Synthetic Polymers (Plastics and Fibers)

Synthetic polymers, which transformed organic chemistry after the 20th century, support modern life as plastics, fibers, rubber, and adhesives.

Basic Knowledge

Types of polymerization:
① Addition polymerization (chain polymerization): Alkenes (C=C) react at the center of a chain reaction. No by-product is formed.
Examples: ethylene → polyethylene, vinyl chloride → polyvinyl chloride (PVC), propylene → polypropylene (PP), styrene → polystyrene (PS), tetrafluoroethylene → polytetrafluoroethylene (PTFE, Teflon).
② Condensation polymerization: Water (or a small molecule) is eliminated as bonds form between two types of functional groups.
Examples: terephthalic acid + ethylene glycol → polyester (PET), hexamethylenediamine + adipic acid → nylon 66, ε-caprolactam ring opening → nylon 6.
③ Copolymerization: A mixture of two or more monomers. SBR (styrene + butadiene, synthetic rubber), ABS resin (acrylonitrile + butadiene + styrene).
④ Ring-opening polymerization: A cyclic monomer opens and polymerizes (nylon 6, polyethylene oxide, etc.).

Distinguish between thermoplastics (linear/branched; soften when heated and can be molded; reusable) and thermosetting plastics (three-dimensional networks; cannot be remelted once cured).
Thermoplastics: PE, PP, PVC, PS, nylon, PET, PMMA (acrylic).
Thermosetting plastics: phenolic resin (Bakelite), urea resin, melamine resin, epoxy resin, unsaturated polyester.

Synthetic fibers: nylon (aliphatic polyamide), polyester (PET), acrylic fiber (polyacrylonitrile), vinylon (polyvinyl acetate + formaldehyde treatment), etc.
Regenerated fibers: Natural cellulose is dissolved and re-formed to produce rayon, cupra, and acetate.
Synthetic rubber: Polymerization of isoprene → polyisoprene (the same as natural rubber), butadiene + styrene → SBR (tires), chloroprene → neoprene. Vulcanization (addition of sulfur and cross-linking) improves elasticity.

Further Study (Principles and Applications)

Nylon: Synthesized by Carothers of DuPont in 1935 as a synthetic fiber replacing silk. It was initially used for women's stockings, parachutes, and toothbrushes, and was a strategic material during World War II. Its slogan was “stronger than steel and finer than a spider's web.”
PET: Used for plastic bottles (beverages), polyester fibers (clothing), X-ray film, and automobile parts. In Japan, PET-bottle recycling is a successful example of separate collection and resource recovery.
Bakelite: Synthesized by Baekeland in 1907, it was the world's first fully synthetic resin. Its electrical insulation properties led to its use in telephone bodies and early electronic-device housings.
Functional polymers:
・Ion-exchange resins: -SO₃H (cation exchange) or -N⁺R₃ (anion exchange) groups are introduced into styrene-divinylbenzene copolymers. Used for producing pure water, water purification, and analysis.
・Superabsorbent polymers: Cross-linked polyacrylic acid compounds that absorb hundreds of times their own mass in water. Used in disposable diapers and desert greening.
・Biodegradable plastics: Polylactic acid PLA (derived from corn), PHB, etc. Proposed solutions to the marine-plastic problem.
・Conductive polymers: Polyacetylene, etc. (Hideki Shirakawa's 2000 Nobel Prize); foundations for organic LEDs and organic solar cells.
Environmental problems: Microplastics (plastic fragments 5 mm or less) cause marine pollution, bioaccumulation, and adsorption of toxic substances. Reducing plastics and developing alternative materials are urgent worldwide.

Points of Caution (Common Confusions and Frequent Errors)

① Nylon 6 and nylon 66: Nylon 6 is made from a single caprolactam molecule (C6×1), while nylon 66 is made from two types of monomers (C6+C6). Their properties are similar, but their methods of production, costs, and characteristics differ. ② Vinylon: Polyvinyl acetate is saponified and then treated with formaldehyde for acetal formation; it is a synthetic fiber independently developed by Japanese chemist Ichiro Sakurada. ③ Distinguishing addition and condensation polymerization: If the monomer has C=C, it is addition polymerization; if it has two types of functional groups (COOH+OH, COOH+NH₂), it is condensation polymerization. ④ Polyethylene has low-density (LDPE, highly branched) and high-density (HDPE, linear) forms, which have different properties.

Practice

1. Explain the difference between addition and condensation polymerization using polyethylene and nylon 66 as examples.
2. Write the synthesis equation for nylon 66 and show the structure of its repeating unit.
3. State the structural differences between thermoplastics and thermosetting plastics and give three representative examples of each.