Cyclic oligomers, polyamides

FIGURE 17.19 2D contour plot of a modified polyamide 6, first dimension LCCC, second dimension SEC. Area 1 unmodified chains, area 2 modified chains, area 3 cyclic oligomers. (See color plate.)... 

The presence of even small amounts of cyclic oligomers can be detrimental for the utilization of a polymer if the cyclics migrate out of the product during its use. Many commercial processes remove cyclic by extraction (e.g., with steam in polyamide production) or thermal devolatilization (polysiloxane). 

The polyamide-6 (Nylon-6) so obtained has a crystallite melting point around 216 °C. It still contains monomer and low-molecular-weight cyclic oligomers which can be removed by extraction with water or lower alcohols.These oligomers can be separated and identified chromatographically (see Example 4-9). 

Polyamides prepared by cationic polymerization of lactams contain a significant fraction of cyclic oligomers. Reactions leading to formation... 

Most of the quantitative studies of the cyclic oligomer concentrations of polyamides have been concerned with nylon-6 (I50-I55), although some preliminary work has... 

Montaudo, G., Mass Spectral Determination of Cyclic Oligomers Distributions in Polymerization and Degradation Reactions, Macromolecules, 24, 5289, 1991. Ballistreri, A., Garozzo, D., Giuffrida, M., Impallomeni, G., and Montaudo, G., Primary Thermal Decomposition Processes in Aliphatic Polyamides, Polym. Deg. and Stab., 23, 25, 1988. 

Thermal degradation of various aliphatic polyamides was also studied by Py-EIMS, Py-field ionization (FI), and field desorption (FD)-MS. The Py-EIMS studies demonstrated that the polylactams mainly decomposed to cyclic oligomers, whereas the favored decomposition pathways of diamine-dicarboxylic acid-type polyamides were the cis elimination reaction and the cleavage of the amide bond, which also occurred in the polylactams with a large number of methylene groups, such as nylon 12. On the other hand, Py-FIMS of various diamine-dicar-... 

While deriving the dynamic batch-reactor models in Tables 7.3 and 7.4, the equal-reactivity-of-functional-groups assumption was made, so that kp is the rate constant for all forward linear polyamidation reactions and Ay is the rate constant for all reverse hydrolysis reactions involving amide links on linear chains. Let the rate constant for the forward cychzation reaction be Ay and the rate constant for hydrolysis of an amide link on cyclic oligomers be A . In practice all of these rate constants depend on the reactor temperature and... 

Conversely, the thermal degradation reactions of polyamides such as PA-6,10, for example, produce mostly linear or cyclic oligomeric fragments and monomeric units. Primary polyamide chain scission (C(O)-NH or NH-CH2 bonds), hydrolysis, homolytic scission, intramolecular C-H transfer and ds-elimination are all suggested from the product distribution as possible mechanisms 502570. For PA-12, it has been found that lactams (cyclic monomer and dimer) are the major primary products of the thermal degradation however, olefinic nitriles and a-olefins were also found. In the case of PA-6,12 the formation of cyclic oligomers has been found to occur as a result of thermal degradation, but also some dinitriles have been identified 502570. ... 

The chain-ring equilibria are often observed in systems of polyamides. Independently of the route of preparation of polymers (polycondensation, polymerization of lactams, or other cyclic amides, including cyclic oligomers) the ring-chain interconversions are often important and cannot be neglected. However, the equilibrium conditions usually are not reached and often only kinetic distributions can be observed. 

Carothers work included both, KC and TC polymerization, but neither he nor his coworker Rory realized and discussed this difference. Under the conditions used by Carothers, the syntheses of polyesters and polyamides were TC polycondensations, whereas the syntheses of polyalkanes from 1,10-dibromodecane and sodium metal were perfect KCPs (see Chap. 3). In 1950, Stockmayer and coworkers published two papers [1,2] which dealt with the formation of cylics in ring-chain equilibria during synthesis or thermal degradation of polymers. The first paper was focused on the calculation of distribution functions of the cyclic oligomers, whereas the second... 

But even for narrowly distributed polymers, the accuracy of determining accurate molecular mass distributions depends on several parameters. In addition, the ionization probabilities of species with different masses and/or functionalities (e.g., end groups) can be completely different. Thus, 1 1 mixtures of polymers with different end groups or different topologies (e.g., linear and cyclic oligomers, often observed in polyesters and polyamides (PAs)) but nearly identical molecular masses CcUi result in MALDI mass spectra that are dominated by peaks representing only either stmcture. Some efforts have been made to overcome this problem, for example, by a mathematical correction. ... 

Although rigid-rod poly(p-phenyleneterephthalamide) analogues having alkyl side chains did not contain cyclic polymers, the polycondensation of silylated m-phenylenediamine and aliphatic dicarboxyhc acid chloride afforded cyclic polyamides predominantly (Scheme 49) [187]. Furthermore, cyclic polymers were also produced in polycondensations for polyesters, poly(ether ketone)s, polyimides, and polyurethanes [183]. These examples are the products in polycondensation of AB monomers or in A2 + B2 polycondensations, but cyclization of oligomer and polymer was also confirmed in polycondensation of AB2 monomers [ 188-195] and in A2 + B3 [ 196-202] and A2 + B4 polycondensations [203-206], which afford hyperbranched polymers. 

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