27.2 Classification of Polymers

Polymers can be classified in two ways: based on their physical response to heating resulting in how they are used and based on their method of polymerization resulting from how they are prepared.

Plastics

The term plastics is used to describe polymers that are capable of being molded (formed into a shape) or are pliable. (Hein et al., 2014)

Thermoplastic and Thermosetting Polymers

Polymers are classified by their physical behaviour when heated. Thermoplastic polymers are soften when heated.  They are plastics that become firm again when cooled. Thermoplastic polymers are common because the cycle of heating and cooling is common in the forming and recycling process and allows the polymer to be reformed. (Libretexts, 2022)

In thermoplastic polymers, monomer molecules are joined end to end in a linear chain with little to no cross linking between the chains. (Hein et al., 2014)  In Figure 27.2a. and Figure 27.2b., the thermoplastic polymer (also called an elastomer in this image) shows multiple strands of polymer chains but no connection between any of the strands so the polymer can be stretched.  Polyethylene is an example of a thermoplastic polymer. (Hein et al., 2014) 

Addition and Condensation Polymers

Addition polymerization and condensation polymerization are two modes of polymerization reactions in the formation of polymers.

In an addition polymerization, the monomer molecules bond to each other without the loss of any other atoms. Addition or chain-growth polymerization involves the rearrangement of bonds within the monomer in such a way that the monomers link up directly with each other.  Figure 27.2c. shows the formation of ethylene through addition polymerization. Addition polymers from alkene monomers or substituted alkene monomers are the biggest groups of polymers in this class. Ring opening polymerization can occur without the loss of any small molecules.

In condensation polymerization, two different monomers combine with the loss of a small molecule, usually water. Most polyesters and polyamides (nylon) are in this class of polymers. Figure 27.2d. shows the condensation polymerization of Nylon 6,6.  A large number of important and useful polymeric materials are not formed by addition polymerization, but proceed instead by conventional functional group transformations of polyfunctional reactants. These polymerizations often (but not always) occur with loss of a small byproduct, such as water, and generally (but not always) combine two different components in an alternating structure.