21.16:
Molecular Weight of Step-Growth Polymers
Recall that step-growth polymerization involves bifunctional monomers that undergo stepwise condensation to form long‐chain polymers.
The stepwise construction of polymers is associated with a slow increase in the molecular weight of the polymer.
During the initial stages of polymerization, small chains of dimers, trimers, and tetramers are formed.
These small chains undergo condensation with monomers or other small chains to form a large number of low-molecular-weight oligomers. This process continues until 99% of the monomers are consumed.
Only at this polymerization stage do larger chains react with each other to form high-molecular-weight polymers.
Consequently, high-molecular-weight polymer chains are formed only in the late stages of the reaction.
Typically the polydispersity index for step-growth polymers is greater than one. Therefore, they exhibit a broad molecular weight distribution curve.
21.16:
Molecular Weight of Step-Growth Polymers
Step growth polymerization involves bi or multifunctional monomers. Bifunctional monomers react to form linear step growth polymers, whereas multifunctional monomers react to form non-linear or branched polymers.
As the step-growth polymerization involves step-wise condensation of monomers, the molecular weight also builds up eventually. Consequently, high molecular weight polymers are obtained at the late stages of the polymerization, where 99% of monomers have been consumed.
The extent of the reaction can be obtained from the Carothers equation.
Here, Xn describes the average chain length, P describes the extent of the reaction, N0 is the number of molecules at the beginning of the polymerization, and N is the number of molecules left in the reaction after some time.
The equation above shows that in step-growth polymerization, a high monomer conversion is required to achieve a high degree of polymerization.
The polydispersity index (PDI) is a measure of the broadness of molecular weight distribution in a given polymer sample. PDI of a polymer is the ratio of the weight and number average of the molecular weight of the polymer. The PDI is unity if all the polymer molecules are of the same size.
In the case of step-growth polymers, the PDI is 2, indicating a broad molecular weight distribution.
Suggested Reading
- Polymeric Foams StructurePropertyPerformance. DOI: http://dx.doi.org/10.1016/B978-1-4557-7755-6.00002-1
- Molecular Weight Distribution In Linear Step-Growth Polymers.