Nylons

The nylons are named by the number of carbon atoms in the repeat units. The materials formed by ring opening polymerizations of lactams, therefore, carry only one number in their names, like, for instance, nylon 6 that is formed from caprolactam. By the same method of nomenclature, a nylon prepared by condensing a diamine with a dicarboxylic acid, like, for example, hexamethylene diamine with adipic acid, is called nylon 6,6. It is customary for the first number to represent the number of carbons in the diamine and the second number to represent the number of carbons in the diacid. A discussion of various individual nylons follows. Not all of them are industrially important. At present, we only know how to prepare Nylon 1 by anionic polymerization of isocyanates:

Potassium and sodium cyanide catalyze the reaction. It can be carried out between 20 and 100C. An example is the following preparation [47]:

The resultant polymer has a molecular weight of one million. When the methyl group is replaced by butyl, the product is a tough film former, but depolymerizes in the presence of some catalysts. Many other interesting high molecular weight polymers with various substitutions can be formed by this reaction at low temperature. Polymerization of N-carboxy-a-amino acid anhydrides results in formations of Nylon 2. This reaction is also discussed in Chap. 5. These polymers are mainly of interest to protein chemists in model studies of naturally occurring poly (a-amino acids). Nylon 3 can be synthesized by intramolecular hydrogen transfer polymerization of acrylamide [48]:

The fibers from nylon 3 are reported to resemble natural silk [50]. They possess high water absorbency and good light and oxygen stability. The polymer, however, is too high melting for melt spinning, or for molding and extrusion [46]. Nylon 3 fibers can be spun, however, from special solutions containing formic acid [52]. It is difficult to synthesize b-propiolactam. A synthetic route, however, was found for substituted propiolactams like b-butyrolactams [50]. The compounds form by nucleophilic additions of carbonylsulfamoyl chloride to olefins [51]:

The above shown b-butyrolactam polymerizes readily by anionic mechanism, yielding a very high molecular weight polymer. This lactam preparation reaction is quite general. It can be carried out on propene, 1-butene, 1-hexene, and styrene [50]. Although substituted lactams are harder to polymerize [53], the four-membered lactams exhibit such a strong tendency toward ring opening that even substituted b-propiolactams polymerize well [50]. The rate of polymerization, however, does tend to decrease with the number of substituents. Nylon 4 or polypyrrolidone is an attractive polymer for use in fibers. The original syntheses of nylon 4 from 2-pyrrolidone were carried out by alkaline catalyzed ring opening polymerizations promoted by N-acylpyrrolidone [49]. The products from these reactions melt between 260 and 265C.

They are unstable at these temperatures and cannot be melt spun. Fibers, however, were prepared by dry spinning from hydrocarbon suspensions [49]. Later, it was found that when the anionic ring opening polymerizations of 2-pyrrolidone are activated by CO2 in place of the N-acyl derivative, the resultant higher molecular weight product has much better heat resistance [54]. This “new” nylon 4, reportedly, can be melt spun. Nylon5orpoly(a-piperidone) can be prepared by ring opening anionic polymerization of valerolactam [53]. The reaction requires very pure monomer to yield a high molecular weight polymer [53]:

One route to valerolactam is from cyclopentadiene:

Valerolactam can also be polymerized with the aid of coordination catalysts to a high molecular weight polymer using alkali metal-Al(C₂H₅)₃ catalysts or alkali metal alkyl-Al(C₂H₅)₃ catalysts [55]. The polymerizations require relatively long times. Nylon 6 is obtained via ring opening polymerization of caprolactam:

This polymer developed over the years into an important commercial material. As a result, many preparatory routes were developed for the starting material and the polymerization reaction was studied thoroughly. The most commonstarting materials for preparations of caprolactam are phenol, cyclohexane, and toluene. Some caprolactam is also made from aniline. In these synthetic processes, the key material is cyclohexanone oxime. The route based on phenol can be shown as follows:

