The Role of Self‐Dilution in Step‐Growth Polymerizations

In step‐growth polymerizations, the molar concentration of reactive linear species (oligomers and polymers) decreases with higher conversions and finally reaches zero at 100% conversion. This self‐dilution favors cyclization at the expense of chain‐growth. Cyclization reduces the average lengths of the linear species, and thus, induces a kind of “self‐acceleration”. Both effects together overcompensate the decreasing cyclization tendency resulting from increasing chain lengths. This influence of the self‐dilution is also operating in the case of “ab n ” monomers, so that at 100% conversion (defined for the “a” functional groups) all hyperbranched polymers will have a cyclic core. With modifications, the “law of self‐dilution” also applies to “a 2  + b 3 ” or “a 2  + b 4 ” polycondensations. Furthermore, the “law of self‐dilution” is valid for both kinetically‐ and thermodynamically‐controlled polycondensations.