Chemistry Reference
In-Depth Information
The term degree of polymerization is also used in some contexts in the polymer
literature to mean the number of monomer residues in an average polymer mole-
cule. This number will be equal to the one in the definition stated above if the
repeating unit is the residue of a single monomer. The difference between the two
terms is explained in more detail in Section 7.4.2, where it will be more readily
understood. To that point in this topic the definition given at the beginning of this
section applies without qualification. We use the abbreviation DP for the degree of
polymerization defined here and X for the term explained later. (The coining of a
new word for one of these concepts could make this topic clearer, but it might con-
fuse the reader's understanding of the general literature where the single term
degree of polymerization is unfortunately used in both connections.)
The relation between degree of polymerization and molecular weight, M,of
the same macromolecule is given by
M
5
ð
DP
Þ
M
0
(1-2)
where M
0
is the formula weight of the repeating unit.
1.3
Polymerization and Functionality
1.3.1
Polymerization
Polymerization is a chemical reaction in which the product molecules are able to
grow indefinitely in size as long as reactants are supplied. Polymerization can
occur if the monomers involved in the reaction have the proper functionalities.
1.3.2
Functionality
The functionality of a molecule is the number of sites available for bonding to
other molecules under the specific conditions of the polymerization reaction.
A bifunctional monomer can be linked to two other molecules under appropri-
ate conditions. Examples are
triphenyl phosphine
catalyst
dry hexane
n CH
2
O
CH
2
O
n
polyformaldehyde
(1-3)
1-12
heptane, 60°C
x CH
2
= CH
2
CH
2
-CH
2
(1-4)
x
Al (CH
2
-CH
3
)
3
/TiCl
4
catalyst
polyethylene
