Step-growth polymers polyurethanes

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. 

Step-growth polymers, the second major class of polymers, are prepared by reactions between difunctional molecules, with the individual bonds in the polymer formed independently of one another. Polycarbonates are formed from a diester and a diol, and polyurethanes are formed from a diisocyanate and a diol. 

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

Condensation polymers, which are also known as step growth polymers, are historically the oldest class of common synthetic polymers. Although superseded in terms of gross output by addition polymers, condensation polymers are still commonly used in a wide variety of applications examples include polyamides (nylons), polycarbonates, polyurethanes, and epoxy adhesives. Figure 1.9 outlines the basic reaction scheme for condensation polymerization. One or more different monomers can be incorporated into a condensation polymer. 

Step-growth, or condensation, polymers are usually formed in a reaction between two monomers, each of which is at least difunctional. Polyesters, polyamides, polyurethanes, and epoxy resins are typical examples of step-growth polymers. These polymers grow by steps or leaps rather than one monomer unit at a time. 

Many of our common polymers are made by this type of reaction, including polyesters, nylons, polycarbonates, polyurethanes, and polyaramids (e.g., Kevlar). Many natural polymers, including cellulose, starch, proteins, and polynucleotides are also step-growth polymers. 

Interfaciarpolymerization can be used to make many types of step-growth polymers such as polyamides, polyesters, polycarbonates, and polyurethanes. Although most step-growth polymers are prepared by a melt process, somd specialty polymers are prepared by the interfacial technique, allowing rapid reaction at low temperatures. 

The synthesis of step-growth polymers—polytimides such as nylon and Kevlar, polyesters such as Dacron, polyurethanes such as spandex, and polycarbonates such as Lexan (Section 30.6)... 

Polyurethane (Section 30.6C) A step-growth polymer that contains many — NHC(=0)0— bonds in its backbone, formed by reaction of a diisocyanate and a diol. 

Dodge, Polyurethanes and Polyureas Synthetic Methods in Step-Growth Polymers, 197, John Wiley Sons, Inc., New York (2003). 

Polymers in this category are synthesized by routes similar to the routes used to synthesize regular step-growth polymers. The difference, however, is that the monomer units contain a metal-metal bond. A sample step-growth polymerization reaction is shown in Eq. 7.1, which illustrates the reaction of a metal-metal bonded dialcohol with hexamethylene diisocyanate (HMDI) to form a polyurethane.4... 

The repeat groups for several well-known step-growth polymers of industrial importance are shown in Table 7.1. Step-growth polymerizations that produce small byproduct molecules (aU examples in Table 7.1, except for polyurethane) are commonly called condensation polymerization. 

Condensation polymer (See Step-growth polymer. Section 17.12, and Special Topic C in WileyPLUS) A polymer produced when bifunctional monomers (or potentially bifunctional monomers) react with each other through the intermolecular elimination of water or an alcohol. Polyesters, polyamides, and polyurethanes are all condensation polymers. 

In general, step-growth polymers are usually condensation polymers, while chain-growth polymers are usually addition polymers, but there are exceptions, such as polyurethanes. Polyurethanes are made up of repeating urethane moieties, also sometimes called carbamate moieties. 

This process is not a condensation, because it does not involve the loss of a small molecule. Polyurethanes are technically addition polymers, but nevertheless, they are step-growth polymers because the growing polymer chain has two growth points rather than one. This example illustrates that step-growth polymers cannot always be classified as condensation polymers. Polyurethanes are used for insulation in the construction of homes and portable coolers. 

Polyurethanes (PUR) are step-growth polymers synthesized by the reactions of polyols and polyisocyanates, generally in the presence of basic or organometallic catalysts. 

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