Chemistry Chapter 4: Polymers

Overview of Polymers

• Polymers are macromolecules composed of many repeating units (monomers).
• Monomers are the smallest unit in a polymer chain.
• Polymerization is the process of forming polymers from monomers.

Types of Polymers

• Homopolymers consist of one type of monomer.
• Copolymers are formed from two or more different types of monomers.

Classification of Polymers

Origin

• Natural Polymers: Include proteins and polysaccharides (e.g., agar, alginate).
• Synthetic Polymers: Produced industrially or in labs through chemical reactions (e.g., acrylic resin).

Spatial Configuration

• Linear Polymers: Straight-chain structure.
• Branched Polymers: Side chains extend from the main chain.
• Cross-linked Polymers: Chains are interconnected.

Thermal Behavior

• Thermoplastic Resins: Soften when heated, harden upon cooling (physical change).
• Thermoset Resins: Harden irreversibly through a chemical reaction upon heating.

Polymerization Reactions

• Polymerization is the chemical process that converts monomers into polymers.
• Condensation Polymerization: Involves two molecules combining, releasing a smaller molecule (e.g., water).
• Addition Polymerization (Chain Reaction): Involves the combination of monomers without the release of by-products.

Stages of Addition Polymerization

• Activation and Initiation:

  • Requires free radicals, which are reactive due to unpaired electrons.
  • Initiators must be activated by heat, chemicals, or light.

• Propagation:

  • Process where active monomers grow into a polymer chain.
  • Involves continuous reactions until all monomers are consumed.

• Termination:

  • Chain growth ends through:
    • Direct Coupling: Two active chains form one stable chain.
    • Hydrogen Atom Transfer: Transfers a hydrogen atom between chains to form two stable structures.

Factors Affecting Polymerization

• Release of heat, making the reaction exothermic.
• Shrinkage occurs as volume decreases during polymerization.
• Residual monomers may remain since the reaction never fully completes.

Inhibition and Retardation of Polymerization

• Impurities in monomers can inhibit or slow down polymerization by reacting with free radicals.
• Hydroquinone can inhibit polymerization without an initiator and slow it down in its presence.
• Substances like eugenol or excess oxygen also inhibit polymerization.