Polymer step growth

The first completely synthetic polymer, nylon, is a polyamide that forms by step growth and condensation. Nylon 6 is a homopolymer made from 6-aminohexanoic acid, whereas Nylon 66 is a copolymer. Kevlar is closely related to nylon and is classified as an aramid because of the presence of 

Will a different pattern be observed for changes in the molecular weight of the polymer over time for chain-growth versus addition polymers  

The analogous strategy for synthesizing metal-metal bond-containing polymers also uses difunctional, cyclopentadienyl-substituted metal dimers. A sample step-growth polymerization reaction is shown in equation 3, which 

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). 

Yet another step-growth synthesis strategy is to react the difunctional dimer molecules with prepolymers. Equation 7 shows an example of this technique.16 (As received from the manufacturer, prepolymers are often ill-defined 

copolymers can likewise be synthesized by using prepolymers and another organic difunctional molecule (eq. 8).33 

In another example of a step-growth polymerization reaction that yielded polymers containing metal-metal bonds, Moran and co-workers reported the 

---

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

To see why the assumption of equal reactivity is so important to step-growth polymers, recall from Table 1.2 the kind of chemical reactions which produce typical condensation polymers ... 

At the other end of the reaction, deviations from idealized rate laws are attributed to secondary reactions such as degradations of acids, alcohols, and amines through decarboxylation, dehydration, and deamination, respectively. The step-growth polymers which have been most widely studied are simple... 

Polyesters and polyamides are two of the most studied step-growth polymers, as well as being substances of great commercial importance. We shall consider polyesters in the next section, and polyamides in Sec. 5.6. 

As with polyesters, the amidation reaction of acid chlorides may be carried out in solution because of the enhanced reactivity of acid chlorides compared with carboxylic acids. A technique known as interfacial polymerization has been employed for the formation of polyamides and other step-growth polymers, including polyesters, polyurethanes, and polycarbonates. In this method the polymerization is carried out at the interface between two immiscible solutions, one of which contains one of the dissolved reactants, while the second monomer is dissolved in the other. Figure 5.7 shows a polyamide film forming at the interface between an aqueous solution of a diamine layered on a solution of a diacid chloride in an organic solvent. In this form interfacial polymerization is part of the standard repertoire of chemical demonstrations. It is sometimes called the nylon rope trick because of the filament of nylon produced by withdrawing the collapsed film. 

Next let us examine the effect of monomers with functionality greater than 2 on step-growth polymers. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

Some Cross-linked Step-Growth Polymers... 

In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

Once the potential associated with this aspect of molecular architecture is recognized, the principles of the last section coupled with the richness of organic (and inorganic) chemistry suggest numerous synthetic possibilities. We shall not attempt to be comprehensive in discussing this facet of polymer chemistry instead we cite only a few examples of step-growth polymers which incorporate... 

As a step-growth polymer made under equiUbrium conditions, PET has a molecular weight distribution very close to the theoretical value of 2.0. 

The alkene and diene polymers discussed in Sections 7.10 and 14.6 are called chain-growth polymers because they are produced by chain reactions. An initiator adds to a C=C bond to give a reactive intermediate, which adds to a second alkene molecule to produce a new1 intermediate, which adds to a third molecule, and so on. By contrast, polyamides and polyesters are called step-growth polymers because each bond in the polymer is formed independently of the others. A large number of different step-growth polymers have been made some of the more important ones are shown in Table 21.2. 

Table 21.2 Some Common Step-Growth Polymers and Their Uses...

The best known step-growth polymers are the polyamides, or nylons, first prepared by Wallace Carothers at the DuPont Company by heating a diamine with a diacid. Por example, nylon 66 is prepared by reaction of adipic acid (hexanedioic acid) with hexamethylenediamine (.1.,6-hexanediamine) at 280 °C. The designation "66" tells the number of carbon atoms in the diamine (the first 6) and the diacid (the second 6). 

Draw structures of the step-growth polymers you would expect to obtain from the following reactions ... 

Step-growth polymers, such as polyamides and polyesters, are prepared by reactions between difunctional molecules. Polyamides (nylons) are formed by reaction between a diacid and a diamine polyesters are formed from a diacid and a diol. 

The step-growth polymer nylon 6 is prepared from caprolactam. The reaction involves initial reaction of caprolactam with water to give an intermediate open-chain amino acid, followed by heating to form the polymer. Propose mechanisms for both steps, and show the structure of nylon 6. 

Step-growth polymers are produced by reactions in which each bond in the polymer is formed stepwise, independently of the others. Like the polyamides (nylons) and polyesters that we saw in Section 21.9, most step-growth polymers... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Identify the montuner units from which each of the following polymers is made, and tell whether each is a chain-growth or a step-growth polymer. 

Nylon (Section 21.9) A synthetic polyamide step-growth polymer. 

Polyurethane (Section 31.4) A step-growth polymer prepared by reaction between a diol and a diisocyanate. 

Step-growth polymer (Sections 21.9, 31.4) A polymer in which each bond is formed independently of the others. Polyesters and polyamides (nylons) are examples. 

Polyamides and Polyesters Step-Growth Polymers 818 21.10 Spectroscopy of Carboxylic Acid Derivatives 822... 

Synthetic Methods in Step-Growth Polymers. Edited by Martin E. Rogers and Timothy E. Long 2003 John Wiley Sons, Inc. ISBN 0-471-38769-X... 

INTRODUCTION TO SYNTHETIC METHODS IN STEP-GROWTH POLYMERS... 

STRUCTURE-PROPERTY RELATIONSHIPS IN STEP-GROWTH POLYMERS 3... 

---