Conformational Distributions near and on the Substrate during Surface-Initiated Living Polymerization: A Lattice-Based Kinetic Monte Carlo Approach

Abstract

One of the challenges in the field of surface-initiated polymerization (SIP) is gaining access to conformational distributions allowing one to quantify the degree of brush/mushroom character during synthesis. Here, we put forward a novel kinetic Monte Carlo (kMC) tool to be successful in this respect, focusing on chain-to-chain deviations on and near the surface accounting for varying reaction probabilities and combining conventional kMC modeling with a modified version of the bond fluctuation model. The potential of the tool is illustrated for living SIP addressing the effect of shielding on the efficiency of surface initiation and propagation. It is shown that at higher reaction times shielding for propagation leads to the increased formation of hindered shorter chains, causing the formation of a bimodal number chain-length distribution (CLD) for tethered chains compared to the always unimodal number CLD for free larger chains near the surface. Moreover, it can be evaluated at any synthesis time if an individual chain possesses a mushroom, brushlike, or brush conformation. It is demonstrated that an optimal (average) initiator surface coverage exists, leading to a sufficiently high chain grafting density and a maximization of the brush character provided that an initiator with the correct (surface) initiation reactivity is selected. The developed tool is important for the multiangle design of future SIP processes focusing on optimization in reaction time, control over CLD, and conformational features in view of the desired application.

The Competition of Termination and Shielding to Evaluate the Success of Surface-Initiated Reversible Deactivation Radical Polymerization

“…The conformation adopted by surface-tethered polymer chains is governed by a competition between the high entropic cost for these chains to stretch out to their maximum length and the excluded volume interactions between their segments. This competition is to a first approximation influenced by the average chain grafting density (σ chains ), which is linked to the average initiator coverage, and more in detail by the spectrum of values of the radius of gyration ( R g,surf ) of the individual polymer chains [ 14 , 15 , 16 ].…”

“…Conventionally, two main or average regimes of three-dimensional (3D) conformations have been introduced to describe the polymer layer on a surface, i.e., the so-called mushroom and brush regime [ 17 , 18 , 19 ]. Recent work of Arraez et al [ 15 ] focusing on surface initiated living polymerization (SIP) with chain initiation and propagation at the surface and in the solution highlighted that a classification based on a single average can be biased. The polymer layer cannot be described solely by introducing an average conformation but should be at least described by percentages of chains belonging to the two aforementioned conformation regimes, also considering a third intermediate regime termed brush-like to highlight a mixed contribution of the two main regimes [ 15 , 20 ].…”

“…Recent work of Arraez et al [ 15 ] focusing on surface initiated living polymerization (SIP) with chain initiation and propagation at the surface and in the solution highlighted that a classification based on a single average can be biased. The polymer layer cannot be described solely by introducing an average conformation but should be at least described by percentages of chains belonging to the two aforementioned conformation regimes, also considering a third intermediate regime termed brush-like to highlight a mixed contribution of the two main regimes [ 15 , 20 ]. Based on literature data [ 14 , 21 ] and dedicated analysis considering complete simulated R g,surf distributions, these authors proposed to classify each chain according to: …”

“…In addition, the determination of σ chains is usually more onerous than measuring the (average) molar mass [ 14 , 42 ], therefore making it more difficult to access detailed experimental information on the conformations of the surface-tethered polymer chains. In contrast, experimental analysis of the chains in solution is relatively straightforward but not recommended [ 15 , 42 , 43 , 44 ], as the kinetics for chain formation in the solution and on the surface are completely different, due to a very strong impact of confinement effects exerted by the presence of the surface. For example, the likelihood that two active chains at the surface can terminate with each other is much lower than if those two chains are free in the solution, as conceptually illustrated in Figure 2 (bottom).…”

Molecular Imprinting of Bisphenol A on Silica Skeleton and Gold Pinhole Surfaces in 2D Colloidal Inverse Opal through Thermal Graft Copolymerization

“…Additionally, typical synthetic methods, such as surface-initiated atom transfer radical polymerization (SI-ATRP) [ 8 ] and electrochemical polymerization [ 9 ] have been implemented for the development of surface-imprinted films as BPA-detecting sensors. The SI-ATRP as one of the “grafting from” methods has extensively been used for surface modification on metals, SiO 2 , and polymers and has successfully controlled graft polymerization on the surface [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Following MIP synthetic strategies, the choice of efficient analytical equipment is necessary for the proper analysis of the sensing properties on MIP films based on confidential instrumentation.…”