What Limits the Chain Growth from Flat Surfaces in Surface‐Initiated ATRP: Propagation, Termination or Both?

Abstract

Surface‐initiated controlled radical polymerization, such as ATRP, has been proven to be a powerful method for preparation of well‐controlled functional grafted polymers. There are thousands of experimental works published, by varying polymers, surfaces, and applications. In comparison, theoretical developments are very lacking. Many fundamental questions still remain to be answered. These questions include, but not limited to: What determine the surface initiator efficiency? How many chains per square nanometer can be fully grown? What is the maximum thickness can polymer grow to? If chains are simultaneously grown from surface and solution, which has higher molecular weight? Answers to these questions are most helpful for innovation and further development in this important area. In this work, we employed recently developed theories to explain experimentally observed kinetic profiles of polymer layer thickness growth. What limits the growth of surface chains? Is it the monomer starvation in grafting layer that slow down propagation? Or is it the termination of radicals that stop the chain growth? It was found that no existing models could fully explain the often contradictory experimental observations. It is our hope that this work will provoke further discussions and inspire more effort in resolving the fundamental issues of this area.

Factors Affecting Grafting Density in Surface‐Initiated ATRP: A Simulation Study

“…Some experimental studies have reported increasing catalyst concentration in SI‐ATRP result in a decrease of grafting rate, while another reported the opposite trend . Based on a previous study, these trends on the effect of catalyst concentration could not be explained through either simplified propagation model or termination model . These kinetic models assumed a known grafting density unaffected by the reaction conditions, which we believed could explain these opposite trends.…”

“…Kinetic models have also been used as a simple and straightforward approach to investigate the polymer growth in surface‐initiated controlled radical poly­merization systems . However, a prior knowledge of grafting density must be assumed when using such approach.…”

“…Moreover, it is often assumed that grafting density remains unaffected by changes in the reaction conditions. Our group recently investigated the difference in growth kinetics of surface polymers predicted by the two schools of theories, propagation versus termination , using simplified kinetic models . The predicted kinetics based on these models are compared with the experimental trends from literatures.…”

“…[ 20 ] Kinetic models have also been used as a simple and straightforward approach to investigate the polymer growth in surface-initiated controlled radical polymerization systems. [ 8,[21][22][23][24][25] However, a prior knowledge of grafting density must be assumed when using such approach. Moreover, it is often assumed that grafting density remains unaffected by changes in the reaction conditions.…”

“…Our group recently investigated the difference in growth kinetics of surface polymers predicted by the two schools of theories, propagation versus termination , using simplifi ed kinetic models. [ 25 ] The predicted kinetics based on these models are compared with the experimental trends from literatures. The comparison showed that neither of the two models could fully predict the reported trends.…”