A by-product of the above reaction is ammonium sulfate. To avoid the necessity of disposing of ammonium sulfate, many caprolactam producers sought other routes to the oxime. One approach is to form it directly by reacting cyclohexanone with ammonia and hydrogen peroxide in the presence of tungstic acid catalyst [56]:

The reaction is conducted in water and the product oxime is extracted with an organic solvent. Another process is based on photo-nitrosyl chlorination. Here cyclohexane is converted in one step to cyclohexanone oxime hydrochloride [57]:

Another process uses ketene to form cyclohexene acetate [58]:

Among some more recent developments is a one-step synthesis of caprolactam from cyclohexanol [59]:

There are other processes for caprolactam syntheses as well; however, a thorough discussion of this subject belongs in books dedicated to the subject. The mechanism of the reaction of ring opening polymerization of lactams is discussed in Chap. 5. Several important side reactions accompany this polymerization. One is formation of cyclic oligomers [58]. The cyclic oligomers, soluble in water and alcohol mixtures, range in size from cyclic dimers to cyclic nonamers [60–62]. Formation of these compounds may be governed by equilibrium [63]. The polyamide will also thermally oxidize upon prolonged exposure to heat and air. Another important side reaction is decarboxylation that occurs at high temperatures. This is a result of interaction of a carboxyl group with a molecule of caprolactam or with an amide group [58]:

Polymerizations of caprolactam should be conducted in inert atmospheres to prevent oxidative decompositions. These can result in formations of carbon monoxide, carbon dioxide, acetaldehyde, formaldehyde, and methanol. Caprolactam can even oxidize in air at temperatures between 70 and 100C [64], according to the following scheme:

Much of nylon 6 is used in producing fibers. Polycaprolactam prepared by water catalyzed polymerizations is best suited for this purpose. It can also be used in molding, though anionically polymerized caprolactam can be used as well [65]. The polymerizations are carried out both in batch and in continuous processes. Often tubular flow reactors are employed. Atypical polymerization reaction is carried out as follows. Caprolactam, water (5–10% by weight of monomer), and acetic acid (about 0.1%) are fed into the reactor under nitrogen atmosphere. The reaction mixture is heated to about 250C for 12 h. Internal pressure is maintained at 15 atmospheres by venting off steam. The product of polymerization is extruded as a ribbon, quenched, and chopped into chips. It consists of about 90% polymer and about 10% low molecular weight compounds and monomer. The polymer is purified by either water leaching at 85C or by vacuum extraction of the undesirable by-products at 180C. Castings of nylon 6 are commonly formed in situ in molds. Here the preparation of the polymer by anionic mechanism is preferred. The catalyst systems consist of 0.1–1.0 mole percent of acetyl caprolactam and 0.15–0.5 mole percent sodium caprolactam. The reaction temperature is kept between 140 and 180C. An exotherm can raise it as much as 50C as the polymerization proceeds. Nylon 7 and nylon 9 are part of a process developed in Russia to form polyamides for use in fibers. The process starts with telomerization of ethylene [66]. A free-radical polymerization of ethylene is conducted in the presence of chlorine compounds that act as chain-transferring agents. The reaction is

Carried out at 120–200Ctemperatureand400–600atmospheres pressure. The preferred chain transferring agents for this reaction are CCl₄ and COCl₂[66]:

The resultant chloro alkanes are then hydrolyzed with sulfuric acid:

After hydrolyses, the products are treated with ammonia:

The amino acid is condensed to a lactam and subsequently polymerized. Table 7.3 shows the composition of the telomers in the above free-radical polymerization [66]. As seen from the table, the economics of producing nylon 7 by this process is not as favorable as one may wish. An advantage, however, to producing nylon 7 is that the polymer contains little monomer and can be spun without washing or extraction, as is required with nylon 6.

