[rogixmalix]

Can anyone help me with this exercise?

Nylon is widely used for fiber making, with a wide variety of
differ slightly, but always containing amide groups and methylene chains. The reaction below
exemplifies the general formation of nylon-n, m, where n and m indicate the number of carbons between the functions in
each initial reagent. As Technical Responsible for an Industry Quality Control Lab
manufacturing of textile components, how would it be possible to ensure that the formation reaction of nylon-6,6 was
successful using infrared absorption spectroscopy? It would be possible to identify the
chain size using absorption spectroscopy in the ultraviolet region or the region of the
infra-red? Justify your answer.

Picture corresponding to the exercise: https://imgur.com/a/w0ugPNO

[Corribus]

Using polyethylene as an example, shorter chains have more methyl groups so in principle you should be able use FTIR to say something about MW. But in practice it's not that simple. Although there are many things you can learn about polymer structure from FTIR, molecular weight isnt an easy one to do.

[hollytara]

I think they are asking if you can use IR to distinguish between say Nylon 6,6 and Nylon 6,10 for example. 

The answer is "yes" - if you have authentic samples to use for comparison, it is relatively simple to match spectra.

But it is not as easy to predict the spectra from the structures alone.  There are some software packages that can do a decent job. 

They may be asking if you can tell if the synthesis is successful (for example, 1,6-hexanediamine and 1,6-hexanedicarbonyl chloride to nylon 6,6). 

Again - the answer is yes - the carbonyl absorbance of the acid chloride and the amide are sufficiently different.

Establishing MW from IR - not possible.

[pgk]

You cannot conclude about the quantitative formation of Nylon 6/6 because the ratio between carbonyl and proton acompanied, nitrogen groups doesn’t change during polymerization and thus, molecular weight variations are not clearly visible in IR spectroscopy neither in UV spectroscopy.
PS: Regardless if having a different absorbance, low amounts apidoyl chloride impurities (if being a starting material) are not visible in IR spectroscopy, due to carbonyl bands overlapping.

[hollytara]

Actually, since adipoyl chloride carbonyl is at 1800 cm-1, (https://webbook.nist.gov/cgi/cbook.cgi?ID=C111502&Type=IR-SPEC&Index=1#IR-SPEC)  and nylon 6,6 carbonyl is at 1600 cm-1 (https://www.piketech.com/files/pdfs/Analysis_of_Polymers_ATR_FTIR_AN.pdf - see blue spectrum in Figure 4).  This is sufficient separation - and the spectrum of Nylon is pretty clear above 1600 - that any unused monomer above a few percent might be visible - trace amounts might still be undetected.  Adipoyl chloride is pretty reactive with water, and the adipic acid has a carbonyl much closer to the 1600 cm-1 Nylon carbonyl stretch. 

[wildfyr]

Excellent answers everyone, however a gentle mod reminder that the OP did not demonstrate his own attempt to solve the question. Next time please coax them to a bit more thinking before providing your excellent expertise.

[pgk]

Wildfyr, thanks for the gentle reminder but until the next time:
Hollytara, my previous reply referred to whether the polymerization was complete or not, as well as to conclusions about the molecular weight of the polymer.
Indeed, you can distinguish apidoyl chloride and nylon 66 at ≈1800 cm-1 and 1630-1640 cm-1, respectively but you cannot conclude about the end of nylon 66 polymerization by IR spectroscopy.
To detail, apidoyl chloride absorbs at 1790 cm-1 in CCl4 solution 10%, as reported in the NIST webbook. However and as being an impurity, apidoyl chloride is now dissolved in the more polar nylon 66 (solid solvent) and thus, the carbonyl wavelength number is decreased. Moreover and as being in low amounts, the carbonyl wavelength number is further decreased, due to massive formation of hydrogen bonds with the amide protons. Furthermore, the (solid) solvent is a dicarboxylic acid derivative and thus, significant interactions with the baseline decrease the intensity of the corresponding (small) carbonyl peak that tends to be overlapped by the neighboring (giant) amide peak.
Consequently, you cannot conclude about the end of nylon 66 polymerization by IR spectroscopy.
1). Solvent-Independent Anharmonicity for Carbonyl Oscillators, Journal of Physical Chemistry B, 121(10), 2331-2338, (2017)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5453717/
2). Issues in the Quantitation of Functional Groups by FTIR Spectroscopic Analysis of Impactor - Collected Aerosol Samples, Aerosol Science and Technology, 35(5), 899-908, (2001)
https://www.tandfonline.com/doi/pdf/10.1080/02786820126852

PS: In practice and apart exceptional cases, you cannot measure concentrations < 5-6% by IR quantitative spectrometry. 

[hollytara]

Wildfyr - my apologies if we were too specific - I was at first attempting to clarify what the question was asking. 

PGK - my reply specified that unused monomer would only be observed above a few percent - whether it is 3% or 5% is a trivial difference.
 
Industrial preparation of nylon does not usually use adipoyl chloride with hexamethylene diamine but instead uses the salt of adipic acid and hexamethylene diamine, which is actually a much more difficult IR problem than adipoyl chloride! 

The original question asks if nylon formation was "successful" not that it had gone to completion - these kinds of condensation polymerizations don't ever go to completion, unless there is an imbalance in the reagents (which then limits the molecular weight, but that is another story).  The adipoyl chloride reagent is usually used when the polymerization is done as an interfacial polymerization in a research lab or as a demonstration.

There may be some change in the carbonyl of adipoyl chloride when mixed with nylon - but will it be more than 50 cm-1?  The JPC article you reference shows a change in acetophenone carbonyl stretch from 1700 cm-1 in hexane to 1680 in DMSO and 1670 in water - will adipoyl chloride in nylon be more than the 30 cm-1 shown here?  Carbamoyl chlorides (like pyrrolidinecarbamoyl chloride) have a stretch at about 1750 cm-1; this isn't really a solvent effect but a direct resonance effect on the carbonyl, which is expected to be greater. 

[pgk]

Indeed, the industrial production of Nylon 66 is based on the polycondensation of adipic acid and hexamethylene diamine and not on the adipoyl chloride. In general, chlorine chemistry is not preferred in the chemical industry due to authorities specifications about environmental safety and waste management. But I did not want to discuss this issue in detail, at the beginning.
On the other hand, polymerizations must be complete (almost quantitative) in order to be successful otherwise, the polymer materials have not strong enough, mechanical properties that are adequate for technical applications. Consequently, incertitude in the range ≈ 3-5% about the polymerization end, is not tolerated.
Getting to the point:
Although esterifications, transesterifications, direct amidations, transmidations and the like are reverse reactions in equilibrium, they occur almost quantitatively during polymerizations. As an indicative example, continuous removal of water during Nylon 66 polymerization pushes the equilibrium to right (Le Chatelier’s principle) and spectacularly increases the reaction yield. Moreover and as the molecular weight increases, the polymer is continuously transformed to a more hydrophobic material that kicks out any remaining amount of water and further pushes the reaction equilibrium to the right with almost quantitative yields.
In general: the more hydrophobic the formed product, the higher the yields of esterification and direct amidation, are; when prepared from hydrophilic starting materials.