Competing exchange and irreversible reactions in a linear co-polycondensation lead to a broad composition window where tunable high molecular weight polymers can be prepared

TL;DR

This paper analyzes a co-polycondensation process with competing reactions, revealing a broad composition window where high and tunable molecular weight polymers can be produced, supported by analytical and Monte Carlo methods.

Contribution

It introduces a combined analytical and simulation approach to understand how competing irreversible and exchange reactions create a stable composition window for high molecular weight polymer synthesis.

Findings

A single peak in number average molar mass at stoichiometry.

A second peak in weight average molar mass at a different stoichiometry.

A broad composition range with high, stable weight average molar mass.

Abstract

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.