[chel45]

Greetings!

I am now mastering the synthesis of a polymer that I need for my work. The polymer is polymerized by step polymerization, which requires reliable chemistry.

Now for me there was a problem in stability of results. The resulting products differ in viscosity, morphology (sometimes flakes, sometimes sponge). I know that it would not be bad to characterize the molecular weight and record the ftir spectra.

Here is the reaction scheme:



In the first reaction, an isocyanate and a diol are mixed to form a polyurethane bond. After that, diamine is used for polymerization. Typically the components are added in a ratio of PCL:BDI:Putrescine 1:2:1. If we consider only the polymerization phase, that the ratio of components is 1:1. Perhaps this ratio is the problem ...

 

And here is the gap. The number average molecular weight can be calculated using the Carothers equation

M_w = M₀ / (1 - p)

where p is the proportion of reacted active groups.

It can be seen that if p tends to 1, then the molecular weight tends to infinity. This is impossible, but the more accurately the ratios of components are selected, the more Mn will increase, at some point being limited by the viscosity of the solution. Therefore, under conditions of exponential growth in molecular weight, any slight variations in the ratios of the components will lead to a significant change in the molecular weight of the resulting polymer.

Below is a plot of Mn versus p (Carothers equation) over the range of interest to me (Mn ~ 50,000 - 200,000).



Do I understand correctly that in order to obtain a molecular weight in a given range, you need to reduce the amount of putrescine. That is, use a mixing ratio of 1:2:0.98 to get Mn ~ 100,000 (for a PCL molecular weight of 2,000). And the ratio is 1:2:0.99 for Mn ~ 200,000?

[hollytara]

You are assuming that your first step works perfectly, and all you get is the trimer: BDI-PCL-BDI.   And the isocyanate reactions are pretty fast.  But you may get some pentamers: BDI-PCL-BDI-PCL-BDI or heptamers etc.  With the excess isocyanate, you will have all isocyanate ends.  You can analyze this - take a little sample of the reaction mixture and quench it with methanol (to get the methyl urethane at the ends) and then analyze by GPC or HPLC (or even TLC might work on small things like this).  If you have different oligomer compositions, you final polymer will be different. 

For your polymerization step, where you take 1:1 oligomer di-isocyanate and diamine you are correct about the Carothers equation and the influence of slight mismatches in stoichiometry.  You of course did the extreme version with a 2:1 stoichiometry and get mostly trimers because fairly quickly all end groups are isocyanate.  It does not take much of a mismatch to limit the Mn pretty severely!  You may find it difficult to measure the quantities of materials with sufficient accuracy to use this effect, however: very small errors (impurities, water, solvents, etc.) can make a big difference.

The other factor that is commonly used to effect MW control is time.  People run the reactions under strictly controlled conditions (temperature, concentrations, stirring) and then quench the reaction at different times and evaluate the MW obtained.  This can generate a MW vs rxn time plot, and allows you to "pick" your MW by stopping the reaction at the correct time.  This is a common way to generate consistent MW in commercial polymerizations. 

[chel45]

You are assuming that your first step works perfectly, and all you get is the trimer: BDI-PCL-BDI.   And the isocyanate reactions are pretty fast.  But you may get some pentamers: BDI-PCL-BDI-PCL-BDI or heptamers etc.  With the excess isocyanate, you will have all isocyanate ends.  You can analyze this - take a little sample of the reaction mixture and quench it with methanol (to get the methyl urethane at the ends) and then analyze by GPC or HPLC (or even TLC might work on small things like this).  If you have different oligomer compositions, you final polymer will be different. 

For your polymerization step, where you take 1:1 oligomer di-isocyanate and diamine you are correct about the Carothers equation and the influence of slight mismatches in stoichiometry.  You of course did the extreme version with a 2:1 stoichiometry and get mostly trimers because fairly quickly all end groups are isocyanate.  It does not take much of a mismatch to limit the Mn pretty severely!  You may find it difficult to measure the quantities of materials with sufficient accuracy to use this effect, however: very small errors (impurities, water, solvents, etc.) can make a big difference.

