[vanillafinger83]

Why does Poly(styrene sulfonate) have 70.000kDa and 206g/mol as its MW? how are they different?

Thank you!!!!!!!

[fledarmus]

Poly(styrene sulfonate) is not a defined single compound. The concept of molecular weight is not a particularly useful one for polymers.

One monomer that can be used to make poly(styrene sulfonate) is sodium styrene sulfonate. This compound has a molecular weight of 206, so I believe that is the second number you mention.

The styrene group in that monomer can undergo an initiation reaction, which then allows it to react with a second styrene group, which can react with a third, and so on until all the monomer in your reaction vessel is used up or until the chain is terminated by other chemical processes. The number of styrenes linked together can be quite high, but the reactions aren't perfectly consistent - each chain initiated will terminate at a different point, usually giving you a gaussian distribution of chain lengths for your final molecule. There are ways of controlling how long the chains will get and partially controlling the spread of chain lengths in the final mixture, but for polymers you ordinarily report an average molecular weight. The "70,000 kDa" you report would be absolutely enormous (an average molecular weight of 70,000,000) - I suspect it's actually 70 kDa? Or was that actually a decimal number you are reporting?

Either way, my guess is that the 206 g/mol is the molecular weight of the monomer, and the "70.000 kDa" is the average molecular weight of that particular batch of polymer.

[vanillafinger83]

thanks for clarifying all for me!!! long time question solved. 70.000 i meant was 70, not 70,000.

btw...why is one sodium styrene sulfonate has a molecular weight of 206g/mol whereas the polymer has 70kDa which is only a tiny tiny fraction of 206g??

70kDa is about 1.16e-19 gram, then why is the average weight of polymer a lot smaller than the molecular weight of monomer unit?

[vanillafinger83]

also if I want to make 0.1M of PSS solution with water. then should I be using the MW of Sodium styrene sulfonate or the average molecular weight of 70kDa which is for PSS?

[Arkcon]

Well, kDa is not a unit in the definition of molarity, so you will have to convert first.

[Polytriazole]

Polymer scientists define the unit Da, or Dalton as 1 g/mol.  So when we say a polymer has a molecular weight of 70 kDa, we mean its MW is 70,000 g/mol.  That's really just an average, as each chain has a slightly different length.

When making a solution of a given molarity, it's usually the polymer chain that's treated as the relevant chemical unit.