Polyamides, formation

Other Preparative Reactions. Polyamidation has been an active area of research for many years, and numerous methods have been developed for polyamide formation. The synthesis of polyamides has been extensively reviewed (54). In addition, many of the methods used to prepare simple amides are appHcable to polyamides (55,56). Polyamides of aromatic diamines and aUphatic diacids can also be made by the reaction of the corresponding aromatic diisocyanate and diacids (57). 

Condensation polymers containing the dioxolane ring have been prepared (76MI11102) from 4,5-bis(ethoxycarbonyl)-l,3-dioxolane (90). Polyamide formation occurs rapidly at room temperature in protonic solvents (Scheme 24). 

Besides polymerization, another type of polyreaction can be used for stabilizing model membrane systems. Recently, Fukuda et al.28) described polyamide formation via polycondensation in monolayers at the gas/water interface (definition of mono-layers see Sect. 3.2). Long-chain esters of glycine and alanine were polycondensed to yield non-oriented polyamide films of polyglycine and polyalanine. 

All polycondensation reactions were followed by FT-IR-spectroscopy. Polyamide formation could be demonstrated by the disappearance of the ester C = 0 stretching mode and simultaneous formation of amide C = 0 stretching modes32>. Additional proof for polymer formation is given by gel permeation chromatography. From the elution volumes a molecular weight between 2,000 and 10,000 can be estimated. 

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.

Rate coefficients and activation energies for polyamide formation in m-cresol"... 

Other monomers, such as methacrylamide and ethylenesulfonamide, react similarly. Useful catalysts include potassium tert-hutoxide, sodium methoxide, and phenylmagnesium bromide. Recent evidence suggests that both vinyl polymerization and polyamide formation may occur simultaneously. The mechanism has been the subject of some controversy, particularly with regard to the nature of the end groups and whether a chain reaction or step reaction is involved. The original literature or a recent review (12) may be consulted for details. 

The first method of making isocyanate-based foams was based on the reaction of a carboxyl-terminated polyester with an organic diisocyanate, e.g., toluene diisocyanate. The simultaneous reactions resulting in carbon dioxide generation and polyamide formation produced cellular plastics. 

Polyamide formation is entirely analogous to polyesterification and all of the earlier considerations apply. For polymers of the type AABB, nylon-6,6 is the best example. Thus poly(hexamethylene adipamide) is formed by the condensation reaction between hexam-ethylene diamine and adipic acid ... 

None of the enzymatic polymerizations have as yet been looked at with computational chemistry techniques. This is all the more surprising as the literature of the last five to ten years provides us with numerous examples of particularly computational simulations of enzyme-catalyzed ester and amide formation and cleavage. This prompted us to look into these enzymatic reactions with particular emphasis on polyamide formation and in this article we outline our initial results of a computational chemistry approach towards the mechanism of CALB-catalyzed polyamide formation. 

In contrast to this mechanistic concept, Schwab et al. [9] surprisingly found, that P-alanine itself is not a substrate for the CALB-catalyzed polyamide formation. This can be understood in the light of a recent publication by Hollmann et al. [16] who could demonstrate that organic acids having a pKa value of <4.8 irreversibly inhibit CALB, presumably by protonation of His224, which in turn prevents the deprotonation of Ser s necessary as part of the catalytic process. P-Alanine has a first pKa of 3.6 [17] which clearly is below the critical value. Therefore, if P-alanine is generated in larger amounts is will reduce the overall enzyme activity by deactivation. Furthermore, Schwab et al. [4] only achieved low... 

As mentioned earlier neither [3-alanine nor the P-alanine ester can be used for chain elongation and they rather slow down the polyamide formation from P-lactam when simultaneously employed in enzymatic polymerizations. As a consequence, the p-lactam molecule is not only the initiating structure but also the elongating building block. 

