In polycondensation, the polymer molecules are formed by reaction between monomers which each have two or more reactive groups. The process is best understood by considering first a simple condensation reaction between molecules each having only a single reactive group. Acetic acid, for example, will react with ethanol to give the ester ethylacetate, and water is eliminated:
CH,COOH + C21150H CH,C00C2H5 + H20
acetic acid ethanol ethyl acetate water
The reaction is between the acid group (-COOH) of the acetic acid molecule and the alcohol or hydroxyl group (-OH) of the ethanol molecule. If the acid molecule were to have two acid groups and the alcohol were to have two alcohol groups it would be theoretically possible to build up long-chain molecules, and this is what happens in polycondensation. The polyester Dacron is made by reacting ethylene glycol (HO-CH2CH2-0H) with terephthalic acid (HOOC-C6H4-COOH) and consists of a chain having the following structure:
If the starting materials have only two reactive groups, linear polycondensates are obtained, but if they have more than two such groups, branched-chain or crosslinked plastics are formed. The structure of a crosslinked plastic can be thought of as a three-dimensional network. It is a characteristic of polycondensation reactions that low molecular weight compounds (water in the case of polyesters) are eliminated during polymerization.
Polyaddition is a stepwise process involving monomers having at least two reactive groups. In contrast to polycondensation, no low molecular weight compounds are eliminated during the reaction, which involves only the migration of a hydrogen ATOM. A good example of polyaddition is the formation of polyurethane plastics from a monomer having at least two isocyanate (-NCO) groups and a monomer having at least two alcohol groups (-OH).
The isocyanate groups react with the alcohol groups by addition to form linkages having the following structure -NH-C0-0-. In industry, polycondensation and polyaddition reactions usually take place in a melt of the reactants themselves or in the presence of a solvent which is added. Acidic or basic catalysts may also be present in the reaction medium. Typical of polycondensation plastics are the formaldehyde resins. When formaldehyde is condensed with urea and melamine, amino resins, such as Kaurit and Urecoll, are obtained. If condensation is carried out with phenols, phenolic resins are formed, of which the best-known are the Bakelite plastics. The condensation is usually carried out in an aqueous medium. By varying the reaction conditions, for example, the temperature or the pH, a large range of branched polymers can be produced, some of which are soluble in water or alcohols and can be later crosslinked to make thermoset polymers.
Various polyester plastics are also produced by polycondensation. As already explained, the building blocks are alcohols and organic acids, which are linked to each other during condensation by means of ester bonds. One of the most commercially important linear polyesters is polyethylene terephthalate, the starting material for synthetic fibers such as Terylene and Trevira. Alkyd resins, such as Alkydal and Glyptal, and unsaturated polyester resins such as Palatal and Crystic, are branched or crosslinked polycondensates.
If adipic acid (HOOC-(CH2)4-COOH) and hexamethylene diamine (H2N(CH2)6NH2) are polycondensed, Nylon 66 is obtained; if the starting material is caprolactam, Nylon 6 is formed. Both of these are polyamides.
Among the most important products obtained by polyaddition are the polyurethanes. The chemistry of these plastics is extremely complicated, but the starting materials are always isocyanates and alcohols. Depending on the type of starting material selected, linear, branched or crosslinked polyurethanes can be produced. Polyurethane foam, widely used as an insulating material in the building industry, is manufactured by polyaddition in the presence of water and with an excess of the isocyanate monomer. Among the best-known polyurethane plastics are Desmophen, Desmodur and Pluracol.