Polyamides repeating unit

As noted above, proteins are polyamides in which a-amino acids make up the repeat units, as shown by structure [III] ... 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

These polymers, typical of polyamides with fewer than four main chain carbon atoms in the repeating unit, decompose before melting and have to be processed from solution. Several of the polymers may, however, be spun into fibres. Over thirty years ago Courtaulds produced silk-like fibres on an experimental commercial scale from poly-(L-alanine) and from poly-(a-methyl-L-glutamate). The latter material is also said to be in use as a synthetic leather in Japan. The... 

We ve seen on several occasions in previous chapters that a polymer, whether synthetic or biological, is a large molecule built up by repetitive bonding together of many smaller units, or monomers. Polyethylene, for instance, is a synthetic polymer made from ethylene (Section 7.10), nylon is a synthetic polyamide made from a diacid and a diamine (Section 21.9), and proteins are biological polyamides made from amino acids. Note that polymers are often drawn by indicating their repeating unit in parentheses. The repeat unit in polystyrene, for example, comes from the monomer styrene. 

Draw structures that show the repeat units of the polyamide and the polyester arising from these monomers. 

A repeat unit is the sequence of atoms that is repeated to build a polymer chain. This definition, although very simple, distinguishes between X-Y and X-XY-Y units. For instance, the repeat units of polyamide 6 and polyamide 6610 are ... 

A structural unit or monomer can be defined as the residue from each monomer molecule, whatever its chemical species. In the case of X-Y monomers, a structural unit is identical to a repeat unit (e.g., in polyamide 6), while in the case of X-X, Y-Y monomers, there are two structural units per repeat unit (e.g., in polyamide 66). In the latter case, the mass of a structural unit is obtained by averaging the masses of the two different chemical species. For polyamide 6 and polyamide 66 the masses of structural units are thus 113 in both cases. 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

Special terminology based on trade names has been employed for some polymers. Although trade names should be avoided, one must be familiar with those that are firmly established and commonly used. An example of trade-name nomenclature is the use of the name nylon for the polyamides from unsubstituted, nonbranched aliphatic monomers. Two numbers are added onto the word nylon with the first number indicating the number of methylene groups in the diamine portion of the polyamide and the second number the number of carbon atoms in the diacyl portion. Thus poly(hexamethylene adipamide) and polyfhexamethylene sebacamide) are nylon 6,6 and nylon 6,10, respectively. Variants of these names are frequently employed. The literature contains such variations of nylon 6,6 as nylon 66, 66 nylon, nylon 6/6, 6,6 nylon, and 6-6 nylon. Polyamides from single monomers are denoted by a single number to denote the number of carbon atoms in the repeating unit. Poly(e-caprolactam) or poly(6-aminocaproic acid) is nylon 6. 

Further variation is possible in the polymer structure and properties by using mixtures of the appropriate reactants such that the polymer chain can have different R and R groups. Thus polyamide structures of types XXXV and XXXVI are possible variations on structures XXXIII and XXXIV, respectively. A polymer such as XXXV or XXXVI has two different repeat units and is referred to as a copolymer the process by which it is synthesized is... 

However, only (a) and (b) are constitutional repeating units, describing the polymer s constitution precisely and completely. The polyamide prepared from hex-amethylenediamine and adipic acid has the following constitutional repeating unit ... 

Polyamides are macromolecules whose constitutional repeating units are joined by amide groups, for example ... 

Because the repeating units of polyamides with an even number of methylene groups fit together better than those of polyamides with an odd number of ntethjrhme groups, the Tm of nylons with even numbers of carbon atoms is greater than that of comparable nylons with odd numbers of carbon atoms. The Tm of nylon 4 is less than that of nylon 3. 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

A sample polymerization reaction, showing the synthesis of a polyurethane, is shown equation 3. Using similar synthetic strategies, various polyurethanes, polyureas (eq. 4), and polyamides (eq. 5) have been synthesized.14-17 Note that the step polymers in these reactions have a metal-metal bond in every repeat unit. Copolymers are straightforwardly synthesized by adding appropriate difunctional organic molecules into the reaction mixture (e.g., eq. 6). 

Thermal lability of aliphatic polyamides, in general, is influenced by the potential for ring-formation during chain degradation, and this is particularly the case with PA 6.6, in which the adipate repeat unit enables the formation of a six-membered intermediate along the polymer chains with eventual formation of cyclopentanone and its derivatives.29... 

An additional, important, type of chain polymers is the polyamides, polyesters, and polyurethanes [195-197], Among the polyamides, nylon is a very important member. In Figure 2.38, the repeating unit of one of the member of the nylon family, that is, the repeating unit of polyhexamethylene adipamide, or nylon 6,6 is represented [8,195]. As was previously commented, nylon is applied in stockings, ropes, fabrics, and has other uses. 

These synthetic polyamides are made up of the same repeating unit but will inevitably have a range of molecular weights as the polymer length will vary. This is a different story from that of the natur polyamides—peptides and proteins—that you met in Chapter 49. Those polymers were made of twenty or so different monomers (the amino acids) combined in a precise order with a precise stereochemistry and all molecules of the same protein have the same length. Nonetheless, some of their uses are almost identical both nylon and wool are polyamides, for example. 

The polycondensation of l,l -dicarboxycobalticenium chloride with aromatic diamines in molten antimony trichloride at 150-175 °C gave polyamides containing the cobalticenium tetrachloroantimonate(V) salt system in the repeating unit (Eq. 5)9. ... 

Nylon 6 and nylon 6,6 are two commercially produced polyamides. How are they synthesized and how do their chemical repeat units differ (Note you won t find a description of the synthesis of nylon 6 in the book. Check other sources )... 

---