An early synthesis of nylon 8 used cyclooctatetraene that was formed from acetylene and then converted to nylon 8 as follows:

The acetylene was later replaced by butadiene for economic reasons. Butadiene is cyclodimerized, then hydrogenated to cyclooctane, and the oxime is prepared directly from cyclooctane by photonitrosation:

Nylon 9 or poly(w-pelargonamide) is produced in Russia together with nylon 7, poly (aminoenanthic acid) as described above. In the U.S., Kohlhase et al. [67] developed a route to nylon 9 via ozonolysis of unsaturated fatty acids like those that can be obtained from soybean oil. The glycerol fatty acid esters of oleic, linoleic, and linolenic acids are transesterified with methanol to form methyl esters. The esters are then cleaved via ozonolysis to yield methylazelaldehyde and by- products that are removed. The purified product is reacted with ammonia and then reduced over Raney nickel to yield a methyl ester of the amino acid:

After hydrolysis and purification, the free amino acid is converted to high molecular weight polymers [68]. To date, nylon 9 has not been commercialized in the U.S., though the polymer has a high melting point of 209°C and is more flexible than nylon 6. It is also lower in water absorption. Nylon 11 was originally synthesized in France. The monomer, @-amino-undecanoic acid, is obtained from methyl ricinoleate that comes from castor oil. Methyl ricinoleate is first cleaved thermally to heptaldehyde and methyl undecylenate:

The ester is then hydrolyzed and converted to an amino acid:

Polycondensation is conducted in the melt under nitrogen at 215°C for several hours. The polymer is transparent in its natural form. It has high-impact resistance, low moisture absorption, and good low temperature flexibility. It is also manufactured in the U.S. Nylon 12 is produced in U.S., Japan, and Europe with the original development coming from Europe. All current manufacturing processes of this polyamide, formed by ring opening polymerization of lauryl lactam, are based on cyclododecatriene. This ring compound can be obtained by

trimerization of butadiene using Ziegler-Natta type catalysts. One patent reports using polyalkyl- titanate and dialkylaluminum monochloride [69]:

The cyclododecatriene is then converted to lauryl lactam by different processes. One of them consists of hydrogenation of the cyclic triene, followed by oxidation to a cyclic ketone, conversion to an oxime, and rearrangement by the Beckmann reaction to the lactam:

Another process utilizes photonitrosation:

There are still other processes, but they lack industrial importance. Nylon 12, like nylon 11, exhibits low moisture absorbency, good dimensional stability, and good flexibility at low temperatures.

Preparations of other nylons were reported from time to time in the literature. For one reason or another, however, they have not developed into industrially important materials. Thus, for instance, for some time now it has been known that nylon 13 can be prepared from erucic acid that is found in crambe and rapeseed oils. The polymer is supposed to be quite similar to nylon 11, though lower melting. The melting points of the nylons describe above are summarized in Table 7.4.

Nylon 6,6 is a condensation product of hexamethylene diamine and adipic acid. This polyamide was originally synthesized in 1935 and first produced commercially in 1938. It is still one of the major commercial nylons produced today. Because high molecular weight is required for such polymers to possess good physical properties, it is necessary to follow exact stoichiometry of the reactants in the condensation. To achieve that, the practice is to initially form a “nylon salt,” prior to the polymerization. To do this, equimolar quantities of adipic acid and hexamethylene diamine are combined in aqueous environment to form solutions of the salt. The end point is controlled electrochemically. An alternate procedure is to combine the diacid with the diamine in boiling methanol. A 1:1 adduct precipitates out, is filtered off, and dissolved in water.