In the book, Condensation Polymers By Interfacial and Solution Methods, by P.W. Morgan,67 interfacial polyamide formation is stated to occur in the organic phase, that is, on the organic solvent side of the interface. Several proofs are presented in support of this statement. For instance, monofunctional acyl halides added to the difunctional acyl halide in the organic phase always lowered polymer molecular weights. However, monofunctional amines added to the difunctional amines in the aqueous layer did not always show this effect. In this latter case, partition coefficients became a factor, particularly when relative reactivities of the amines were comparable. Mass transfer rate of diamine across the interface into the organic phase was noted to be the rate-controlling step at all concentrations of diamine. 

In general, the reactions for polyamide formation are seldom catalyzed. The reactions to produce thermosets, such as melamine-urea or phenol-formaldehyde, require a basic or an acidic catalyst. Some polyurethane formation reactions are catalyzed by basic reactants. Equation 3.11 applies when the rate constant of the externally catalyzed reaction (k ) is much larger than that of the self-catalyzed... 

Even though water has a low solubility in carbon dioxide, it has been shown that it can also be effectively removed with supercritical carbon dioxide in formation of nylon 66 (a polyamide) [34]. In this polyamide formation, because of the reactivity of carbon dioxide with amines, instead of using hexamethylene diamine, reaction was carried out with a nylon salt. Carbon dioxide was shown to lower the melting point of the nylon salt and permit polymerization to proceed at lower temperatures. At temperatures around 270 and over a reaction time of about 3 hr at 3000 psi polyamides of high molecular weight (Mn = 25,000) have been produced. 

Two main types of polymerization procedures were employed. The traditional methods (Figure 2, lower path) use high temperatures (>200°C) for conversion in the melt and solid-state (7) or in polyphosphoric acid (PPA) solution (2,6). The milder, two-step procedure (Figure 2) involves polyamide formation and isolation followed by thermal cyclization to the polybenzoxazole. 

Polyamide formation can be demonstrated by FT-IR spectroscopy (disappearance of ester and formation of amide bands) and gel permeation chromatography of the polypeptides... 

Lacking a better alternative, an empirical dependence of rate parameters on conversion, for a given starting composition, must be tried. Examples of this approach will be discussed along with their respective chemical systems (bulk polyester and polyamide formation). It is seen that the glass effect could be taken into account using a similar procedure. 

The polyamide formation initially follows the enzymatic acylation of Seri05 by p-lactam. The reaction of the acyl-enzyme intermediate with water releases p-alanine, and the reaction with the growing oligomer yields poly (P-alanine). It has been shown that this mechanism is applicable only to i5-lactam and not to i5-alanine polymerization. " ... 

The reaction is usually carried out at high temperatures (of about 200°C) in a polar solvent, such as tetramethylene sulfone, and the polyamide formation can be accelerated by the addition of l-phenyl-3-methyl-2-phospholene 1-oxide as catalyst. However, in the case of a two-step process, the reaction time of the first step must be carefully controlled, since the catalyst can also play a role in the formation of carbodiimides from two terminal isocyanate groups [36], These carbodiimides can then further react and lead to crosslinking [36], In most cases [34-39], the polymers are prepared with 4,4 -methylene bis(phenyl-isocyanate) (MDI), using adipic acid, isophtahc acid, azelaic acid, or a mixture of two of them (in order to accelerate the solubilization of the polyamide phase in the solvent) and a polyether based on tetramethylene oxide, ethylene oxide, or a mixture of propylene oxide and ethylene oxide. 

Figure 1. Effect of CO pressure on aromatic polyamide formation between diiodoaromatics and dibromoaromatics. Reaction in DMAc (0.33 M), 90°C, 3% PdCl2L2, 6% PPhs and 1.2 equiv DBU. bromo, 1 atm iodo, 1 atm A bromo, 20 psig CO A iodo, 20 psig CO O bromo, 90 psig CO iodo, 90 psig CO. produced with permission from ref 7a.

---