Chem 421: Introduction to Polymer Chemistry

Step and Chain Polymerization

Step Polymerization	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 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.

• 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 polyurethanes:

  

• Chain polymerization (ring opening of heterocycle) with loss of CO₂ to form polypeptide.

  

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