The other factor that is commonly used to effect MW control is time.  People run the reactions under strictly controlled conditions (temperature, concentrations, stirring) and then quench the reaction at different times and evaluate the MW obtained.  This can generate a MW vs rxn time plot, and allows you to "pick" your MW by stopping the reaction at the correct time.  This is a common way to generate consistent MW in commercial polymerizations.

Thank for the answer. I see the process is very very sensitive. This is a bit difficult for a person who is not a chemist, but a biophysicist)

I see that the amount of BDI-PCL-BDI could be decreased in the first reaction. So it is no sense to decrease the amount of diamine in the second? Because for example if PCL will be decreased by 2% I may get the ratio 0.985:0.98 that may cause instability (of course it could be understood only after chromatography)? Or not? Could you give a practical advice with amount of diamine to use in the next try?

As I am doing the polymer for myself and not for commercial application, I'll be satisfied with just consistent Mw at the end of every synthesis. I think I can achieve this nut fixint all the parameters of the process. But for information I'll do the chromatography after the first reaction and at the end.

[chel45]

It seems that dimers and trimers of BDI-PCL-BDI could be formed in the first reaction as during one synthesis I got the perfect polymer that worked perfectly for my purposes (nanofibers). But after other synthesis the polymer was stiffer and shrink much more that tells about different chemical composition. I guess that in the second case there were more urea bonds than in the first one. The urea bond are stiffer and form more hydrogen bonds that results in shrinkage. Is it a reasonable explanation?

[wildfyr]

Yes, it is very reasonable!

To back up hollytara, it is a near certainty that oligomers form from alcohol-isocyanate reaction. You can only use the Carothers equation in such a situation where you take two difunctional monomers without side reactions and run the polymerization to high conversion.

in your graph do you see p on the x-axis? your graph is only showing above 90% conversion! For good reason. At low conversion, there are all sorts of dimers, trimers, etc. At very high conversion, we see high MW build. However to get to high conversion, we can't have side reactions. Esters and alcohols are great for this, as the byproduct is only water, and we run the reactions in a manner to force water out, and not much other stuff can happen.

Isocyanates react with water, and some water is probably in your diol and especially your putresceine. This will create dead polymer chain ends. If we really want to dig, it will in fact create carbamate chain ends, which being unsubstituted will degrade into amines and screw up your stoichiometry even more.

Anyways, sounds like things worked out, but you also learned a new subject .

[hollytara]

You don't really care about all these details - you just want a material that works!

For your first step, to maximize the amount of simple trimers do this:  Take a solution with 2 equivalents of the BDI and add dropwise the 1 equivalent of diol.  This way when a diol hits the reaction, it sees a large excess of BDI and forms the trimer.  It will only be at the end that this material sees some isocyanates that are part of trimers and may form larger oligomers.

If you can purify your trimer - say by precipitation from hexane, while the BDI remains dissolved - then you can use a larger excess like 4 equivalents, and only get trimers that you can obtain pure. 

Then you can pick the ratio of diamine you want to use - maximize MW by trying to be exactly 1:1 or control MW by picking a different ratio (0.98 to get isocyanate ends or 1.02 to get amine ends).  Once you have a system that works, be sure and always make the material exactly the same way!

[wildfyr]

^^^ this girl polymerizes.

One problem with purifying is that isocyanates are not terribly stable to humid air so doing more than one pot and generally handling them may be worse than just letting nature take its course unless you really know what you're doing.

You should consider drying your chemicals to minimize the impact of water. the Putresceine and any solvents are first suspects. PCL is pretty nonpolar, it should be OK. Its probably worth also checking the purify of your BDI, as it is reacting with water a little bit every time you open the container so a problem may be getting worse.