Polymerization hydrolytic

Hydrolytic polymerization of e-caprolactam to form nylon 6 (Sec. 2-8f) is carried out commercially in both batch and continuous processes by heating the monomer in the presence of 5-10% water to temperatures of 250-270°C for periods of 12 h to more than 24 h [Anton and Baird, 2002 Zimmerman, 1988]. Several equilibria are involved in the polymerization [Bertalan et al., 1984 Sekiguchi, 1984]. These are hydrolysis of the lactam to e-amino-caproic acid (Eq. 7-56), step polymerization of the amino acid with itself (Eq. 7-57), and initiation of ring-opening polymerization of lactam by the amino acid. The amino acid is 

The overall rate of conversion of e-caprolactam to polymer is higher than the polymerization rate of e-aminocaproic acid by more than an order of magnitude [Hermans et al., 1958, I960]. Step polymerization of e-aminocaproic acid with itself (Eq. 7-57) accounts for only a few percent of the total polymerization of e-caprolactam. Ring-opening polymerization (Eq. 7-58) is the overwhelming route for polymer formation. Polymerization is acid-catalyzed as indicated by the observations that amines and sodium e-aminocaproate are poor initiators in the absence of water and the polymerization rate in the presence of water is first-order in lactam and second-order in COOH end groups [Majury, 1958]. 

Although step polymerization of e-aminocaproic acid with itself is only a minor contribution to the overall conversion of lactam to polymer, it does determine the final degree of polymerization at equilibrium since the polymer undergoes self-condensation. The final 

Amidines are formed in hydrolytic polymerizations of lactams but do not limit the polymer molecular weight. Molecular weight buildup is not impeded since the carboxyl end groups of growing polymer are quite reactive toward amidine groups [Bertalan et al., 1984]. 

The mechanism of chain growth in hydrolytic polymerization is similar to that of the cationic process under anhydrous conditions 21). In the first step the amide bond in the lactam is cleaved by H20 and is converted to primary amine and carboxylic acid functions  

Although water induces the polymerization, the true initiating species is the amino-caproic acid formed by hydrolysis. This acid is usually added to accelerate the polymerization. The preliminary formation of aminocaproic acid by hydrolysis of eCLM explains the increase in polymerization rate with increaseing [H20]. Some other carboxylic acid derivatives are also used in practice e.g. the hexamethylene diamine salt of the adipic acid (so called 6,6 salt). 

Once the aminoacid is formed polycondensation can contribute to chain growth  

In the polymerization of eCLM only a few percent of lactam is incorporated into the polymer through hydrolysis and bimolecular condensation. Most monomer enters the polymer through stepwise addition, i.e. the cationic chain growth 22)  

The kinetics and molecular weight distribution in the hydrolytic process were studied and described in detail by Reimschuessel27,28). The molecular weights and the equilibrium conversion decrease with an increase of temperature but this influence is not very important due to the narrow temperature range (220-260 °C) applied in practice. Polymerization is usually carried out with 1-5 % initiator (H20, aminocaproic acid, 6,6 salt) for a few hours. The process involves three steps l,29). In the first one 90% polymer with molecular weight Mn = 8000 to 14000 is formed. The second step consists of condensation of these chains into polymers with Mn = 18 000 to 

This reaction can be initiated by strong bases (metal hydrides, alkali metals, metal amides, metal alkoxides, and organometallic compounds), protonic acids, or by water. Water is often used as the initiator (cf. Problem 10.7) for industrial polymerization of lactams and the process is referred to as hydrolytic polymerization. Anionic initiation with bases is preferred when polymerization is to be carried out in the mold itself for converting monomer directly into a molded object. Cationic initiation with acids, however, is not so useful because both the extents of conversion and polymer molecular weights are significantly lower (Odian, 1991). 

The hydrolytic polymerization (Bertalan et al., 1984 Sekiguchi, 1984) of e-caprolactam can be carried out, on a laboratory scale, by heating a mixture of the monomer and water (2% by wt.) m a thick-walled polymerization tube with suitable safety precautions (Sorensen and Campbell, 1968). The water initiates the reaction by attacking the carbonyl group in the ring to form the open chain species, e-aminocaproic acid  

This is followed by step polymerization of the amino acid with itself  

The initial ring-opening [Eq. (10.35)] and subsequent propagation steps [Eqs. 

Employing the usual simplifying assumption, that the reactivity of the end groups are equal and independent of the chain length of the respective molecules, 

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It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

Starting with the cyclic lactams, by the hydrolytic polymerization method, three... 

Hydrolytic enzymes, 10 303 Hydrolytic polymerization, 19 748 Hydrolytic process, influences on, 10 281 Hydrolytic reactions, selective, 16 400 Hydrolytic stability, of ethylene-tetrafluoroethylene copolymers, 18 322-325... 

The Influence of Excess Citrate. J Am Chem Soc 89 5559 333b. Spiro TG, Pape L, Saltman P (1989) The Hydrolytic Polymerization of Ferric Citrate. I. The Chemistry of the Polymer. J Am Chem Soc 89 5555... 

Variations in ferritin protein coats coincide with variations in iron metabolism and gene expression, suggesting an Interdependence. Iron core formation from protein coats requires Fe(Il), at least experimentally, which follows a complex path of oxidation and hydrolytic polymerization the roles of the protein and the electron acceptor are only partly understood. It is known that mononuclear and small polynuclear Fe clusters bind to the protein early in core formation. However, variability in the stoichiometry of Fe/oxidant and the apparent sequestration and stabilization of Fe(II) in the protein for long periods of time indicate a complex microenvironment maintained by the protein coats. Full understanding of the relation of the protein to core formation, particularly at intermediate stages, requires a systematic analysis using defined or engineered protein coats. 

