Alternating copolymerization of γ-selenobutyrolactone with episulfides for high refractive index selenium-containing polythioesters

High-Refractive-Index Cross-Linked Cyclic Olefin Polymers with Excellent Transparency via Thiol-Ene Click Reaction

High-refractive-index polymers are important optical materials in optoelectronics. Conventional cyclic olefin polymers (COPs), possessing many excellent optical properties, are a class of highly promising optical materials; however, one of the greatest obstacles is their low refractive index of n = 1.52-1.54. Here, one efficient strategy of first incorporating high molar refraction groups, including carbazolyl and indolyl moieties, into unsaturated COPs via ring-opening metathesis polymerization (ROMP) and then introducing another high molar refraction sulfur atom by a subsequent thiol-ene click reaction is presented. The obtained cross-linked COPs bearing both an aromatic group and sulfur possess significantly higher refractive indices (n = 1.611-1.684 at 589 nm) and highly optical transparency (approximately 95%) in the range of vis-NIR. This provides a way toward potential applications of new-generation optical materials.

Recent Progress in Sulfur-Containing High Refractive Index Polymers for Optical Applications

The development in the field of high refractive index materials is a crucial factor for the advancement of optical devices with advanced features such as image sensors, optical data storage, antireflective coatings, light-emitting diodes, and nanoimprinting. Sulfur plays an important role in high refractive index applications owing to its high molar refraction compared to carbon. Sulfur exists in multiple oxidation states and can exhibit various stable functional groups. Over the last few decades, sulfur-containing polymers have attracted much attention owing to their wide array of applications governed by the functional group of sulfur present in the polymer repeat unit. The interplay of refractive index and various other polymer properties contributes to successfully implementing a specific polymer material in optical applications. The focus on developing optoelectronic devices induced an ever-increasing need to integrate different functional materials to achieve the devices' full potential. Several devices that see the potential use of sulfur in high refractive index materials are reviewed in the study. Like sulfur, selenium also exhibits high molar refraction and unique chemical properties, making it an essential field of study. This review covers the research and development in the field of sulfur and selenium in different forms of functionality, focusing on the chemistry of bonding and the optical properties of the polymers containing the heteroatoms mentioned above. The strategy and rationale behind incorporating heteroatoms in a polymer matrix to produce high-refractive-index materials are also described in the present review.

Copolymerization Involving Sulfur-Containing Monomers

Incorporating sulfur (S) atoms into polymer main chains endows these materials with many attractive features, including a high refractive index, mechanical properties, electrochemical properties, and adhesive ability to heavy metal ions. The copolymerization involving S-containing monomers constitutes a facile method for effectively constructing S-containing polymers with diverse structures, readily tunable sequences, and topological structures. In this review, we describe the recent advances in the synthesis of S-containing polymers via copolymerization or multicomponent polymerization techniques concerning a variety of S-containing monomers, such as dithiols, carbon disulfide, carbonyl sulfide, cyclic thioanhydrides, episulfides and elemental sulfur (S8). Particularly, significant focus is paid to precise control of the main-chain sequence, stereochemistry, and topological structure for achieving high-value applications.

Recent Advances in Sequence-Controlled Ring-Opening Copolymerizations of Monomer Mixtures

Transforming renewable resources into functional and degradable polymers is driven by the ever-increasing demand to replace unsustainable polyolefins. However, the utility of many degradable homopolymers remains limited due to their inferior properties compared to commodity polyolefins. Therefore, the synthesis of sequence-defined copolymers from one-pot monomer mixtures is not only conceptually appealing in chemistry, but also economically attractive by maximizing materials usage and improving polymers' performances. Among many polymerization strategies, ring-opening (co)polymerization of cyclic monomers enables efficient access to degradable polymers with high control on molecular weights and molecular weight distributions. Herein, we highlight recent advances in achieving one-pot, sequence-controlled polymerizations of cyclic monomer mixtures using a single catalytic system that combines multiple catalytic cycles. The scopes of cyclic monomers, catalysts, and polymerization mechanisms are presented for this type of sequence-controlled ring-opening copolymerization.

Easy Access to Diverse Multiblock Copolymers with On-Demand Blocks via Thioester-Relayed In-Chain Cascade Copolymerization

Multiblock copolymers are envisioned as promising materials with enhanced properties and functionality compared with their diblock/triblock counterparts. However, the current approaches can construct multiblock copolymers with a limited number of blocks but tedious procedures. Here, we report a thioester-relayed in-chain cascade copolymerization strategy for the easy preparation of multiblock copolymers with on-demand blocks, in which thioester groups with on-demand numbers are built in the polymer backbone by controlled/living polymerizations. These thioester groups further serve as the in-chain initiating centers to trigger the acyl group transfer ring-opening polymerization of episulfides independently and concurrently to extend the polymer backbone into multiblock structures. The compositions, number of blocks, and block degree of polymerization can be easily regulated. This strategy can offer easy access to a library of multiblock copolymers with ≈100 blocks in only 2 to 4 steps.

Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.

Heterocycle/Heteroallene Ring-Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers

Heteroatom-containing polymers have strong potential as sustainable replacements for petrochemicals, show controllable monomer-polymer equilibria and properties spanning plastics, elastomers, fibres, resins, foams, coatings, adhesives, and self-assembled nanostructures. Their current and future applications span packaging, house-hold goods, clothing, automotive components, electronics, optical materials, sensors, and medical products. An interesting route to these polymers is the catalysed ring-opening copolymerisation (ROCOP) of heterocycles and heteroallenes. It is a living polymerization, occurs with high atom economy, and creates precise, new polymer structures inaccessible by traditional methods. In the last decade there has been a renaissance in research and increasing examples of commercial products made using ROCOP. It is better known in the production of polycarbonates and polyesters, but is also a powerful route to make N-, S-, and other heteroatom-containing polymers, including polyamides, polycarbamates, and polythioesters. This Review presents an overview of the different catalysts, monomer combinations, and polymer classes that can be accessed by heterocycle/heteroallene ROCOP.