Chem 421: Introduction to Polymer Chemistry

Step and Chain Polymerization

Step Polymerization Chain Polymerization
Step scheme Chain scheme
  • Any two molecular species can react.
  • Monomer disappears early.
  • Polymer MW rises throughout.
  • Growth of chains is usually slow (minutes to days).
  • Long reaction times increase MW, but yield of polymer hardly changes.
  • All molecular species are present throughout.
  • Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.
  • Growth occurs only by addition of monomer to active chain end.
  • Monomer is present throughout, but its concentration decreases.
  • Polymer begins to form immediately.
  • Chain growth is usually very rapid (second to microseconds).
  • MW and yield depend on mechanism details.
  • Only monomer and polymer are present during reaction.
  • Usually (but not always) polymer repeat unit has the same atoms as had the monomer

Condensation vs. Addition

Carothers originally classified polymers based on a comparison of the atoms in the monomer to the atoms in the polymer repeat unit.

There are far too many exceptions for this to be useful.

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