Nylon, polymerization mechanism

CAROTHERS. WALLACE H. 0896-1937). Bom in Iowa. Carothers obtained his doctorate in chemistry at the University of Illinois. He joined the research staff of Du Pom in 1928, where lie undertook the development of polychioroprene (later called neoprene) that had been initialed by Nieuland" research on acetylene polymers. Carother s crowning achievement was the synthesis of nylon, the reaction product of hexamethylenetetramine and adipic acid. Carother s work in the polymerization mechanisms of fiber like synthetics of cyclic organic stoic-lures was brilliant and productive, and he is regarded as one of the most original and creative American chemists of the early 20"1 cenlury. 

Both industrial demand and academic interest stimulated detailed research of the synthesis of lactam polymers and elucidation of the polymerization mechanisms. Until recently, nylon 6 has been the only industrially produced lactam polymer and, therefore, most of the extensive investigations are dealing with the polymerization of caprolactam [2]. The mechanisms of the main elementary reactions derived for caprolactam are applicable to other lactams as well, except to the / /-substituted ones. 

This is a highly polar polymer and crystalline due to the presence of amide linkages. To achieve effective intercalation and exfoliation, the nanoclay has to be modified with some functional polar group. Most commonly, amino acid treatment is done for the nanoclays. Nanocomposites have been prepared using in situ polymerization [85] and melt-intercalation methods [113-117]. Crystallization behavior [118-122], mechanical [123,124], thermal, and barrier properties, and kinetic study [125,126] have been carried out. Nylon-based nanocomposites are now being produced commercially. 

Nylon-6-clay nanocomposites were also prepared by melt intercalation process [49]. Mechanical and thermal testing revealed that the properties of Nylon-6-clay nanocomposites are superior to Nylon. The tensile strength, flexural strength, and notched Izod impact strength are similar for both melt intercalation and in sim polymerization methods. However, the heat distortion temperature is low (112°C) for melt intercalated Nylon-6-nanocomposite, compared to 152°C for nanocomposite prepared via in situ polymerization [33]. 

Because membrane filtration is the only currently acceptable method of sterilizing protein pharmaceuticals, the adsorption and inactivation of proteins on membranes is of particular concern during formulation development. Pitt [56] examined nonspecific protein binding of polymeric microporous membranes typically used in sterilization by membrane filtration. Nitrocellulose and nylon membranes had extremely high protein adsorption, followed by polysulfone, cellulose diacetate, and hydrophilic polyvinylidene fluoride membranes. In a subsequent study by Truskey et al. [46], protein conformational changes after filtration were observed by CD spectroscopy, particularly with nylon and polysulfone membrane filters. The conformational changes were related to the tendency of the membrane to adsorb the protein, although the precise mechanism was unclear. 

The polymerization process for nylon 6 consists primarily of the three types of reaction illustrated in Fig. 23.6. Each of the reactions is reversible, tvith the equilibrium of the products being controlled primarily by the concentration of water in the reaction vessel. The reaction is initiated by the hydrolytic ring opening of caprolactam to form 6-aminohexanoic acid, as shown in Fig. 23.6 a). Chain extension of the type shotvn in Fig. 23.6 b) dominates when water is abundant (10 to 20%) in the reaction mixture. At lower water levels (2 to 5%) chains grow primarily by the mechanism shown in Fig. 23.6 c). In order to limit the average molecular... 

Y. Ou, F. Yang, J. Chen, Interfacial interaction and mechanical properties of nylon 6-potassium titanate composites prepared by In-situ polymerization, Journal of Applied Polymer Science, vol. 64, pp. 2317-2322,1998. 

Polymerization by a ring-opening reaction is confined to cyclic monomers which contain at least one heteroatom. The mechanism is very often a polyaddi-tion-type with a product which has a polycondensation-type character. For example, ethylene oxide and other cyclic esters can be polymerized into linear chains by this type of reaction. An even more complicated example of this type of polymerization reaction is the polymerization of e-caprolactam into Nylon 6 (PA 6). 

A related system is that of the lipid-bilayer corked capsule membranes which are formed from ultrathin (about 1 pm thick), spongy, 2.0- to 2.5-mm-diameter, more-or-less spherical nylon bags in which multiple bilayers are immobilized (Fig. 43) [343-345]. They were considered to combine the advantages of mechanical and chemical stabilities of polymeric membranes with the controllable permeabilities of surfactant vesicles. Polymerization of the bilayers, in situ,... 

