Introduction to Chalcogen-containing polymers

Justin M. Chalker

Rongrong Hu

Jeffrey Pyun

The ability to impart diverse properties and functions into macromolecules is critical for the future of polymer science.¹ One strategy is to install unique atoms or functional groups that convey a desired property or reactivity to the polymer. As a group of promising polymer materials, chalcogen-containing polymers have rapidly expanded in recent years, owing to their many intriguing and useful physical, optical, and material properties.² For example, polyphenylene sulfides (PPSs) and polyphenylene sulfones (PPSUs) are well-known high-performance, super-engineering plastics with high thermal stability, excellent chemical resistance, good mold precision, and high stiffness and modulus. In this themed collection for Polymer Chemistry, we focus on the synthesis, structures, and functions of a large variety of sulfur- and selenium-containing polymers, including sulfur-rich polymers prepared from inverse vulcanization or related approaches, polysulfites, polysulfones, poly(disulfide)s, polythioethers, polydithiocarbonates, polymonothiocarbonates, polythioamides, poly(thiazolidin-2-imine)s, and conjugated polythiophenes, which have been synthesized from readily available chalcogen-containing monomers, such as elemental sulfur, SO₂, thiols, carbonyl sulfide, S/Se-containing vinyl monomers, cyclic thiocarbonates, diisocyanates, and so on. Additionally, selenium³ and sulfur^4–6 in polymers enable diverse properties not possible in other macromolecules, imparting unique capabilities in gold extraction, high-refractive-index optics applications, redox activity for rechargeable batteries, intracellular drug delivery, and multi-responsive photonic crystals. The themed collection contains 4 review articles, 2 communications, and 20 research papers from across the globe, curated to illustrate the growing and current interests in this field and the wide variety of fundamental and applied polymer projects that are motivated by the unique chemistry of chalcogen-containing polymers.

In the case of sulfur-containing polymers, there is an opportunity to tap into underused feedstocks in the synthesis of functional polymers. Millions of tonnes of elemental sulfur are produced in excess each year in petroleum refining,⁷ so there is an opportunity to convert this material into value-added polymers. Pyun and co-workers have spurred a renaissance in the use of elemental sulfur as a monomer with the development of a copolymerisation method termed inverse vulcanisation.⁸ In this collection, the reach of this concept is reflected in the diverse applications of inverse vulcanisation in accessing sorbents for heavy metal remediation (Théato, https://doi.org/10.1039/d2py00773h and Chalker, https://doi.org/10.1039/d2py00903j), mechanically enhanced composites (Hasell, https://doi.org/10.1039/d2py00321j), novel adhesives made from sulfur and low-cost or renewable comonomers (Jenkins, https://doi.org/10.1039/d2py00418f), and cathode materials for Li–S batteries (Fu, https://doi.org/10.1039/d2py00823h). The effects of thermal treatment of sulfur-rich polymers, and the resulting thermomechanical properties, are studied by Kim and Wie (https://doi.org/10.1039/d2py01390h). Insights into the complex mechanistic details of inverse vulcanisation are also provided by Kuwabara and Kanbara (https://doi.org/10.1039/d2py00774f).

To pursue the diverse structures and properties of chalcogen-containing polymers, there is a need to develop efficient and controlled methods for their synthesis and establish techniques to control their physical properties. Multicomponent polymerization (MCP) is a powerful tool to construct chalcogen-containing polymers from simple monomers. For example, Hu and Tang report the efficient synthesis of aromatic polythioamides with well-defined structures from the MCP of elemental sulfur, aromatic amines and aldehydes (https://doi.org/10.1039/d2py01560a).

Besides elemental sulfur, sulfur dioxide is a harmful byproduct of many industrial activities and its use as a monomer holds potential for a number of applications. These opportunities are demonstrated and reviewed by Ren (https://doi.org/10.1039/d2py00185c) and Wu (https://doi.org/10.1039/d2py00685e), respectively. Through copolymerization of SO₂ and epoxides or olefins, polysulfites or polysulfones could be accessed. Such efforts in valorisation of underused feedstocks in polymer synthesis are important from both an environmental and economic perspective.

Sulfur- and selenium-containing vinyl monomers (either electron-rich or electron-deficient) have brought access to polysulfides and polysulfoxides, and Brendel summarizes the progress of these chalcogen-containing polymers from the versatility of S-vinyl monomers (https://doi.org/10.1039/d2py00850e and https://doi.org/10.1039/d2py00598k). Complex polymer architectures such as degradable thioester core-crosslinked star-shaped polymers are also synthesized by using a bifunctional thiomethacrylate crosslinker from reversible addition–fragmentation chain transfer polymerisation (Becer, https://doi.org/10.1039/d2py00901c).

