The role of self-dilution in step-growth polymerizations

Abstract

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 "abn" 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 "a2 + b3" or "a2 + b4" polycondensations. Furthermore, the "law of self-dilution" is valid for both kinetically- and thermodynamically-controlled polycondensations. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.