Macromolecules: A Comprehensive Guide to Polymers

Macromolecules

Contents

Polymers, also known as macromolecules, are large molecules with high molecular masses, reaching millions of atomic mass units (amu). They are formed by the repetition of smaller units called monomers, linked together by covalent bonds.

Polymer chains interact with each other through van der Waals forces, hydrogen bonding, or hydrophobic interactions.

Classification of Polymers

Polymers can be classified based on several criteria:

Origin

  • Natural polymers: Rubber, polysaccharides (cellulose, starch), proteins, nucleic acids
  • Synthetic polymers: Plastics, synthetic fibers, polyurethane, bakelite

Composition

  • Homopolymers: Composed of a single type of monomer
  • Copolymers: Composed of two or more different types of monomers

Structure

  • Linear polymers: Monomers are joined in a straight chain
  • Branched polymers: Monomers can be attached at three or more sites, creating branches

Reaction to Heat

  • Thermoplastics: Soften when heated and regain their properties upon cooling, allowing for repeated molding
  • Thermosets: Harden upon heating and cannot be reshaped after cooling

Copolymerization

Copolymerization involves the polymerization of two or more different monomers. Copolymers can be:

  • Alternating
  • Block
  • Random
  • Branched or grafted

Types of Polymerization

Addition Polymerization

In addition polymerization, the polymer’s molecular mass is a multiple of the monomer’s molecular mass, as monomers join without losing any atoms. The process typically involves three steps:

  1. Initiation: A catalyst generates a free radical from a monomer.
  2. Propagation: The free radical reacts with other monomers, extending the chain.
  3. Termination: The chain reaction stops when free radicals react with impurities or combine.

Condensation Polymerization

In condensation polymerization, a small molecule, such as water, is eliminated when monomers join. The polymer’s molecular mass is not an exact multiple of the monomer’s molecular mass. Examples of condensation polymers include:

  • Polyethylene glycol
  • Silicones
  • Bakelite
  • Polyesters
  • Polyamides (e.g., nylon)

Natural Polymers

Examples of natural polymers include:

  • Rubber
  • Polysaccharides (e.g., starch, cellulose, glycogen)
  • Proteins
  • Nucleic acids (DNA and RNA)

Polysaccharides

Polysaccharides are formed by the condensation of glucose molecules. Examples include starch and cellulose, which differ in their three-dimensional structures.

Proteins

Proteins are formed by the condensation of amino acids, resulting in complex structures such as alpha helices and beta sheets.

Nucleic Acids

Nucleic acids, such as DNA and RNA, are formed by the condensation of nucleotides. DNA consists of two strands of deoxyribonucleotides linked by hydrogen bonds between complementary base pairs (adenine with thymine, and guanine with cytosine), forming a double helix structure.