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Polymers, Synthesis 755

SCHEME 2 Linear step-growth processes: polyesterifications.

react with one another, whether they are dimers reacting Ester interchange processes will proceed and produce
with monomers, trimers, or higher species. In the gen- methanol, which is removed. Ideally, the structure is
eral case, one considers an n-mer reacting with an m-mer bis(hydroxyethyl) terephthalate. Actually, some higher
to produce the next higher oligomer. This whole process oligomers may also be formed at this stage, but the point
must continue until P begins to approach 0.99 or higher. is that the new structure has both hydroxyl and ester
A second illustration of a linear step-growth polymer- functionality in the same molecule.
ization is the synthesis of polyesters via ester interchange Sometimes it is common to term such a new monomer
processes (Scheme 4). Here, one utilizes the dimethyl an AB system. This is contrasted with a so-called AA +
ester of terephthalic acid with, initially, an excess of ethy-
lene glycol. Historically, the choice of the dimethyl
ester was based on considerations of monomer purity.
Thus, in early industrial practice, it was not feasible
to produce nearly 100% pure terephthalic acid. Since
terephthalic acid cannot be distilled, early investigators
took recourse to producing the dimethyl ester, which
could be vacuum-distilled and also crystallized, thus
allowing one to meet the criterion of extremely high
monomer purity. The dimethyl ester then will react with
a diol, such as ethylene glycol, in the manner indicated.

SCHEME 3 Examples of step-growth polymerization. Any two
species in the reaction mixture can react with one another. SCHEME 4 Synthesis of polyesters via ester interchange.