temperature, reactant concentration, and most importantly, removal of water as the monomers condense
controlling the concentration of monomer and time and also depends on extent of reaction..
Polycondensation polymerization is a stepwise process in which polyfunctional monomers react with concurrent elimination of small molecules like H2O, NH3, and NaCl in a repetitive manner . Indefinite growth of the polymer occurs at both ends. The condensation reaction terminates when one of the reacting monomers is completely consumed in the reaction.
I shall give some examples for controlling polycondensation polymerization:
We can control the obtained polymer structure. If bifunctional monomers (e.g. diols) are reacted with dicarboxylic acids , then a polymer with linear structure is obtained. If a triol is reacted with the dicarboxylic acid, then a 3D crosslinked network polymeric structure will result. Of course, polyesters are made by both reactions.
We can control reactivity upon forming polyesters. Reaction of dicarboxylic acid halides (more reactive than diacids) with diols or triols is much more facile. No need for heating or catalysis.
We can control the average molecular mass of the resulting polyester. After mixing a diol or triol with a dicarboxylic acid, we can inject little monocarboxylic acid (e.g. acetic acid) to the reaction flask. This small acid will cause chain termination ~~~~~~~~O-CO-CH3 upon reacting with the accessible alcohol groups.
* High purity of monomers and well-chosen solvents. The presence of impurities in monomers does not allow to reach the optimal conditions of polymer formation, as defined by the Carothers equation which is used to determine the degree of polymerization in polycondensation reactions.
* Temperature
* Concentration of monomers
* Monomer addition order (sometimes is critical to avoid the side reactions)
* Reaction reversibility
If you are using two monomers with different end groups, for instance a diol and a diacid, stoichiometry is a key factor.
Stochiometric ratio, monomer functionality, temperature, etc. It all depends on what you want to achieve. The final molecular weight is highly dependent in all of the above.
To begin with I am recommending you the seminal work of Carothers even though much more work has been done on this topic since.
To some extent, all the above comments have given you some tips on most of the parameters that need to be controled in order to achieve high molecular weights. Practically you'll need to control:
- Stochiometry best realized by ring opening polymerization
- catalysis in a large number of cases
- temperature history
- pressure history that has not been mentionned yet
- good mixing conditions
Best of luck.
The reaction must be carried out at high degree on conversion. Usually up to 99%
- Badges
- Science topic