Condensation polymers, stoichiometry

In a third approach, supramolecular polymers are based on the reversibility of metal-coordination bonding. These polymers are the closest analogues to conventional macromolecules, because most of the polymers disclosed make use of strong bonding,16 in which the reversibility can be tuned by chemical means only. However, appropriate choice of the metal ion can give rise to bonding that resembles that of the other two approaches. The DP of the polymers in the case of the coordination polymers is similar to that of the condensation polymers, and achieving exact stoichiometry is of distinct importance here. 

Two basic types of condensation polymers are possible. Nylon-6,6 is the typical example of the poly condensation of two bifunctional monomers, adipic acid and hexamethylenediamine. This type of polycondensation is generally referred to as the AABB reaction. In contrast, polycondensation of one difunctional monomer is referred to as an AB reaction. The AABB-type polycondensation has the advantage that the stoichiometry is fixed by salt formation. In contrast, AB polyamides are usually prepared by ring-opening polymerization of a cyclic lactam. This reaction requires only a catalytic amount of water, because the water required for ring opening is replenished by the simultaneously occurring polycondensation ... 

Achieving adequate stoichiometry and by-product removal to obtain high molecular weight polymers. This is applicable mainly to condensation polymers. 

Wallace Carothers, duPont s famous polymer chemist, proposed classifying polymers by reference to the stoichiometry of the polymerization reaction. If the entire monomer molecule ends up in the polymer, he called it an addition polymer, whereas if there is a byproduct, often water, the primary product is called a condensation polymer. He thus considered vinyl polymers to be addition polymers and polyesters to be condensation polymers. However, it was later learned that it is possible to make some addition polymers by reactions in which there is a byproduct and to make some condensation polymers by reactions in which there is no byproduct. Paul Flory, who started his career as the theoretician in Carother s duPont research group, later proposed that the reaction mechanism be used as the basis for classifying polymers. In this scheme, in a step polymerization, any two reactive molecules can combine, so polymerization occurs uniformly throughout the reaction mixture. In chain growth polymerization, on the other hand, monomer units are added only to species containing an active center or initiator, which can be a free radical, an ion, or an active catalyst site. Condensation polymers are usually produced by step reactions, and addition polymers are usually made by... 

Example 13.3 The conversion of a self-condensing reaction can be limited to give polymers with finite lengths. How does the polydispersity of these polymers compare with those in Example 13.2 where the reaction went to completion with imperfect stoichiometry Make the comparison at the same average chain length. 

Many important step-growth polymerizations are run under conditions in which the reverse reaction between the polymer and condensation products is significant [as in reactions (a) and (b) in Fig. 5-2]. In these cases, the stoichiometry of the reactants must be carefully controlled, and a close balance is needed between the concentrations of functional groups of opposite kinds. 

The traditional way to classify polymerization reactions has been by stoichiometry—i.e., whether or not the molecular weight of the polymer produced is an exact multiple of the molecular weight of the monomer (exclusive of end groups). If so, the reaction has been termed an addition polymerization if weight was lost during polymerization, it was named condensation polymerization. Vinyl polymerization (Reaction 5) and polyurea formation (Reaction 6) are examples of the former, whereas polyesterification (Reaction 7) and formation of poly(a-amino acids) from N-carboxyanhydrides (Reaction 8) are examples of the latter. 

Finally, phenols have a tendency to autooxidize and so form quinones and thereby condense to form ill-defined polymers. The label on the bottle and the stoichiometry and structure do not completely correspond. Thus, the measured enthalpy of combustion and the derived enthalpy of formation are for an impure sample. 

The melt condensation of acid and hydroxyl functional group normally requires exact stoichiometry, elevated temperature, and a long reaction cycle. Such a route would not be possible to utilize to produce block polymers from lactones and other vinyl monomers. However, a rather facile route leading to polyester formation can be realized by the ring-opening polymerization of lactones as seen from the scheme ... 

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