Callister - An introduction - 8th edition

15.20 Polymerization • 609
Addition polymerization is used in the synthesis of polyethylene, polypropylene,
poly(vinyl chloride), and polystyrene, as well as many of the copolymers.
Concept Check 15.11
Cite whether the molecular weight of a polymer that is synthesized by addition
polymerization is relatively high, medium, or relatively low for the following
situations:
(a) Rapid initiation, slow propagation, and rapid termination
(b) Slow initiation, rapid propagation, and slow termination
(c) Rapid initiation, rapid propagation, and slow termination
(d) Slow initiation, slow propagation, and rapid termination
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
condensation
polymerization
Condensation Polymerization
Condensation (or step reaction) polymerization is the formation of polymers by stepwise
intermolecular chemical reactions that may involve more than one monomer
species. There is usually a small molecular weight by-product such as water that is
eliminated (or condensed). No reactant species has the chemical formula of the repeat
unit, and the intermolecular reaction occurs every time a repeat unit is formed.
For example, consider the formation of the polyester poly(ethylene terephthalate)
(PET), from the reaction between ethylene glycol and terephthalic acid; the intermolecular
reaction is as follows:
Ethylene glycol
H H
+
HO C C OH HO
Terephthalic acid
O
O
C
C
OH
H
H
H
H
O
O
HO
C
C
O
C
C
OH H 2 O
(15.9)
H
H
This stepwise process is successively repeated, producing a linear molecule. The
chemistry of the specific reaction is not important, but the condensation polymerization
mechanism is. Furthermore, reaction times for condensation are generally
longer than for addition polymerization.
For the previous condensation reaction, both ethylene glycol and terephthalic
acid are bifunctional. However, condensation reactions can include trifunctional or
higher functional monomers capable of forming crosslinked and network polymers.
The thermosetting polyesters and phenol-formaldehyde, the nylons, and the
polycarbonates are produced by condensation polymerization. Some polymers, such
as nylon, may be polymerized by either technique.