Page 60 - Carrahers_Polymer_Chemistry,_Eighth_Edition

2 Polymer Structure

(Morphology)

The size and shape of polymers are intimately connected to their properties. The shape of polymers
is also intimately connected to the size of the various units that comprise the macromolecule and
the various primary and secondary bonding forces that are present within the chain and between

chains. This chapter covers the basic components that influence polymer shape or morphology.
We generally describe the structure of both synthetic and natural polymers in terms of four lev-
els of structure. The primary structure describes the precise sequence of the individual atoms that
compose the polymer chain. For polymers where there is only an average structure, such as proteins,
polysaccharides, and nucleic acids, a representative chain structure is often given.
The structure can be given as a single repeat unit such that the full polymer structure can be
obtained by simply repeating the repeat unit 100, 500, and 1,000 times depending on the precise
number of repeat units in the polymer chain. For poly(vinyl chloride), PVC, this is

R—(—CH —CH—)—R or R—(—CH —CH(Cl)—)—R or —(CH —CH—)
2
2
2
(2.1)
Cl Cl
—
—
Or some fuller description of the primary structure may be given such as that below for three
repeat units of PVC where the particular geometry about each chiral carbon is given.
R
R
H H H
(2.2)
Cl Cl Cl

The ends may or may not be given depending on whether they are important to the particu-
lar point being made. Thus, for the single PVC repeat unit given above the end groups may be as
follows:

CH —CH—(—CH —CH—) —CH —CH Cl
n
2
3
2
2
(2.3)
—
—
Cl Cl
Natural polymers also have general repeat units such as for cellulose (2.4):
OH
H O O n R
H
OH H
R H (2.4)
H OH
The secondary structure describes the molecular shape or conformation of the polymer chain.
For most linear polymers this shape approaches a helical or “pleated skirt” (or sheet) arrangement

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