The polymerization of lactams (cyclic amides) can be initiated by bases, acids, and water [Reimschuessel, 1977 Sebenda, 1976, 1978 Sekiguchi, 1984]. Initiation by water, referred to as hydrolytic polymerization, is the most often used method for industrial polymerization of lactams. Anionic initiation is also practiced, especially polymerization in molds to directly produce objects from monomer. Cationic initiation is not useful because the conversions and polymer molecular weights are considerably lower. 

An alternative mechanism has been proposed by Schneider (1988) who considers that ferritin could be also filled via a transient, mononuclear Fe " species. This species is similar to Fe in size, but is more versatile in its interaction with the protein shell. Experiments have shown that as the pH of a system containing diferric-trans-ferrin and ferritin was lowered very slowly from 7.5 to 5.0, monomeric Fe was released from the transferrin and redeposited in the ferritin (Glaus, 1989). Calculations of the iron flux across the cell membrane and estimates of the rates of interaction of the mononuclear species with ferritin and with the cell mitochondria indicated that the steady state concentration of the mononuclear Fe species would be sufficiently low for this species alone to enter the protein shell and be deposited as the iron core. Uptake of this species by the protein shell is about fiftyfold faster than the rate of hydrolytic polymerization or even of the dimerization of Fe (tiy2 1 vs. 50 ms). This hypothesis suggests an interesting direction for further research. 

Hydrol5dic polymerization in the ferric citrate system can be prevented if sufficient excess citrate is present in solution 66). Approximately 20 millimolar excess citrate is sufficient to supress pol3mier-ization of 1 millimolar iron, as indicated by dialysis and spectrophotomet-ric measurements. From pH titration in high citrate solutions, it was concluded 66) that a dicitrate complex of iron is formed at high pH. Presumably formation of the dicitrate chelate is competitive with hydrolytic polymerization. The fraction of polymer formed in ferric citrate solutions was found to decrease smoothly as the citrate content was increased up to 20 millimolar. The nuclear relaxation rate of the water protons in ferric citrate solutions increases with the citrate concentration. 

Anionic ring opening polymerization of lactams to generate polyamides has been studied quite extensively by Sebenda [8-10], Sekiguchi [11], and Wichterle [12-13], among others, in academia, and by Gabbert and Hedrick [14] and by us [23-25] in industry. By far, caprolactam is the most studied lactam and the nylon 6 prepared by this route compares favorably in properties with that prepared by conventional hydrolytic polymerization. 

We saw in Section 13.6 that hydrolysis and subsequent polymerization of aqueous metal cations can lead to the precipitation of gels. In the case of Fe(H20)63+ in mildly acidic solutions, the polymerization sequence of Eqs. 13.25 and 13.26 and Fig. 13.6 first reversibly forms cationic colloidal spherules, 2-4 nm in diameter, with the structure of 7-Fe0(0H) [double chains of Fe(0,0H)6 octahedra] on a timescale of about 100 s. These lose H+ and harden over several hours and then, over several days, form aged polymer rods, then rafts, and ultimately, after several months, needles of solid goethite [cc—FeO(OH)].1,2 Thus, aging is an important feature of hydrolytic polymerization. 

The polymeric nature of zirconium in aqueous systems is similar to that of titanium compounds. However, zirconium compounds tend to be significantly more stable towards hydrolytic polymerization. 

The base catalysed caprolactam polymerization differs substantially in various aspects from the wellknown industrial polymerization process, which is introduced by water, and the mechanism of which is governed by endstanding carboxylic and amino groups. Therefore it is evident, that the broad experience gathered in the hydrolytic polymerization of caprolactam cannot be applied to the base polymerization, which was developed separately and which requires an independent theoretical treatment. 

As compared with the hydrolytic polymerization, the base catalysed polymerization is essentially more sensitive towards impurities. The inhibition or retardation by traces of various foreign substances follows from the extremely low concentrations of the imide formed by the disproportionation reaction ranging about in the order of magnitude 10 2mol-%, and from the high reactivity of the imide. The imides are the key intermediate in the polymerization mechanism and hence every substance which is able to react with imides acts either as an inhibitor or as a retarder of the base catalysed polymerization of caprolactam (67). 

Borane polymers were prepared by the Rohm and Haas Corp. by using the Friedel-Crafts addition to an olefinic linkage followed by a typical silicone hydrolytic polymerization. 

Although several cyclic amides (lactams) can be polymerized by cationic mechanism, this method of polymerization is of little practical importance because the anionic or hydrolytic polymerization provides much more convenient route to corresponding polyamides. Polyamides obtained by cationic polymerization of lactams are less stable and oxidize faster than those obtained by anionic polymerization [213). 

Undoubtedly the most unlikely substrate for ferritin is U02 +, however, the imphcations of biomimetically synthesizing uranium oxide loaded ferritin could have use in neutron capture therapy. The synthetic approach utilized ion binding of U02(02CCH3)2 according to a known stoichiometry of 12 ions per ferritin molecule, followed by hydrolytic polymerization of metal ions within apo-ferritin. Characterization by TEM analysis confirmed dense cores of polymerized uranyl oxyhydroxide particles of 6 nm in diameter. 

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