Chem 421: Introduction to Polymer Chemistry
Step and Chain Polymerization
- 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
- 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
Condensation vs. Addition
Carothers originally classified polymers
based on a comparison of the atoms in the monomer to the atoms in the polymer
- Condensation polymers had fewer atoms in the repeat unit (i.e., some small
molecule was emitted during polymerization).
- Addition polymers had the same atoms as their monomers.
There are far too many exceptions for this to be useful.
- Step polymerization by addition of alcohols to diisocyanates to form
- Chain polymerization (ring opening of heterocycle) with loss of
CO2 to form polypeptide.
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