Step-growth polymerization

Polycondensation is converted into polymers (plastics ) is a frequently occurring condensation reaction, the monomers. Thus, a monomer can participate in the reaction, it must possess at least two functional groups that are particularly responsive (e.g.,- OH,- COOH,- NH2 ,-CHO ... ). In this case, various kinds are typically monomers ( for example, diols and dicarboxylic acids, which condense to form esters ) are reacted with each other. Polycondensation proceeds stepwise ( step-growth reaction), stable but still reactive intermediates (oligomers ). The oligomers react with one another and finally form a macromolecule. The products are called polycondensation. Apart from plastics, there are also a number of natural polymers, such as poly-silicic acids caused by polycondensation.

In contrast to polymer recovery by chain or polyaddition of at least one by-product released during the polycondensation. These by-products (eg water, ammonia, lower alcohols, hydrogen chloride) must be continuously removed, otherwise the polycondensation stops for thermodynamic reasons, in case of low degree of polymerization.

In contrast to the chain polymerization, which is carried by chain growth, the reaction conversion rate in polycondensation must be at least 99% to obtain a polycondensate of high molar mass. Furthermore, the ratio of the amounts of respective monomer used must be adapted to the predetermined by the reaction stoichiometric ratio as accurately as possible, otherwise you get to a point where all oligomers have the same active ends and can no longer react with each other ( Carothers equation ).

Historical Information

The first polycondensation the German chemist and Nobel laureate Adolf von Baeyer in 1872. He described the polycondensation reaction of phenol and formaldehyde to Bakelite and thus laid the foundation for today's polymer chemistry. Bakelite was produced on a large scale for the first time in 1909 by Leo Hendrik Baekeland and has been used for decades in many areas. It is still made today.

Application

The polycondensation is an important process in polymer chemistry, the number of important polymers, such as phenol resin (e.g., Bakelite ), polyesters and polyamides are prepared. The polycondensation is of great importance in the production of adhesives, such as phenol-formaldehyde adhesives, and in the production of brake linings for motor vehicles.

Examples

Phenoplast

By reaction of phenol with an aldehyde is formed, composed of a catalyst, a first intermediate product.

This intermediate reacts with the elimination of water, again with formaldehyde and phenol to a macromolecule:

This is a condensation reaction, as in the continuous polymer formation repeatedly splits off water. There will be a copolymerization and a spatial network and is called polycondensation. In this case, a phenolic resin produced (including phenolic resin, phenol-formaldehyde condensate or called Bakelite ).

Polyester

By reaction of carboxylic acids ( here terephthalic acid) with diols ( compounds containing two alcohol groups here ethanediol ) is formed with elimination of water, a polyester ( e.g., polyethylene terephthalate (PET) ). Running this reaction including the reaction product as the starting material from a plurality of stages, it is a polycondensation.

If instead ethanediol example, glycerol used as the starting material, there is a spatial networking and the emergence of a thermosetting plastic.

Polyamides

Diamines react with dicarboxylic acids by polycondensation polyamides (eg, nylon )

By heating a mixture of adipic acid and hexamethylenediamine, so there is a polycondensation of 6,6-nylon.

Technical Procedures

• Solution polycondensation
• Melt polycondensation
• Interfacial polycondensation
• Solid-state polycondensation
• Fällungspolykondensation