So I am taking diols, dissolving them into chloroform and pyridine, and adding a solution of chloroform/triphosgene to the reaction over a period of time. I am failing to get a high number average of molecular weights. Is it possible that having redisual carboxylic acid end groups, as opposed to diols, is leading to a low Mn?
When phosgene is reacted with a diol such as Bisphenol A, the product is the famous polycarbonate "Lexan" & the side product is HCl
If a carboxylic acid is present, then it will react with phosgene producing acid chloride as the equation shows:
RCO2H + COCl2 → RC(O)Cl + HCl + CO2
Therefore, the acid chloride will start to compete with phosgene in reacting with the diol. This means that a low molecular weight polymer will form certainly.
High molecular weight polymer is obtained when there are the diol & phosgene only, when there is HCl scavenger "using pyridine is fine", when the equipment is very clean, and when the required time for the condensation polymerization is given.
So I was able to take the polymer powder and dissolve it in CHCL3, and add equal volumes of acetone and IPA, 1:1:1 and wash out the residual monomer, success.
Dr. Matar,
Thank you for the insight! So are you saying that the Acid Chloride that results from residual carboxylic acid in the monomer will actually chain terminate the polymer?
Do you have any ideas on how to reduce the excess carboxylic acids to diols? Id rather not have to use something like Lithium aluminum hydride. I am making a LMW PLLA diol as one of the starting materials and I think that its possible that it has residual acid end groups.
The acid chloride will react with the diol in a different path to that of phosgene-diol reaction, therefore the original path will have chain termination. As you know, shorter chains imply lower MW.
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