Ring-opening polymerization (ROP) with cyclic monomers is a widely adopted synthetic approach for chalcogen-containing polymers. For example, the rich chemistry of γ-thiolactone has endowed it with great potential in polymer synthesis, and Illy and Mongkhoun have summarized the utilization of γ-thiolactone in polymer synthesis as a thiolation agent, linker, ROP monomer, reactant in stepwise polymerization, component of initiating systems, and so on, bringing various polymer architectures of polythioethers (https://doi.org/10.1039/d2py00731b). Ring-opening copolymerization of five-membered cyclic carbonates and carbonyl sulfide could also afford polythioethers (Zhang, https://doi.org/10.1039/d2py01014c). Buchard reports a series of cyclic (thio)carbonate monomers derived from carbohydrate D-glucal, and their ROP could afford a batch of polydithiocarbonates and polymonothiocarbonates (https://doi.org/10.1039/d2py01366e).

Moreover, polycondensations have also been presented to provide straightforward synthesis of chalcogen-containing polymers. Mutlu reports the polymerization between 1,1′-carbonyldiimidazole and thiols to access intriguing polydithiocarbonates (https://doi.org/10.1039.d2py00990k). Functional poly(monothiocarbonate) copolymers were also synthesized by Detrembleur from diamines, dithiols and a CO₂-sourced activated dicyclic carbonate (https://doi.org/10.1039/d2py00307d). Polymers with more complicated structures, such as thiacyclic polymers and poly(thiazolidin-2-imine)s, were synthesized by the polymerization of aziridine-based cycloaddition polymerization with isocyanates (Zhang, https://doi.org/10.1039/d2py00569g). Selenium-containing linear and brush polymers were synthesized through diselenide–yne chemistry by Pan (https://doi.org/10.1039/d2py00621a).

The chemical reactivity of sulfur- and selenium-containing polymers is also diverse. Chalcogen–chalcogen bonds (S–S, S–Se, etc.) in these polymers can be broken and reformed by diverse stimuli, including heat, light, chemical reagents or mechanical force.^9,10 The dynamic nature of these systems has inspired advances in polymer modification, repair, and recycling.^11–13 In this collection, Chalker uses this chemistry to develop a rapid, microwaved-induced S–S metathesis reaction to mould magnetic-responsive polysulfide composites into recyclable machine components (https://doi.org/10.1039/d2py00903j). Sulfur and selenium functional groups in polymers can exhibit complex redox chemistry, with many accessible oxidation states. This reactivity has been harnessed for diverse applications of chalcogen-containing polymers in sensing and drug release. For example, Ghosh compares branched and linear poly(disulfide) systems in the delivery and glutathione-triggered release of therapeutic cargo (https://doi.org/10.1039/d2py00896c). The redox activity of these chalcogen-containing polymers has also been leveraged in a number of energy storage applications that inform the future of battery technologies,¹⁴ which is discussed in a timely review in this collection by Fu (https://doi.org/10.1039/d2py00823h).

The affinity of sulfur for metals has led to a number of applications of chalcogen-containing polymers in metal sensing, remediation, and precious-metal recovery.^15,16 In this collection, advances in using sulfur-rich polymers for mercury remediation (https://doi.org/10.1039/d2py00903j) and gold recovery (https://doi.org/10.1039/d2py01560a) are reported, respectively.

The potential for chalcogen-rich polymers in optics and electronics is also exemplified in this collection. For instance, the incorporation of sulfur and selenium into polymers can increase the refractive index¹⁷ and modulate luminescence properties¹⁸ in comparison to more traditional polyolefins and condensation polymers. For example, polythioamides possess some of the highest refractive indices reported for synthetic organic polymers (https://doi.org/10.1039/d2py01560a). Li and Zhu take advantage of selenium's versatile chemistry and optical properties to make photonic crystals that respond to reactive oxygen species, pH changes and light (https://doi.org/10.1039/d2py00654e). The electronic structure of conjugated polymers can also be tuned through the incorporation of sulfur and selenium, which supports advances in flexible electronics, solar cell technology, and organic light-emitting diodes.¹⁹ In this collection, polythiophenes and polyselenophenes are explored computationally by Wang to provide a molecular level understanding of their properties (https://doi.org/10.1039/d2py00960a). Matsumoto and Nishino report an assessment of the mechanical properties of polythiophene block copolymers, with promising insights into accessing flexible semiconducting polymers (https://doi.org/10.1039/d2py00765g). Tomita describes a hybrid conjugated polymer containing polythiophene and titanacycle units, and its intriguing optoelectronic properties (https://doi.org/10.1039/d2py00452f). Seferos reports degradable π-conjugated polythiophenes with a 1,2,4-oxadiazole linker synthesized by direct heteroarylation polymerization (https://doi.org/10.1039/d2py00984f).

The diverse polymer structures, reactivity, and applications described in this themed collection are a testament to the versatility of chalcogen-containing polymers. In this Editorial we have highlighted a few of the many discoveries reported in this collection and we encourage you to explore the online compilation in its entirety here. We thank all the authors for their contributions to this themed collection and advancing our knowledge of chalcogen-containing polymers. We hope that this compilation is instructive to those new to the field, and that the novel findings spur future work in this intriguing area of polymer science.

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