Introduction

Jia-Liang Yang1, Xiao-Han Cao1, Xing-Hong Zhang1, and Patrick Theato2,3

1MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

2Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesser Str. 18, D-76131 Karlsruhe, Germany

3Soft Matter Synthesis Laboratory, Institute for Biological Interfaces III (IBG3), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

1 Scope of This Book

Various elements in nature are continuously cycled through a variety of chemical, biochemical, and physical processes. As the sixth abundant nonmetal element in the earth, sulfur and its natural cycle are closely related to the ecological balance and human life (Figure 1). The ocean is a major part of natural sulfur sources. Sulfur exists in the form of sulfates in the ocean and can also be converted to hydrogen sulfide (H2S) or carbonyl sulfide (COS), which are released to the atmosphere. In addition to marine sources, natural releases from land including volcanic eruptions, and human activities also emit a variety of sulfur-containing gases into the atmosphere, including methyl sulfide ((CH3)2S), H2S, COS, carbon disulfide (CS2), methyl mercaptan (CH3SH), and dimethyl disulfide (CH3SSCH3) [1,2]. Sulfur-containing compounds in the atmosphere undergo a series of photochemical reactions and return to the ocean and land through rain. Herein we want to address another conversion route for sulfur-containing compounds: the synthesis of sulfur-containing polymers [3,4] (Figure 1).

Polymer materials play an incredibly important role in modern life. The richness of polymer materials derives from the diversity of their constituent elements, such as carbon, nitrogen, oxygen, silicon, and sulfur. Unlike other chemical commodities, sulfur is mostly "involuntary" produced. It mainly arises from petroleum refineries and natural gas processing, thus creating a global surplus of sulfur. Production of sulfur reached more than 60 million tons in 2013 with a rising trend [5]. Global sulfur capacity tends to be in excess thus the use of sulfur needs to find a new direction. Actually, sulfur has been recognized as a valuable chemical agent since antiquity: employed as far back as 1600 B.C.E. by the Egyptians to bleach cotton fabric. Since the "chemical" invention of sulfur in explosives by the Chinese, the use of sulfur is closely linked with the development of chemical science. From the raw material of sulfuric acid to rubber industry auxiliaries, the mid-19th century witnessed the first applications of sulfur in polymer chemistry such as the vulcanizing agent of rubber. After the development of polymer chemistry in the last century, sulfur and sulfur-containing moieties have been increasingly involved in the construction of various polymers.

Figure 1 Sulfur cycle in nature and the formation of sulfur-containing polymers.

The purpose of this book is to summarize the latest development of sulfur-containing polymers, focusing on the utilization of sulfur-containing compounds and corresponding polymerization methods, analyzing the intrinsic relationship between polymer structure and its properties and applications. This book covers the state-of-the-art syntheses of a spectrum of sulfur-containing polymers from various sulfur resources including elemental sulfur, CS2, COS, and mercaptan. In-depth mechanistic understanding relating to the chain polymerization is particularly presented for the readers. Of course, various types of sulfur-containing polymers, including poly(thioester)s, poly(thioether)s, poly(thioamide)s, poly(thiocarbonate)s, poly(thiourethane)s, and poly(thiourea)s with linear or hyperbranched (dendrimer) architectures, are discussed. High-tech applications of these sulfur-containing polymers in energy, optical and biomaterials are discussed in detail. This book will provide the latest knowledge in this area in a comprehensive way to the readers including students in the colleges, engineers in research organizations in the polymer community.

