Chemistry Reference
In-Depth Information
functionality of 1 on a monomer with functionality 2. The radical is a
transient species, however, and the net result of the chain of reactions is
linkage of styrene units with each other so that the process is effectively
polymerization of these bifunctional monomers.
3.
Functionality is defined only for a given reaction. A glycol, HOROH, has a
functionality of 2 in esterification or ether-forming reactions, but its
functionality is 0 in amide-forming reactions. The same is true of 1,3-
butadiene.
HH
CH
2
=
C
C
=
CH
2
1-16
which may have a functionality of 2 or 4, depending on the particular double-
bond addition reaction.
4.
The condition that monomers be bi- or polyfunctional is a necessary, but not
sufficient, condition for polymerization to occur in practice. Not all reactions
between polyfunctional monomers actually yield polymers. The reaction must
also proceed cleanly and with good yield to give high-molecular-weight
products. For example, propylene has a functionality of 2 in reactions
involving the double bond, but free-radical reactions do not produce
macromolecules whereas polymerization in heptane at 70
CwithanAl
(CH
2
CH
3
)
2
Cl/TiCl
3
catalyst does yield high polymers.
5.
Functionality is a very useful concept in polymer science, and we use it later
in this topic. There are, however, other definitions than the one given here.
All are valuable in their proper contexts.
1.3.3
Latent Functionality
Important commercial use is made of monomers containing functional groups that
react under different conditions. This allows chemical reactions on polymers after
they have been shaped into desired forms. Some examples follow.
1.3.3.1
Vulcanization
Isoprene,
H
CH
2
=
C
C
=
CH
2
CH
3
1-17
