Polycondensation

a process for the production of polymers from bifunctional and polyfunctional compounds (monomers), accompanied by the elimination of low-molecular weight by-products (for example, water, alcohols, and hydrogen halides). A typical example of polycondensation is the synthesis of a complex polyester:

n HOAOH + n HOOCA′COOH

⇄[— OAOOCA′CO —]_n + 2n H₂O

where A and A′ are glycol and dicarboxylic acid groups, respectively. The process is called homopolycondensation if the minimum possible number of monomer types for a given case participates. This number is usually two, as in the reaction shown above, although it may be unity, for example:

n H₂NACOOH ⇄[— HNACO —]_n + n H₂O

If at least one monomer more than the number required for the given reaction participates in polycondensation, the process is called copolycondensation. Polycondensation in which only bifunctional compounds participate leads to the formation of linear macromolecules and is called linear polycondensation. If molecules with three or more functional groups participate in polycondensation, three-dimensional structures are formed and the process is called three-dimensional polycondensation. In cases where the degree of completion of polycondensation and the mean length of the macromolecules are limited by the equilibrium concentration of the reagents and reaction products, the process is called equilibrium (reversible) polycondensation. If the limiting factors are kinetic rather than thermodynamic, the process is called nonequilibrium (irreversible) polycondensation.

Polycondensation is often complicated by side reactions, in which both the original monomers and the polycondensation products (oligomers and polymers) may participate. Such reactions include the reaction of monomer or oligomer with a mono-functional compound (which may be present as an impurity), intramolecular cyclization (ring closure), and degradation of the macromolecules of the resultant polymer. The rate competition of polycondensation and the side reactions determines the molecular weight, yield, and molecular weight distribution of the polycondensation polymer.

Polycondensation is characterized by disappearance of the monomer in the early stages of the process and a sharp increase in molecular weight, in spite of a slight change in the extent of conversion in the region of greater than 95-percent conversion.

A necessary condition for the formation of macro-molecular polymers in linear polycondensation is the equivalence of the initial functional groups that react with one another.

Polycondensation is accomplished by one of three methods: (1) in a melt, when a mixture of the initial compounds is heated for a long period to 10°-20°C above the melting (softening) point of the resultant polymer; (2) in solution, when the monomers are present in the same phase in the solute state; (3) on the phase boundary between two immiscible liquids, in which one of the initial compounds is found in each of the liquid phases (interphase polycondensation).

Polycondensation processes play an important role in nature and technology. Polycondensation or similar reactions are the basis for the biosynthesis of the most important biopolymers— proteins, nucleic acids, and cellulose. Polycondensation is widely used in industry for the production of polyesters (polyethylene terephthalate, polycarbonates, and alkyd resins), polyamides, phenol-formaldehyde resins, urea-formaldehyde resins, and certain silicones. In the period 1965–70, polycondensation acquired great importance in connection with the development of industrial production of a series of new polymers, including heat-resistant polymers (polyarylates, aromatic polyimides, polyphe-nylene oxides, and polysulfones).