Polymers - Classification of Polymers

  • Polymers are large molecules made up of repeating subunits called monomers
  • Classification of polymers can be based on the modes of polymerization
  • There are four main types of polymerization modes:
    • Addition or chain-growth polymerization
    • Condensation or step-growth polymerization
    • Copolymerization
    • Cross-linking

Addition or Chain-Growth Polymerization

  • In this type of polymerization, monomers react to form a polymer chain
  • The reaction occurs through the addition of monomers to an active center, such as a free radical or anionic site
  • Example: Formation of polyethylene from ethylene monomers
  • Equation: CH2=CH2 ⟶ (-CH2-CH2-)n

Condensation or Step-Growth Polymerization

  • In this type of polymerization, monomers react to form a polymer with the elimination of small molecules such as water or alcohol
  • The reaction involves the condensation of monomers to form a polymer chain
  • Example: Formation of nylon-6,6 from adipic acid and hexamethylenediamine
  • Equation: HOOC-(CH2)4-COOH + NH2-(CH2)6-NH2 ⟶ (-OC-(CH2)4-CO-NH-(CH2)6-NH-)n + nH2O

Copolymerization

  • Copolymerization involves the polymerization of two or more different monomers
  • The resulting polymer contains two or more types of repeat units
  • Types of copolymers:
    • Random copolymers
    • Alternating copolymers
    • Block copolymers
    • Graft copolymers

Random Copolymers

  • Random copolymers have randomly distributed monomer units along the polymer chain
  • Example: Styrene-butadiene rubber (SBR) copolymer
  • Random arrangement of monomer units: (-CH2-CH2-CH=CH-CH2-CH2-CH=CH-CH2-)n

Alternating Copolymers

  • Alternating copolymers have alternative arrangement of monomer units along the polymer chain
  • Example: Poly(ethylene-alt-propylene) copolymer
  • Monomer arrangement: (-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-)n

Block Copolymers

  • Block copolymers have long blocks of one monomer followed by long blocks of another monomer
  • Examples: Styrene-butadiene-styrene (SBS) block copolymer
  • Block arrangement: (-CH2-CH2-CH=CH-CH2-CH2-CH=CH-CH2-)n-(-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-)n

Graft Copolymers

  • Graft copolymers have one main chain with side chains of a different monomer
  • Example: Polypropylene-graft-poly(ethylene oxide) graft copolymer
  • Main chain: (-CH2-CH2-CH2-CH2-CH2-CH2-CH-CH3-)n
  • Side chain: (-CH2-CH2-O-CH2-CH2-)n

Cross-Linking

  • Cross-linking involves the formation of covalent bonds between polymer chains
  • The process results in the formation of a three-dimensional network structure
  • Cross-linked polymers exhibit increased strength, stability, and resistance to solvents and heat
  • Examples of cross-linked polymers:
    • Polyethylene cross-linked with peroxides
    • Vulcanized rubber

Summary

  • Polymers can be classified based on the modes of polymerization
  • Addition or chain-growth polymerization involves the addition of monomers to form a polymer chain
  • Condensation or step-growth polymerization involves the condensation of monomers with the elimination of small molecules
  • Copolymerization results in the formation of polymers with two or more types of repeat units
  • Types of copolymers include random, alternating, block, and graft copolymers
  • Cross-linking involves the formation of covalent bonds between polymer chains, leading to a three-dimensional network structure

Addition or Chain-Growth Polymerization

  • Monomers react to form a polymer chain
  • Occurs through the addition of monomers to an active center
  • Active centers can be free radicals or anionic sites
  • Example: Formation of polyethylene from ethylene monomers
    • Equation: CH2=CH2 ⟶ (-CH2-CH2-)n

Condensation or Step-Growth Polymerization

  • Monomers react to form a polymer with the elimination of small molecules
  • Involves the condensation of monomers to form a polymer chain
  • Small molecules like water or alcohol are eliminated
  • Example: Formation of nylon-6,6 from adipic acid and hexamethylenediamine
    • Equation: HOOC-(CH2)4-COOH + NH2-(CH2)6-NH2 ⟶ (-OC-(CH2)4-CO-NH-(CH2)6-NH-)n + nH2O

Copolymerization

  • Polymerization of two or more different monomers
  • Resulting polymer contains two or more types of repeat units
  • Types of copolymers:
    • Random copolymers
    • Alternating copolymers
    • Block copolymers
    • Graft copolymers

