21.9 Polyamides and Polyesters: Step-Growth Polymers

Step-growth polymers form through reactions between two different monomers with reactive functional groups. This process creates long chains of molecules like nylons and polyesters, used in everyday products from clothing to water bottles.

These polymers have unique properties that make them useful in various applications. For example, nylon is strong and elastic, while PET is lightweight and wrinkle-resistant. Understanding how these polymers form and behave is key to developing new materials.

Formation and Properties of Step-Growth Polymers

Formation of step-growth polymers

• Step-growth polymerization process involves the reaction between two different monomers each containing two reactive functional groups (difunctional monomers)
  • Common reactive functional groups include amines ($-NH_2$), carboxylic acids ($-COOH$), and alcohols ($-OH$)
  • Examples of difunctional monomers: hexamethylenediamine, adipic acid, ethylene glycol, terephthalic acid
• Polymerization proceeds in a stepwise manner forming dimers, trimers, and eventually long polymer chains
  • Each step forms a new covalent bond between the functional groups of the monomers
  • Average molecular weight of the polymer increases as the reaction progresses
• Polyamides (nylons) form through the reaction between a diamine and a dicarboxylic acid creating amide bonds ($-CONH-$)
  • Example: nylon 6,6 forms from hexamethylenediamine and adipic acid
• Polyesters form through the reaction between a diol (dihydric alcohol) and a dicarboxylic acid creating ester bonds ($-COO-$)
  • Example: polyethylene terephthalate (PET) forms from ethylene glycol and terephthalic acid
• High monomer conversion (>99%) required to achieve high molecular weight polymers in step-growth polymerization
  • Stoichiometric balance between the two monomers crucial for obtaining high molecular weights
  • Example: equal molar amounts of diamine and dicarboxylic acid needed for high molecular weight nylon
• Step-growth polymerization often occurs through a condensation reaction, where small molecules (e.g., water) are eliminated during bond formation

Properties of common step-growth polymers

• Nylon (polyamide)
  • Strong, tough, elastic material with good abrasion resistance
  • Applications: textiles (clothing, carpets), ropes, automotive parts (gears, bearings), packaging (food packaging, fishing lines)
  • Examples: nylon 6,6, nylon 6
• Dacron (polyethylene terephthalate, PET)
  • Lightweight, strong, resistant to wrinkling and shrinking
  • Applications: clothing (shirts, pants), upholstery (furniture, car seats), plastic bottles (beverage containers)
  • Commonly used to produce polyester fibers and fabrics
• Lexan (polycarbonate)
  • Transparent, impact-resistant, heat-resistant thermoplastic
  • Applications: bulletproof windows (security barriers), eyewear lenses (safety glasses), electronic devices (phone cases, computer housings)
  • Exhibits excellent optical clarity and dimensional stability

Polymer Characteristics and Synthesis Methods

• Degree of polymerization influences the physical properties of the resulting polymer
• Molecular weight distribution affects the polymer's mechanical and thermal properties
• Interfacial polymerization is a technique used to synthesize some step-growth polymers at the interface of two immiscible liquids
• Melt polymerization is a method where monomers are heated above their melting point to initiate polymerization

Biodegradable Polymers

Structure of biodegradable polymers

• Biodegradable polymers designed to degrade under specific environmental conditions (exposure to microorganisms, water, sunlight)
  • Degradation occurs through chemical reactions: hydrolysis, enzymatic cleavage
  • Degradation products typically non-toxic and assimilated by the environment
• Polylactic acid (PLA)
  • Derived from renewable resources: corn starch, sugarcane
  • Biodegrades into lactic acid metabolized by microorganisms
  • Applications: medical implants (bone screws), sutures, disposable packaging (food containers, utensils)
• Polyhydroxyalkanoates (PHAs)
  • Produced by bacteria as an energy storage mechanism
  • Biodegradable, biocompatible, with properties similar to conventional plastics
  • Applications: medical (tissue engineering scaffolds, drug delivery systems), packaging (films, containers)
• Benefits of biodegradable polymers
  1. Reduce environmental impact by minimizing waste accumulation
  2. Conserve fossil resources by using renewable feedstocks
  3. Improve safety in medical applications (absorbed or excreted by the body)
  4. Enable the development of sustainable packaging solutions