The mechanism for the production of nylon-12 from the lactam is similar to that for nylon. However, in the case of nylon-12, the ring opening is more difficult and the rate of polymerization is slower, at least in part owing to the lower solubility of the lactam in water. A catalyst such as an acid, amino acid, or nylon salt can serve as a ring-opening agent. Nylon-12 can also be produced via anionic polymerization, ie, polymerization using an anhydrous alkali catalyst. This process can be quite fast even at low temperatures, eg, a few minutes at 130°C. 

Here a single number is used to indicate the number of carbon atoms in the original nomomer, i.e., nylon-11 ( eleven" not one-one ), in some instances the cyclic analogue or lactam is more accessible than the amino acid and is polymerized by a ring-opening rather than condensation mechanism ... 

Ergungor describes the application of on-line Raman spectroscopy and neural networks to the simultaneous prediction of temperature and crystallinity of nylon-6 nanocomposites as a function of cooling rate. The authors prefer their neural network approach because they make use of information in the entire spectrum rather than from a few bands as most studies have done.84 Van Wijk etal. of Akzo Nobel obtained a patent on the use of a Raman spectrum of a polymeric fiber to determine dye uptake and other structural or mechanical properties based on previously developed models.85... 

We focus our attention in this experiment on synthetic polymers and the basic mechanism by which some of them are formed. The two most important types of reactions that are employed in polymer manufacturing are the addition and condensation polymerization reactions. The first is represented by the polymerization of styrene and the second by the formation of nylon. 

Note also that nylon-6 was listed in Table 2-2 as the product of the ringopening polymerization of the cyclic monomer caprolactam. So here is an example of the same polymer that can be made from (at least) two different monomers by two different mechanisms. This illustrates the complexity of our classification and nomenclature schemes, because polycaprolactam,... 

Another interesting method of producing radicals in the solid state is the application of mechanical stress to polymeric materials. For example, Campbell and Peterlin (142) have produced free radicals with well-defined spectra by stretching nylon under vacuum within the ESR cavity. Such an effect is fascinating and makes one wonder if similar results can be obtained by subjecting other kinds of solids to various types of mechanical stress or high pressure. 

Wallace Carothers will be the subject of one of our Polymer Milestones when we discuss nylon in Chapter 3. Among his many accomplishments in the late 1920s and early 1930s, Carothers and his coworkers made a major contribution to the discovery and eventual production of the synthetic rubber, polychloroprene. It was synthesized from the diene monomer, chloroprene, CH2=CCI-CH=CHr Chloroprene, which is a very reactive monomer—it spontaneously polymerizes in the absence of inhibitors— was a product of some classic studies on acetylene chemistry performed by Carothers and coworkers at that time. In common with butadiene and iso-prene, in free radical polymerization chloroprene is incorporated into the growing chain as a number of different structural isomers. Elastomeric materials having very different physical and mechanical properties can be made by simply varying the polym-... 

Thus far, we have considered addition polymerization routes - either catalyzed or uncatalyzed. Although this is sufficient to describe the synthesis of common packaging materials such as polyethylene, polypropylene, polystyrene, etc., other classes of polymers such as nylon, PETE, and polyacrylamide are generated through step-growth mechanisms. Although the synthetic pathway for these polymers is more straightforward than addition polymerization, there are many intricate considerations that affect overall polymer properties. 

Organic matrices are divided into thermosets and thermoplastics. The main thermoset matrices are polyesters, epoxies, phenolics, and polyimides, polyesters being the most widely used in commercial applications (3,4). Epoxy and polyimide resins are applied in advanced composites for structural aerospace applications (1,5). Thermoplastics Uke polyolefins, nylons, and polyesters are reinforced with short fibers (3). They are known as traditional polymeric matrices. Advanced thermoplastic polymeric matrices like poly(ether ketones) and polysulfones have a higher service temperature than the traditional ones (1,6). They have service properties similar to those of thermoset matrices and are reinforced with continuous fibers. Of course, composites reinforced with discontinuous fibers have weaker mechanical properties than those with continuous fibers. Elastomers are generally reinforced by the addition of carbon black or silica. Although they are reinforced polymers, traditionally they are studied separately due to their singular properties (see Chap. 3). 

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