Random Copolymers

  • Randomly distributed monomer units along the polymer chain
  • Example: Styrene-butadiene rubber (SBR) copolymer
    • Random arrangement: (-CH2-CH2-CH=CH-CH2-CH2-CH=CH-CH2-)n

Alternating Copolymers

  • Alternative arrangement of monomer units along the polymer chain
  • Example: Poly(ethylene-alt-propylene) copolymer
    • Monomer arrangement: (-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-)n

Block Copolymers

  • Long blocks of one monomer followed by long blocks of another monomer
  • Example: Styrene-butadiene-styrene (SBS) block copolymer
    • Block arrangement: (-CH2-CH2-CH=CH-CH2-CH2-CH=CH-CH2-)n-(-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-)n

Graft Copolymers

  • One main chain with side chains of a different monomer
  • Example: Polypropylene-graft-poly(ethylene oxide) graft copolymer
    • Main chain: (-CH2-CH2-CH2-CH2-CH2-CH2-CH-CH3-)n
    • Side chain: (-CH2-CH2-O-CH2-CH2-)n

Cross-Linking

  • Formation of covalent bonds between polymer chains
  • Results in a three-dimensional network structure
  • Cross-linked polymers exhibit increased strength, stability, and resistance
  • Examples of cross-linked polymers:
    • Polyethylene cross-linked with peroxides
    • Vulcanized rubber

Summary

  • Polymers can be classified based on the modes of polymerization
  • Addition or chain-growth polymerization involves the addition of monomers to form a polymer chain
  • Condensation or step-growth polymerization involves the condensation of monomers with the elimination of small molecules
  • Copolymerization results in the formation of polymers with two or more types of repeat units
  • Types of copolymers include random, alternating, block, and graft copolymers

Summary (continued)

  • Random copolymers have randomly distributed monomer units along the polymer chain
  • Alternating copolymers have alternative arrangement of monomer units along the polymer chain
  • Block copolymers have long blocks of one monomer followed by long blocks of another monomer
  • Graft copolymers have one main chain with side chains of a different monomer
  • Cross-linking involves the formation of covalent bonds between polymer chains, leading to a three-dimensional network structure

21. Types of Polymeric Materials

  • Polymers can be classified into various types based on their physical properties and applications
  • Common types of polymeric materials include:
    • Thermoplastics
    • Thermosetting plastics
    • Elastomers
    • Fibers

22. Thermoplastics

  • Thermoplastics are polymers that can be melted and re-molded multiple times
  • The polymer chains are held together by weak intermolecular forces
  • Examples of thermoplastics:
    • Polyethylene (PE)
    • Polypropylene (PP)
    • Polystyrene (PS)

23. Theromosetting Plastics

  • Thermosetting plastics are polymers that undergo a chemical reaction during curing, which irreversibly hardens the material
  • The polymer chains are linked through strong covalent bonds
  • Once cured, thermosetting plastics cannot be melted or re-molded
  • Examples of thermosetting plastics:
    • Phenolic resins
    • Epoxy resins
    • Polyurethane

24. Elastomers

  • Elastomers are polymers with a high degree of elasticity and flexibility
  • They can undergo large deformations and return to their original shape
  • The polymer chains are cross-linked, providing elasticity
  • Examples of elastomers:
    • Natural rubber
    • Silicone rubber
    • Neoprene

25. Fibers

  • Fibers are polymers that have a long, slender structure
  • They have high tensile strength and are used in textiles and composites
  • Examples of fibers:
    • Polyester
    • Nylon
    • Rayon

26. Polymer Processing Techniques

  • Polymer processing involves converting raw polymers into finished products
  • Common polymer processing techniques include:
    • Extrusion
    • Injection molding
    • Blow molding
    • Compression molding
    • Calendering

27. Extrusion

  • Extrusion involves forcing molten polymer through a die to create a continuous shape
  • Used for producing items such as tubes, pipes, and sheets
  • Example: Production of plastic bags through extrusion of polyethylene

28. Injection Molding

  • Injection molding involves injecting molten polymer into a mold cavity and allowing it to cool and solidify
  • Used for producing complex shapes and high-volume production
  • Example: Production of plastic components for automobiles

29. Blow Molding

  • Blow molding involves inflating a hollow tube of molten polymer in a mold to create a hollow shape
  • Used for producing bottles, containers, and other hollow products
  • Example: Production of plastic bottles for beverages

30. Compression Molding

  • Compression molding involves placing a solid or powdered polymer into a heated mold cavity and applying pressure to shape and cure the material
  • Used for producing parts with intricate shapes and low-volume production
  • Example: Production of electrical switches through compression molding of phenolic resin