Tuesday, 24 May 2016

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

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.

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