polymers

• Polymers


  Large molecular compounds made up of many repeating units called monomers
• Classification of polymers


  • Synthetic
  • Natural
• Polymerization


  Chemical reaction by which the monomers are linked together to form polymers
• Types of polymerization reactions

  • Addition polymerization
  • Condensation polymerization
• Addition polymerization


  1. Initiation

  2. Propagation

  3. Termination
• Initiation


  Initiator molecule is heated to produce free radicals, free radicals react with the monomer breaking the double bond and forming a new radical
• Propagation


  Radical will react with another monomer, repeated many times to form a long chain
• Termination


  Process gets terminated when two long-chained radicals combine to form the polymer
• PEX (cross-linked polyethylene)


  • Stronger polymer by introducing cross-links or molecular bridges between the polymer chains
• High & low density polyethylene

  Strength of polyethylene can be varied by changing the size of the molecules, long chains make polyethylene more viscous and less opaque
• Homopolymer


  Polymer where there is only one type of monomer
• Symmetric monomers


  Monomers where the carbons have the same substituents
• Asymmetric monomers


  Monomers where the carbons have different substituents
• Copolymer


  Polymer formed from two or more different monomers
• Condensation polymerization


  Monomers join at the same time losing a small molecule like water as by-product
• Polyester


  Polymer formed when a monomer with two carboxylic acid groups reacts with a monomer with two -OH groups, forming ester linkages
• Polyesters


  • Polyethylene terephthalate (PET)
• Nylon


  Polyamide formed when a dicarboxylic acid reacts with a diamine
• Linear polymer


  Polymer where the arrangement of atoms is like a long chain, with a backbone and pendant atoms
• Isotactic arrangement


  All the pendant groups or substituents are on the same side of the polymer chain, allowing efficient packing
• Dicarboxylic acid


  Reacts with diamine to form polyamide such as nylon
• Diamine


  Reacts with dicarboxylic acid to form polyamide such as nylon
• Polyamide


  • Nylon
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• Linear polymer


  Polymer where the arrangement of atoms is like that of a long chain
• Backbone


  The long chain in a linear polymer
• Pendant atoms


  Atoms or small groups of atoms attached to the long chain in a linear polymer
• Isotactic arrangement


  • All the pendant groups or substituents are on the same side of the polymer chain
  • They pack efficiently resulting in polymers with high melting point, high crystallinity, and superior mechanical strength
• Syndiotactic arrangement


  • The substituent group alternates from left to right of the carbons
  • They pack less efficiently than isotactic chains
• Atactic arrangement


  • Substituents occur randomly
  • They do not pack well
  • These polymers are rubbery, not crystalline, and are relatively weak
• Branched chain polymers


  • Not linear
  • Less dense and pack loosely compared to linear chains
• Rubber


  • A natural organic polymer formed by the addition of the monomer isoprene
  • Polymer strands of isoprene are crossed linked or bridged by short sulfur chains
• Crosslinks


  Tie or bind the polymer strands together
• When these crosslinked polymers are heated, the strands cannot flow past each other. They do not melt or break apart.
• Plastics


  • Come from the Greek 'plastikos' meaning "to mold"
  • Refer to synthetic polymers
  • Classified into two types: thermoplastics and thermosets
• Thermoplastics


  • Keep their plastic properties
  • They melt when heated and harden when cooled
  • Made of long linear polymer chains that are weakly bonded to each other
• Thermosets


  • Permanently "set" once they are formed
  • They cannot be melted or reshaped
  • If enough heat is added, they will crack or become charred
  • Made up of linear chains that are cross-linked to one another preventing the material from being melted and reformed