Apolyamide is apolymer withrepeating units linked byamide bonds.[1]
Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides areproteins, such aswool andsilk. Artificially made polyamides can be made throughstep-growth polymerization orsolid-phase synthesis yielding materials such asnylons,aramids, andsodium polyaspartate. Synthetic polyamides are commonly used in textiles, automotive industry, carpets, kitchen utensils and sportswear due to their high durability and strength. The transportation manufacturing industry is the major consumer, accounting for 35% of polyamide (PA) consumption.[2]
Polymers ofamino acids are known aspolypeptides orproteins.
According to the composition of their main chain, synthetic polyamides are classified as follows:
| Family | Main chain | Examples | Commercial products |
|---|---|---|---|
| Aliphatic polyamides | Aliphatic | NylonPA 6 andPA 66 | Zytel fromDuPont, Technyl fromSolvay, Rilsan and Rilsamid fromArkema, Radipol fromRadici Group |
| Polyphthalamides | Semi-aromatic | PA 6T =hexamethylenediamine +terephthalic acid | Trogamid T fromEvonik Industries, Amodel fromSolvay |
| Aromatic polyamides, oraramids | Aromatic | Paraphenylenediamine + terephthalic acid | Kevlar andNomex from DuPont, Teijinconex,Twaron andTechnora fromTeijin Aramid, Kermel from Kermel. |
All polyamides are made by the formation of an amide function to link two molecules of monomer together. The monomers can be amides themselves (usually in the form of a cyclic lactam such ascaprolactam), α,ω-amino acids or a stoichiometric mixture of a diamine and a diacid. Both these kinds of precursors give a homopolymer. Polyamides are easily copolymerized, and thus many mixtures of monomers are possible which can in turn lead to many copolymers. Additionally many nylon polymers are miscible with one another allowing the creation of blends.
Production of polymers requires the repeated joining of two groups to form an amide linkage. In this case this specifically involvesamide bonds, and the two groups involved are anamine group, and a terminalcarbonyl component of afunctional group. These react to produce a carbon-nitrogen bond, creating a singularamide linkage. This process involves the elimination of other atoms previously part of the functional groups. The carbonyl-component may be part of either acarboxylic acid group or the more reactiveacyl halide derivative. The amine group and the carboxylic acid group can be on the same monomer, or the polymer can be constituted of two differentbifunctional monomers, one with two amine groups, the other with two carboxylic acid or acid chloride groups.
Thecondensation reaction is used to synthetically produce nylon polymers in industry. Nylons must specifically include a straight chain (aliphatic) monomer. The amide link is produced from an amine group (alternatively known as an amino group), and acarboxylic acid group. The hydroxyl from the carboxylic acid combines with a hydrogen from the amine, and gives rise to water, the elimination byproduct that is the namesake of the reaction.
As an example of condensation reactions, consider that in living organisms,amino acids are condensed with one another by an enzyme to form amide linkages (known aspeptides). The resulting polyamides are known as proteins or polypeptides. In the diagram below, consider the amino-acids as single aliphatic monomers reacting with identical molecules to form a polyamide, focusing on solely the amine and acid groups. Ignore the substituentR groups – under the assumption the difference between the R groups are negligible:
For fully aromatic polyamides oraramids e.g.Kevlar, the more reactiveacyl chloride is used as a monomer. The polymerization reaction with the amine group eliminateshydrogen chloride. The acid chloride route can be used as a laboratory synthesis to avoid heating and obtain an almost instantaneous reaction.[3] The aromaticmoiety itself does not participate in elimination reaction, but it does increase the rigidity and strength of the resulting material which leads to Kevlar's renowned strength.
In the diagram below, anaramid is made from two different monomers which continuously alternate to form the polymer chain. Aramids are aromatic polyamides:
Polyamides can also be synthesized from dinitriles using acid catalysis via an application of the Ritter reaction. This method is applicable for preparation ofnylon 1,6 fromadiponitrile,formaldehyde and water.[4] Additionally, polyamides can be synthesized fromglycols and dinitriles using this method as well.[5]
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