  A co-polycondensation reaction is discussed analytically and by Monte-Carlo
simulations where two reactive units compete for reactions with an alternating
third reactive unit, whereby irreversible reactions replace bonds which are
able to undergo exchange reactions. The resulting number average molar mass,
$M_{\text{n}}$, exhibits only one distinct peak at the stoichiometric condition
of both competitors with the alternating partner. The weight average molar
mass, $M_{\text{w}}$, reaches an additional second peak at the stoichiometric
condition between the dominating competitor and the alternating partner. Both
peaks of $M_{\text{w}}$ surround a range of compositions where a rather high
and approximately constant $M_{\text{w}}$ is obtained. The degree of
polymerization of the dominating and alternating reaction partners is rather
insensitive towards composition fluctuations if the reaction mixture remains
within this composition window. This promotes high molecular weight species and
more homogeneous weight distributions at incomplete mixing conditions. An ideal
reference case (identical reaction rates for all reactions) is solved
analytically to describe these reactions. The position of the stable
composition window and the average molar masses inside this window can be tuned
by choosing appropriate precursor molecules, reaction mixtures or post-tuning
steps at later times.