A 60–75% solution of the salt in water is then fed into a reaction kettle. In a typical batch process, some acetic acid may also be added if it is desired to limit molecular weight (10,000–15,000). The temperature in the reaction kettle is raised to 220C, and due to water and steam in the reactor, internal pressure of about 20 atmospheres develops. After 1–2 h, the temperature is raised to 270–280C. Some steam is bled off to maintain internal pressure at 20 atmospheres. The temperature is maintained and the bleeding out of the steam is continued for 2 h. During that period, the internal pressure is gradually reduced to atmospheric. In some processes, vacuum is applied at this point to the reaction kettle if high molecular weight products are desired. When the reaction is complete, the molten polymer is ejected from the kettle by applying pressure with nitrogen or carbon dioxide. In one continuous process, the desired conditions are maintained while the reaction mixture moves through various zones of the reactor. Tubular reactors are also often employed in continuous polymerizations. Nylon 6.10 is prepared by the same procedure as nylon 6,6 from a salt of hexamethylene diamine and sebacic acid, while nylon 6,9 is prepared from a salt of hexamethylene diamine and azelaic acid. The melting points of various nylons that are formed from diamines and dicarboxylic acids are presented in Table 7.5. One commercial polyamide is prepared by condensation of a cycloaliphatic diamine with a twelve carbon dicarboxylic acid. The diamine, bis(p-amino cyclohexyl)methane, is prepared from aniline:

The diamine is then condensed with dodecanedioic acid, which is obtained from cyclodode- catriene. The structure of this polyamide is as follows:

The polymer has a Tm of 280-290°C and a Tg of 120°C. It exhibits lower moisture pick up than do nylons 6 and 6,6, increased hardness and tensile strength, though lower impact strength. The bulk of this polymer is used in fiber production. The fibers, with a trade name of "Quiana," are claimed to exhibit high luster and a silk like feel.

A number of copolyamides are manufactured commercially to suit various needs. One of them is a polyamide formed by condensation of trimethylhexamethylene diamine with terephthalic acid. The diamine is a mixture of 2,4,4 and 2,2,4 isomers:

The mixture of the two isomers is synthesized from isophorone according to the following scheme:

The mixture of the isomers of trimethyl adipic acids is treated with ammonia, converted to amides, dehydrated to nitriles, and reduced to amines:

This polyamide is prepared somewhat differently. Salts of the diamine isomers with terephthalic acid are only partially polycondensed and the reaction is completed during extrusion [71], because the melt viscosity of the polymer is very high. The product is amorphous and exhibits greater light trasmittancy. It melts at 200C and is sold under the trade name of Trogamid T. Many commercial nylon copolymers are also formed by melt mixing different nylons. Amide interchange reactions occur at melt conditions. At first block copolymers form, but prolonged heating and stirring results in formation of random copolymers. Nylon copolymers are also best prepared directly from mixed monomers.

Nylon polymers generally exhibit high-impact strength, toughness, good flexibility, and abrasion resistance. The principal structural differences between many nylons are in the length of the aliphatic segments between the amide linkages. As a result, the differences in properties depend mainly upon the amount of hydrogen bonding that is possible between the functional groups and on the amount of crystallinity. Also, due to high cohesive energy, nylons are soluble in only a few solvent. The melt viscosity of these materials, however, is generally low In any one series of melting points of polyamides, polymers that contain even numbers of methylene groups between amide linkages fall on a higher curve than those that contain odd ones do [72]. This is due to the crystalline arrangement of the polymeric chains [72, 73]. A zigzag planar configuration of polymers with even number of methylene linkages allows only 50% of the functional groups to form hydrogen bonds. This same configuration, however, allows polymers with odd numbers of methylene linkages to form 100% hydrogen bonding [72, 73]. Polyamides, like nylon 6,6 or nylon 6,10, arrange themselves in pleated sheets during crystallization and hydrogen bonds form between N–H group of one molecule and the C¼O moieties from a neighboring one.

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الآخبار الصحية

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المزيد

الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

Aromatic Polyamides

Special Reactions for Formation of Polyamides

Fatty Polyamides

Polyesters from Lactones

Polycarbonates

Network Polyesters for Surface Coatings

Linear Unsaturated Polyesters

Copolyesters

Synthetic Methods

Linear Saturated Polyesters

Techniques of Polymer Preparation

Ring Formation in Step-Growth Polymerizations