated each step. Examples: nylon, polyester, polyurethane. Industrial: ~400 million tons of polymers annually. Plastics, fibers, rubbers, adhesives — modern materials economy depends on polymerization.
- Addition polymerizationChain growth; monomers add one at a time
- Condensation polymerizationStep growth; small molecule eliminated
- PolyethyleneMost-produced plastic; from ethylene
- NylonPolyamide; condensation polymerization
- Polyester (PET)From ester linkages; bottles, fibers
- Industrial scale~400 million tons polymer/year
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Why polymerization matters
- Plastics. Foundation of plastic industry.
- Fibers. Nylon, polyester clothing.
- Rubber. Natural and synthetic.
- Adhesives. Glues, paints.
- Biology. Proteins, nucleic acids are polymers.
- Medical. Sutures, implants, drug delivery.
- Electronics. Conducting polymers, OLED.
Common misconceptions
- Polymers all same. Diverse — properties depend on structure.
- All polymers synthetic. DNA, proteins, cellulose are polymers.
- Plastics non-degradable. Most slow but degrade eventually.
- One method makes all. Many polymerization mechanisms.
- Condensation always gives water. Sometimes other small molecules.
- Polymers are simple. Complex distributions, structures.
Frequently asked questions
What's a polymer?
Long chain molecule made of repeating units (monomers). High molecular weight (MW): 10⁴-10⁶+ Daltons. Examples: polyethylene (PE), polypropylene (PP), polystyrene (PS), nylon, polyester, rubber, DNA, proteins. Properties: depend on monomer + chain length + arrangement (linear, branched, crosslinked).
How does addition polymerization work?
Chain growth. Initiator generates reactive species (radical, cation, anion). Reactive end attacks monomer → new reactive end. Continues. Monomer adds one at a time. Examples: ethylene → polyethylene. Free-radical: most common, includes most commodity plastics. Coordination (Ziegler-Natta): controls stereochemistry of polypropylene.
How does condensation polymerization work?
Step growth. Two functional groups condense, eliminating small molecule (water, methanol). Builds up gradually. Two monomers required (or single monomer with two different groups). Examples: nylon-6,6 from adipic acid + hexamethylenediamine (eliminate water). PET from terephthalic acid + ethylene glycol. Polyurethane from isocyanates + diols.
What's a copolymer?
Polymer with two or more different monomers. Types: (1) Random — irregular sequence (e.g., styrene-butadiene rubber). (2) Block — long sequences of each (e.g., PEG-block-polylactic acid). (3) Graft — main chain of one type with branches of another. (4) Alternating — regular ABAB sequence. Designed properties: strength + flexibility, etc.
What's polymer molecular weight?
Distribution of MW (polymerization gives chains of different lengths). Number average MW: average per molecule. Weight average MW: weighted by mass. Polydispersity index (PDI) = Mw/Mn — measure of distribution width. Higher MW: stronger; less flexible. Application-tuned.
What about modern controlled polymerization?
Living polymerization: chains grow without termination; controlled MW. Anionic living: classic. Modern: ATRP (atom transfer radical), RAFT (reversible addition-fragmentation chain transfer). Allows: precise MW, narrow distributions, complex architectures (block copolymers, brushes). Key for advanced materials.
How do polymers degrade?
Various pathways. (1) Hydrolysis — esters, amides break (slow but real). (2) Photo-oxidation — UV breaks bonds, free radicals. (3) Thermal — high T breaks chains (depolymerization). (4) Biodegradation — microbes degrade some polymers. Most plastics persist for decades-centuries. New: biodegradable polymers (PLA — polylactic acid).