Chemical kinetic mechanisms and scaling of two-dimensional polymers via irreversible solution-phase reactions.

Ge Zhang,Pavlo Gordiichuk,Yuwen Zeng,Michael S Strano

doi:10.1063/5.0044050

Ge Zhang, Pavlo Gordiichuk + Show 2 more

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https://doi.org/10.1063/5.0044050

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Abstract

Two-dimensional (2D) polymers are extended networks of multi-functional repeating units that are covalently linked together but confined to a single plane. The past decade has witnessed a surge in interest and effort toward producing and utilizing 2D polymers. However, facile synthesis schemes suitable for mass production are yet to be realized. In addition, unifying theories to describe the 2D polymerization process, such as those for linear polymers, have not yet been established. Herein, we perform a chemical kinetic simulation to study the recent synthesis of 2D polymers in homogeneous solution with irreversible chemistry. We show that reaction sites for polymerization in 2D always scale unfavorably compared to 3D, growing as molecular weight to the 1/2 power vs 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane vs rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 106. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for the 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory the feasibility of producing two-dimensional polymers from irreversible polymerization in solution.

Highlights

The problem is that large-scale and facile synthesis of 2D polymers in homogeneous solution remains challenging despite the rapid progress in this field
The analysis described above is supported by kinetic Monte Carlo simulation results
We illustrated that the condition is thought to be strongly unfavorable toward 2D polymers, it is possible to synthesize a large-sized 2D polymer with high yield

Summary

Introduction

The development of polymerization is one of the most significant technological advances in chemistry and materials science. polymers with two-dimensional periodicity have only recently attracted much attention due to their versatile and exotic chemical, mechanical, optical, electronic, and magnetic properties. The problem is that large-scale and facile synthesis of 2D polymers in homogeneous solution remains challenging despite the rapid progress in this field.3,9,14Attempts to polymerize in two dimensions date back to 1935,15,16 and subsequent work has shown the feasibility of crosslinking chain polymers to form single-layered sheets confined to interfaces. Nowadays, monolayer or few-layer 2D polymers can be grown on solid template or fluid interfaces using various reactions, including irreversible C–C coupling and polycondensation. Along another line, two-dimensional covalent organic frameworks (COFs) have been synthesized by reversible bond formation under hydrothermal conditions, which resembles a crystallization process. Recently, bulk synthesis of 2D polymers has been realized by pre-organizing monomers into layered, oriented structures via crystallization or non-covalent assembly. These three methods have all shown great potential in producing macromolecules with long-range order in a 2D plane, but they have certain limitations. Monolayer or few-layer 2D polymers can be grown on solid template or fluid interfaces using various reactions, including irreversible C–C coupling and polycondensation.. Monolayer or few-layer 2D polymers can be grown on solid template or fluid interfaces using various reactions, including irreversible C–C coupling and polycondensation.20–39 Along another line, two-dimensional covalent organic frameworks (COFs) have been synthesized by reversible bond formation under hydrothermal conditions, which resembles a crystallization process.. Bulk synthesis of 2D polymers has been realized by pre-organizing monomers into layered, oriented structures via crystallization or non-covalent assembly.. Bulk synthesis of 2D polymers has been realized by pre-organizing monomers into layered, oriented structures via crystallization or non-covalent assembly.14,48–55 These three methods have all shown great potential in producing macromolecules with long-range order in a 2D plane, but they have certain limitations. Mass production is possible with hydrothermal reactions or crystal polymerization, but the products are usually insoluble powders that have to be exfoliated afterward, and obtaining monolayers of 2D COFs appeared to be a nontrivial task. The reversible linkages in COF (such as imine, boroxine, and boronate ester) are prone to hydrolysis, requiring efforts to convert them into more stable bonds. The need for crystallization constrained the applicable types of monomers for crystal polymerization.

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