Introduction and Nomenclature - Principles of Polymer Chemistry - page 11

1.3.2 Nomenclature of Step-Growth Polymers

The nomenclature for step-growth polymers is more complicated due to the possibility of having
many different repeat units. Usually, the polymers are referred to according to their functional units.
A polyester from ethylene glycol and terephthalic acid is called poly(ethylene terephthalate).
A product from ring opening polymerization, like, for instance, a polymer of caprolactam might be
called polycaprolactam. Here, it is based on the source. The repeat unit is actually not a lactam but
rather an open chain polyamide. Because it is derived from a lactam, it may still carry that name.
In this particular instance, however, it is more common to call the polymer by its generic name,
namely, nylon 6. The same would be true for a polymer from a lactone, like poly(

b -propiolactone).

The IUPAC name for this polymer, however, is:

O

O

n
poly[oxy(1-oxotrimethylene)]

oxo
substituent. In addition, the presence of a 1-oxo substituent requires that parentheses enclose the subunit.

The name is based on a presence of two subunits. Note that the carbonyl oxygen is called an

1.4 Steric Arrangement in Macromolecules

In linear polymers, due to the polymerization process, the pendant groups can be arranged into
orderly configurations or they can lack such orderliness. Propylene, for instance, can be polymerized
into two types of orderly steric arrangement. It can also be polymerized into one lacking steric order.
The same can be true of other monosubstituted vinyl monomers. The steric arrangement in
macromolecules is called tacticit y. Polymers can be isotactic , where all the chiral centers have the
same configuration (see Fig. 1.2 ). By picturing the chain backbone as drawn in the plane of the paper
and by picturing all the phenyl groups as oriented above the plane (Fig. 1.2a ), isotactic polystyrene
can then be visualized. The orderliness can also be of the type where every other chiral center has the
same configuration. Such an arrangement is called syndiotactic (Fig. 1.2b ). A lack of orderliness or
randomness in the steric arrangement is called atactic or heterotactic . Stereospecific polymers can
also be prepared from 1,2 disubstitued olefins. These macromolecules can be distereoisomers, or
ditactic polymers. To describe the arrangement of such polymers, a threo-erythro terminology is
used. An erythrodiisotactic polymer is one possessing alternating substituents—(-CHR 0 CHR-)- .If

we draw the carbon chain backbone in the plane of the paper, then all the R groups would find
themselves on one side of the chain and all the R 0 groups on the other. They would, however, all be on
the same side of the chain in a Fischer or in a Newman projection (Fig. 1.2c ). A threo isomer or a
threodiisotactic polymer is illustrated in Fig. 1.2d .
Polymerization of 1,4-disubstituted butadienes can lead to products that possess two asymmetric
carbon atoms and one double bond in each repeat unit. Such tritactic polymers are named with
prefixes of cis or trans together with erythro or threo (see Fig. 1.3 ).
In polymers with single carbon to carbon bonds, there is free rotation, provided that steric
hindrance does not interfere. This allows the molecules to assume different spatial arrangements or
conformations . Most of the possible isomers, however, represent prohibitively high-energy states.
The three lowest energy states that are most probable [ 8 ] are one trans and two gauche.

n