Page 224 - Carrahers_Polymer_Chemistry,_Eighth_Edition

6 Free Radical Chain

Polymerization

(Addition Polymerization)

Since many synthetic plastics and elastomers and some fibers are prepared by free radical polymer-
ization, this method is important. Table 6.1 contains a listing of commercially important addition
polymers, including those that will be emphasized in this chapter because they are prepared using
the free radical process.
As with other chain reactions, free radical polymerization is a rapid reaction that consists of the
characteristic steps of initiation, propagation, and termination. Free radical initiators are produced
by the homolytic cleavage of covalent bonds as well as numerous radiation-associated methods.

6.1 INITIATORS FOR FREE RADICAL CHAIN POLYMERIZATION
Free radical initiation can occur through application of heat (thermal), ultraviolet (UV) and visible
light (photochemical), ionizing light, redox reagents, electricity (electrochemical), and so on, that is,
any process that creates the essential free radicals.
Light in the UV and visible range can disrupt selected bonds forming free radicals. Such disrup-
tion occurs as we are exposed to sunlight. Suntan treatments often contain certain compounds that
can accept this damaging radiation. Related compounds are also used in foods to give them longer
shelf life. They are generally known as antioxidants. Synthetic antioxidants include benzophenones,
benzils, and certain organic ketones. Thus, diphenylketone decomposes on exposure to UV radia-
tion of the appropriate wavelength forming two free radicals (Equation 6.1).
O O
|| UV || (6.1)
Ph–C–Ph • + •
Ph–C Ph
The advantage of using such photochemical initiations is that polymerization can be carried out
at room temperature.
When molecules are exposed to light of higher energy, shorter wavelength, or higher frequency,

electrons can be removed or added depending on the specific conditions. Usual forms of ionizing
radiation employed industrially and experimentally include neutrons, X-rays, protons, and alpha and
beta particles. Oxidation–reduction, redox, reactions are also often employed to initiate free radical
polymerizations in solution or heterogeneous organic-aqueous systems. Free radicals can be created

by passing a current through a reaction system sufficient to initiate free radical polymerizations.
While application of heat or some other method can rupture the pi bond in the vinyl monomer
causing the formation of a two-headed free radical that can act as a free-radical initiator, perox-
ides and dinitriles are generally employed as initiators. This is a consequence of the general bond
dissociation energy trend of C–H>C–C>C–N>O–O. Dinitrile or azo compounds such as 2,2′-azo-
bis-isobutyronitrile (AIBN) require temperatures of about 70°C–80°C to produce decomposition
with free radical formation. Peroxides such as benzoyl peroxide (BPO) require temperatures higher
o
than 60 C for decomposition and free radical formation. While the dissociation bond energy for C–N
is generally greater than for O–O, the formation of a stable N molecule is the thermodynamic driv-
2
ing force due to an entropy effect, allowing dissociation to occur at typically lower temperatures.
187

9/14/2010 3:39:21 PM
K10478.indb 187 9/14/2010 3:39:21 PM
K10478.indb 187