Step-growth polymerizations polyamides

Even within a particular class of polymers made by step-growth polymerization, monomer composition can be varied to produce a wide range of polymer properties. For example, polyesters and polyamides can be low-Tg, amorphous materials or high-Tg, liquid crystalline materials depending on the monomer composition. 

Nearly all of the polymers produced by step-growth polymerization contain heteroatoms and/or aromatic rings in the backbone. One exception is polymers produced from acyclic diene metathesis (ADMET) polymerization.22 Hydrocarbon polymers with carbon-carbon double bonds are readily produced using ADMET polymerization techniques. Polyesters, polycarbonates, polyamides, and polyurethanes can be produced from aliphatic monomers with appropriate functional groups (Fig. 1.1). In these aliphatic polymers, the concentration of the linking groups (ester, carbonate, amide, or urethane) in the backbone greatly influences the physical properties. 

Linear step-growth polymerizations require exceptionally pure monomers in order to ensure 1 1 stoichiometry for mutually reactive functional groups. For example, the synthesis of high-molecular-weight polyamides requires a 1 1 molar ratio of a dicarboxylic acid and a diamine. In many commercial processes, the polymerization process is designed to ensure perfect functional group stoichiometry. For example, commercial polyesterification processes often utilize dimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) to form the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ. 

Step growth polymerization. Important polymers manufactured by step growth are polyamides (nylons), polyesters, and polyurethanes. 

The most common form of step growth polymerization is condensation polymerization. Condensation polymers are generally formed from simple reactions involving two different monomers. The monomers are difunctional, having a chemically reactive group on each end of their molecules. Examples of condensation polymerization are the formation of nylon 66, a polyamide, and of poly(ethylene terephthalate), a polyester. Because condensation poly-... 

Write a mechanism for a step-growth polymerization, as in the formation of a polyester, polyamide, polyurethane, epoxy resin of phenol-formaldehyde polymer. 

Note that the polymerizations of lactones and lactams give polyesters and polyamides, respectively, polymers that can also be synthesized by conventional step-growth polymerization from noncyclic monomers. Although the polymer prepared from cyclic monomers looks like a condensation polymery in strict terms it is not, because no small-molecule byproduct was pro-... 

This experiment is an example of a step-growth polymerization that takes place at the interface of two immiscible solutions. For this reaction, a diamine dissolved in water reacts with a diacid chloride that is dissolved in an organic solvent. Because neither of the monomers is soluble in the solvent containing the others reaction can occur only at die surface4 or interface between the two solutions. The product is a polyamide, either nylon-6,6 or nylon-6,10, depending upon the number of carbon atoms in the diacid chloride chosen. This activity works well either as a laboratory experiment or as a demonstration. 

Condensation polymers result from formation of ester or amide linkages between difunctional molecules. Condensation polymerization usually proceeds by step-growth polymerization, in which any two monomer molecules may react to form a dimer, and dimers may condense to give tetramers, and so on. Each condensation is an individual step in the growth of the polymer, and there is no chain reaction. Many kinds of condensation polymers are known. We discuss the four most common types polyamides, polyesters, polycarbonates, and polyurethanes. 

Otto Bayer was aware of the work of Staudinger and Carothers. He was particularly impressed with the latter s discovery of polyamides and its implications for the fiber and textile industries. He knew that in the condensation (linear step-growth) polymerization used to prepare nylon and polyesters a small molecule, usually water, is formed and has to be removed. 

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

Step-growth polymers, also called condensation polymers, are formed when mono- -mers containing two functional groups come tc ether and lose a small molecule such as H2O or HCl. In this method, any two reactive molecules can combine, so the monomer is not necessarily added to the end of a growing chain. Step-growth polymerization is used to prepare polyamides and polyesters, as discussed in Section 22.16. 

Nylons are polyamides formed by step-growth polymerization. In Section 22.16A, we learned that nylon 6,6 can be prepared by the reaction of a diacid chloride and a diamine. Nylon 6,6 can also be prepared by heating adipic acid and 1,6-diaminohexane. A Br0nsted-Lowry acid-base reaction forms a diammonium salt, which loses H2O at high temperature. In both methods, each starting material has two identical functional groups. 

Nylon 6 is another polyamide, which is made by heating an aqueous solution of e-caprolactam. The seven-membered ring of the lactam is opened to form 6-aminohexanoic acid, the monomer that reacts with more lactam to form the polyamide chain. This step-growth polymerization thus begins with a single difunctional monomer that has two different functional groups, NH2 and COOH. 

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

Step-growth Polymerization The simplest scheme of this polymerization involves the reaction of a difunctional monomer AB, which contains both functional groups A and B in the molecule. For example, A can be an amine and B a carboxylic acid group. Another scheme involves the reaction between two difunctional monomers of the type AA and BB. In any case, each polymer linkage will have involved the reaction of the functional groups A and B coming from two molecules (monomers or chains). Some examples of polymers synthesized by this mechanism are polyurethane, polyamide, and polyester. 

Step-growth polymerization is a very important method for the preparation of some of the most important engineering and specialty polymers. Polymers such as polyamides [7], poly(ethylene terephthalate) [8], polycarbonates [9], polyurethanes [10], polysiloxanes [11], polyimides [12], phenol polymers and resins, urea, and melamine-formaldehyde polymers can be obtained by step-growth polymerization through different types of reactions such as esterification, polyamidation, formylation, substitution, and hydrolysis. A detailed list of reaction types is shown in Table 3.2. 

The polyamides can be prepared by ring-opening reaction of the correspondent lactam according to its mechanism, this reaction cannot be properly considered as a step-growth polymerization. 

Step-Growth Polymerization Polyurethanes, Polyether-esters, Polyamides... 

Step-growth polymerizations can also be carried out with certain monomers at low temperature by a technique known as interfacial polymerization or interfacial polycondensation Tht reactions (applicable only to fast reactions) are conducted at the interface between two inuniscible liquids. Usually, one of the liquids is water and the other an organic solvent. An example may be a Schotten-Baumann polyamidation reaction. In such an interfacial polymerization, the diamine would be in the aqueous phase and the diacid chloride in the organic phase. The strong reactivity of acid chloride groups with amines allows the reaction to be carried out at room temperature ... 

The second general pattern of polymerization is step-growth or condensation polymerization. While terminal alkenes are the most common monomers in chain-growth polymerization, bifunctional molecules are the characteristic monomers for step-growth polymerization. The polyester-, polyamide-, and polyurethane-forming reactions shown below are examples of step-growth polymerizations ... 

Other commercially important polymers produced by AA and BB step-growth polymerization include aliphatic polyamides of the form ... 

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