21.16 : 逐步生长聚合物的分子量

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.