[nj_bartel]

specifically, polystyrene and nylon-6,6.

how? 

[renge ishyo]

I believe the calculation is performed the same way as before. Although conceptually you might expect a difference, if you treat the repeating units in the polymer as if they were individual molecules the analysis comes out the same.

First, find out what the limiting reactant is; calculate how many moles you have of this. Now convert this to moles of the *monomer* unit that repeats in your polymer product. For example, suppose you were combining A and B to form a polymer with repeating unit -AB-. Let's  say between A and B you find that A is the limiting reactant with 5 moles. Then you also have at maximum 5 moles of -AB- monomers in your product. Convert those 5 moles of -AB- monomers into grams using the molecular weight of a single -AB- monomer, and this will gives you the theoretical yield (in grams) of the polymer as a whole.

[nj_bartel]

Cool!  Thanks.  So you just ignore the ends of the polymer as well as any radical initiators?

For example, in our styrene polymerization, we used 50 mg of benzoyl peroxide, relative to ~2 g styrene.  Would you just consider that quantity to be negligible?

[408]

You do not do stoichiometry for catalysts....

For homopolymers: (ex polystyrene)
Pretend all the end groups are gone, of course this is an oversimplification of what really occurs, but has been 'good enough' for all the polymer work I have done.    Especially when polymers end up having massive molecular weights.

I just calculate the yield of polymer as if the reaction was not a polymerization, but rather the end groups were just falling off.  This means the molecular mass of the polymer is not needed to be known.
I did this for an aryl dibromo compound which was suzuki coupled.

For heteropolymers (ex nylon 6,6)
Same as before, but calculate which is the limiting reagent first.  Then calculate the mass of a theoretical dimer, lacking whatever parts are eliminated upon forming the polymer..

[renge ishyo]

As 408 suggests, you simply fudge it a bit when it comes to the ends of the polymer. It doesn't have to be precise so long as the number of ends of the polymer are much smaller in weight than the weight of the polymer as a whole. Although take heed what he says about nylon. You must use the molecular weight of the unit -AB- which is *not* equal to the sum of the weights of A + B individually (because you lose water).

[nj_bartel]

Thanks 408, I understand the heteropolymer formation, but I'm still not sure what you are saying for the homopolymer (although it feels like it should be essentially the same as heteropolymer).  How do you take into account the amount of radical initiator used, since you don't know how much self terminated and how much became polymer?

[renge ishyo]

The initiator is much lower in concentration than the other reactants (or it should be!). It becomes part of the "ends" of the polymer and it gets neglected in much the same way as for the ends of the heteropolymer. The self terminated initiators (and unreacted initiators) are not part of the product and would not be weighed so you can safely ignore those.

[DrCMS]

Note - this approach to limiting reagents in hetropolymers only works for specific polymerisation types.  If the individual monomers can form homoploymers the approach breaks down.  So it works for nylon but if you were dealing with a styrene/acrylate polymers it would be wrong.

[408]

Quite right DrCMS!  If your polymerization relies on monomers A and B needing to polymerize in alternating fashion(ex nylon), so ....ABABABABAB.... Then that approach is used

for a heteropolymer where A and B can polymerize however they like so ...ABABBBBBBAAABBABABBAABABABBAA..., the approach is the sum of doing the first method I outlined twice, once for each monomer included in the reaction.  Such as for a polyethylene/polystyrene co polymer.

[typhoon2028]

Polymers are large molecules.  It seems to me that just asking what is the theoretical yield of polymerization is too basic of a question.

What is the target molecular weight range?

[mars]

Hi everybody,
How can we calculate the % yield in a self polymeriation reaction.?

 Is it correct if we conisder the same moles of monomer as the moles of one repeat unit and calculate the % yield.


your valuable comments are highly appreciated.
Mars

[fledarmus]

Theoretical yield isn't a particularly useful concept in polymer chemistry. There are other ways of describing the outcome of a polymerization reaction that are more descriptive. Some of these are:

Percent conversion - when you are working with homopolymers or with random copolymers, you generally only want a few initiation events to get large polymers. When the reaction is finished, you typically have long chain polymers (-AAAAAAAAAAA-) and some leftover monomer, as the initiating/replicating sites get quenched before they run out of reagent. Percent conversion tells you how much monomer is leftover.

Average molecular weight - if you have a large number of initiation events in your reaction, you will start a large number of polymer chains, and the resulting chains will be relatively small when they run out of monomer to react with. If you have a small number of initiation events, you will only start a few polymer chains, and they can grow much larger before you run out of monomer. Either way, there will be a statistical variation in the speed with which the reactions occur (not every chain will grow at the same rate) and at the end of the reaction, rather than a single product with a single molecular weight, you will have a statistical spread of products of different molecular weights centered around an average molecular weight.

[yahya]

hi every one
can any one help me how calculate yield for polymer
thanks

[Corribus]

Did you read the post right before yours?