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Topic: Low MW polyacrylamide  (Read 933 times)

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Offline HMDS

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Low MW polyacrylamide
« on: July 11, 2022, 03:59:05 AM »
Hi all,

Just like most I've created my account with a question in mind  :D
I've been doing some exploratory work on acrylamide polimerization using AIBN in organic solvents, with the goal to obtain short length chains of about 20 amide units or less. I'm no polymer scientist myself, so I mainly base myself on what I can find in literature. I'm not sure if the following concept makes any sense at all:

Currently the polymerization takes place in pure ethanol just below boiling pt. A typical screening test uses ~100:1 monomer:initiator (molar) and 2.5:1 EtOH:monomer molar ratio.
At a certain point in time the polymer starts to gradually precipitate into a white bulky mass (preceded by the hazy phase). I'm assuming the polymerization degree gets too high (positive mixing Gibbs) and the growing chain is no longer being able to dissolve in pure Ethanol? From what I find in literature, typical commercial PAAm (MW~10,000) only dissolves into <50% EtOH (aq.) as the solvophobic interaction is too great.

If this is mainly what's happening, could anyone roughly estimate (or point me to some literature) on the MW that can be expected when precipitation occurs? Simply put, at what chain length does a PAAm no longer dissolve properly in Ethanol (branching not taken into account, if I can assume so(?))
And can I trigger precipitation at a lower MW by using a less polar solvent (e.g., IPA or higher alcohols?) that are still compatible with the monomer?

I currently quench the reaction with cold solvent once I see precipitation, followed by filtration and drying. Could I expect significant branching/crosslinking up to this point? We're talking of precipitation after ~10-15min.
Additionally, do I need to quench, or can I just let the reaction continue and don't have to worry about an increase in chain length of the already precipitated polymer? In literature I find that in solution polymerization generally the propagation, which continues in the precipitated swollen particle, doesn't exert a strong effect on the MW of the product (e.g., styrene in benzene), assuming the polymer is not or partially soluble in the solvent. Is that the case here as well?

Thanks in advance, and if there are any additional pointers on getting my desired low MW PAAm in a controlled way, I'm happy to hear it  ;)


HMDS

Offline hollytara

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Re: Low MW polyacrylamide
« Reply #1 on: July 12, 2022, 12:05:24 AM »
So polymerization kinetics follow this rate law:

-d[M]/dt = kp [M] (f kd [I ] / kt )^1/2

here kp is the rate of polymerization, kd is the rate of dissociation of the initiator, kt is the rate of termination.  [M] the concentration of monomer and [I ] the concentration of initiator.  f is the fraction of initiation events that start polymer chains growing - usually around 0.5 

We usually assume that chain initiation, growth through propagation, and termination happen relatively quickly (under a second to a few seconds - not on a time scale of minutes) - so that under a given set of conditions, the MW of the chains is the same whether you quench after 1 minute or 10 minutes.  Chains lengths only change when the monomer concentration [M] changes significantly - say when it is close to being half consumed.  If you let the polymerization go to near completion it changes a lot as [M] drops. 

We can work out the kinetic chain length - how many steps of propagation between initiation and termination.
This turns out to be:

ν = kp [M] / (2 f kt kd [I ]^1/2) 

Kp, kt, kd are functions of temperature primarily.  What is easy to control is [M] and [I ]

to get short kinetic chain lengths, you want low [M] and high [I ].  That means you start a lot of chains (high [I ]), and they dont go far (low [M]) before they meet another growing chain and terminate. 

The other way polymer chemists sometimes control the MW is through a chain transfer agent.  A classic one is toluene - the benzylic hydrogens on the CH3 group are easily abstracted (this stops chain growth) but the benzyl radicals formed will react with monomer to start a new chain.  So even though the chain length from initiation to termination is long, it does not make one polymer chain, but makes several because the chain is transferred. 

I think either adjusting your [M] and [I ] concentrations or adding a chain transfer agent (check this out https://doi.org/10.1002/pola.10594) will work better than trying to find the perfect solvent to precipitate at the desired MW. 

(mod edit to re-introduce the [I ] terms lost by bbs formatting)
« Last Edit: July 26, 2022, 03:18:21 AM by sjb »

Offline HMDS

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Re: Low MW polyacrylamide
« Reply #2 on: July 12, 2022, 03:53:00 AM »
...

Thanks for the info and linked article! I was planning to play around more with the monomer and initiator concentrations/ratios. But a chain transfer agent does sound interesting for further controlling indeed.

When you say that
Quote
We usually assume that chain initiation, growth through propagation, and termination happen relatively quickly (under a second to a few seconds - not on a time scale of minutes) - so that under a given set of conditions, the MW of the chains is the same whether you quench after 1 minute or 10 minutes.
Does that mean that what I'm noticing after x minutes is several polymers gelating into a precipitated mass rather than individual chains growing and precipitating out because of solvophobic effects?


Offline wildfyr

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Re: Low MW polyacrylamide
« Reply #3 on: July 12, 2022, 09:01:49 PM »
Dangit holly, you beat me to it with chain transfer agents.

Offline hollytara

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Re: Low MW polyacrylamide
« Reply #4 on: July 25, 2022, 01:09:00 PM »
Been away for a while - yes, the polymer is likely less soluble than the monomers and may precipitate once it reaches the solubility limit.  Polymer solubility is always driven entirely by enthalpy (entropy of mixing is effectively 0), while small molecules also have entropy to drive dissolution.  So you are probably seeing polymer ppt once it reaches the limit.

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