2 Sulfur-Containing Polymers

2.1 Building Feedstocks

The presence of sulfur atoms in polymer structure, depending on the kind of functional group, can bring some important properties, such as mechanical, electrical, optical, and then adhesion to metals, resistance to heat, chemicals, radiation, bacteria, biocompatibility, and so on. These are some of the functional groups: sulfide (-S-), polysulfide (-Sn-), sulfonyl (-SO2-), sulfinyl (-SO-), sulfo (-SO3H), as well as some organic groups containing sulfur atoms, such as thioester [-(C=O)-S-], monothio- or dithiocarbonate [-O-C(=S)-O-], [-O-(C=O)-S-], or [-S-(C=O)-S-], as well as thiourethane [-NH-(C=O)-S-] or [-NH-(C=S)-O-] and other sulfur-nitrogen groups. Sulfur-containing polymers can be used as high-performance engineering plastics, chemically stable ion-exchange membranes in electromembrane processes, proton-conducting electrolytes, as well as optical, optoelectronic and photochemical materials. Some polymers are employed in biomedical applications, for example, sulfopolymers as biomembranes and blood-compatible materials, polysulfates and polysulfonates as antithrombotic or antiviral agents. The presence of sulfur, particularly in the form of disulfide bridges, has played an important role in biopolymers and self-healing materials.

With a series of synthetic breakthroughs in recent years, new chemistries suitable for the synthesis of various polymers containing sulfur atoms have been developed, and thus many novel materials with promising properties have been reported. As a way to provide sustainable polymers, some sulfur-containing polymers are synthesized from waste of petrochemical resources, which is an advance in waste utilization. Over the past 20 years, researchers have synthesized and prepared a wide variety of sulfur-containing polymers, but there are few books on comprehensive introduction of the synthesis and applications of sulfur-containing polymers. The rapid development of this field can be visualized by the increase of research reports published in recent years (Figure 2). However, there are relatively few monographs on sulfur-containing polymers. Furthermore, books that focus specifically on the synthetic methods and the correlation between polymer microstructure and macroscopic properties are still lacking. This book is intended to meet the development needs in this field, introduce the synthesis methods of various sulfur-containing polymers in detail, and provide directions for the design of new methods and new polymerization systems.

Figure 2 Scientific publications with the key word "sulfur-containing polymers" published from 1990 to 2019 (Web of Science).

Via the in-depth study of sulfur chemistry, diversified chemical transformation strategies have been developed to synthesize various sulfur-containing compounds, including thiourea, episulfide, and a large category of sulfur-containing medicinal compounds. To date, sulfur can also be used directly in the synthesis of sulfur-containing polymers by multicomponent polymerizations (MCPs). Except for sulfur, many sulfur-containing moieties and their comonomers have been developed for the synthesis of sulfur-containing polymers, which are summarized in Figure 3. Noteworthy, sulfur-containing one-carbon (C1) monomers (e.g. CS2, COS) are a kind of important resource as two bulk and cheap compounds from which polythiocarbonates, polythioethers, polythioureas, and poly(thioester)s can be obtained by many polymerization methods. Meanwhile, sulfur-containing moieties with disulfide bond, thioester function groups, or basing on thiophene can also provide sulfur-containing polymers with particular properties. For each type of monomer, researchers have developed and advanced various polymerization methods to achieve an accurate and efficient synthesis of the target products.

2.2 Synthetic Methods

Roughly, Chapters 1, 4, 6, and 8 introduce step polymerization processes for sulfur-containing polymers. MCP is an emerging polymer synthetic method that combines three or more monomers in a one-pot reaction, resulting in a highly efficient, huge structural diversity, high atomic economy, and a well-defined structure polymer. By way of elemental sulfur based MCPs (Figure 4, (1)), polythioamides and polythioureas can be synthesized directly, as summarized by Prof. Rong Rong Hu in Chapter 1. While based on other sulfur containing moieties, many polymers with different microstructures can also be produced by MCPs. Sulfonyl group-containing polymers can be derived from the sulfonyl azide or sulfonyl hydrazide-based MCPs, and the sequence-controlled sulfur-containing polymers can be prepared from the multicomponent tandem polymerizations of thiol or sulfur-containing heterocycle monomers.

Except for MCPs, thiol-based click polymerization features remarkable advantages and has also been nurtured into a powerful technique for the preparation of sulfur-containing polymers (Figure 4, (2)), as introduced in Chapter...