Controlling the shape of the molecular weight distribution for tailored tensile and rheological properties in thermoplastics and thermoplastic elastomers

Tuning the Mechanical Properties of 3D-printed Objects by Mixing Chain Transfer Agents in Radical Promoted Cationic RAFT Polymerization

The utilization of (cationic) reversible addition-fragmentation chain transfer (RAFT) polymerization in photoinduced three-dimensional (3D) printing has emerged as a robust technique for fabricating a variety of stimuli-responsive materials. However, methods for precisely adjusting the mechanical properties of these materials remain limited, thereby constraining their broader applicability. In this study, a facile way is introduced to modulate the mechanical properties of 3D printed objects by mixing two chain transfer agents (CTAs) within a radical-promoted cationic RAFT (RPC-RAFT) polymerization-based 3D printing process. Through systematic investigations employing tensile testing and dynamic mechanical analysis (DMA), the influence of CTA concentration and molar ratio between two CTAs on the mechanical behavior of the printed objects are explored. These findings demonstrate that higher concentrations of CTAs or a greater molar ratio of the more active CTA within the mixed CTAs result in decreased Young's modulus and glass transition temperatures of the printed objects. Moreover, the tensile failure strain increased with the increasing CTA content, i.e., the samples became more ductile. This methodology broadens the toolbox available for tailoring the mechanical properties of 3D printed materials.

Chromatographic Separation: A Versatile Strategy to Prepare Discrete and Well-Defined Polymer Libraries

ConspectusThe preparation of discrete and well-defined polymers is an emerging strategy for emulating the remarkable precision achieved by macromolecular synthesis in nature. Although modern controlled polymerization techniques have unlocked access to a cornucopia of materials spanning a broad range of monomers, molecular weights, and architectures, the word "controlled" is not to be confused with "perfect". Indeed, even the highest-fidelity polymerization techniques─yielding molar mass dispersities in the vicinity of Đ = 1.05─unavoidably create a considerable degree of structural and/or compositional dispersity due to the statistical nature of chain growth. Such dispersity impacts many of the properties that researchers seek to control in the design of soft materials.The development of strategies to minimize or entirely eliminate dispersity and access molecularly precise polymers therefore remains a key contemporary challenge. While significant advances have been made in the realm of iterative synthetic methods that construct oligomers with an exact molecular weight, head-to-tail connectivity, and even stereochemistry via small-molecule organic chemistry, as the word "iterative" suggests, these techniques involve manually propagating monomers one reaction at a time, often with intervening protection and deprotection steps. As a result, these strategies are time-consuming, difficult to scale, and remain limited to lower molecular weights. The focus of this Account is on an alternative strategy that is more accessible to the general scientific community because of its simplicity, versatility, and affordability: chromatography. Researchers unfamiliar with the intricacies of synthesis may recall being exposed to chromatography in an undergraduate chemistry lab. This operationally simple, yet remarkably powerful, technique is most commonly encountered in the purification of small molecules through their selective (differential) adsorption to a column packed with a low-cost stationary phase, usually silica. Because the requisite equipment is readily available and the actual separation takes little time (on the order of 1 h), chromatography is used extensively in small-molecule chemistry throughout industry and academia alike. It is, therefore, perhaps surprising that similar types of chromatography are not more widely leveraged in the field of polymer science as well.Here, we discuss recent advances in using chromatography to control the structure and properties of polymeric materials. Emphasis is placed on the utility of an adsorption-based mechanism that separates polymers based on polarity and composition at tractable (gram) scales for materials science, in contrast to size exclusion, which is extremely common but typically analyzes very small quantities of a sample (∼1 mg) and is limited to separating by molar mass. Key concepts that are highlighted include (1) the separation of low-molecular-weight homopolymers into discrete oligomers (Đ = 1.0) with precise chain lengths and (2) the efficient fractionation of block copolymers into high-quality and widely varied libraries for accelerating materials discovery. In summary, the authors hope to convey the exciting possibilities in polymer science afforded by chromatography as a scalable, versatile, and even automated technique that unlocks new avenues of exploration into well-defined materials for a diverse assortment of researchers with different training and expertise.

Controlling primary chain dispersity in network polymers: elucidating the effect of dispersity on degradation

Although dispersity has been demonstrated to be instrumental in determining many polymer properties, current synthetic strategies predominantly focus on tailoring the dispersity of linear polymers. In contrast, controlling the primary chain dispersity in network polymers is much more challenging, in part due to the complex nature of the reactions, which has limited the exploration of properties and applications. Here, a one-step method to prepare networks with precisely tuned primary chain dispersity is presented. By using an acid-switchable chain transfer agent and a degradable crosslinker in PET-RAFT polymerization, the in situ crosslinking of the propagating polymer chains was achieved in a quantitative manner. The incorporation of a degradable crosslinker, not only enables the accurate quantification of the various primary chain dispersities, post-synthesis, but also allows the investigation and comparison of their respective degradation profiles. Notably, the highest dispersity networks resulted in a 40% increase in degradation time when compared to their lower dispersity analogues, demonstrating that primary chain dispersity has a substantial impact on the network degradation rate. Our experimental findings were further supported by simulations, which emphasized the importance of higher molecular weight polymer chains, found within the high dispersity materials, in extending the lifetime of the network. This methodology presents a new and promising avenue to precisely tune primary chain dispersity within networks and demonstrates that polymer dispersity is an important parameter to consider when designing degradable materials.

Living Cationic Ring-Opening Polymerization of Hetero Diels-Alder Adducts to Give Multifactor-Controlled and Fast-Photodegradable Vinyl Polymers

Precise control of multiple structural parameters associated with vinyl polymers is important for producing materials with the desired properties and functions. While the development of living polymerization methods has provided a way to control the various structural parameters of vinyl polymers, the concomitant control of their sequence and regioregularity remains a challenging task. To overcome this challenge, herein, we report the living cationic ring-opening polymerization of hetero Diels-Alder adducts. The scalable and modular synthesis of the cyclic monomers was achieved by a one-step protocol using readily available vinyl precursors. Subsequently, living polymerization of the cyclic monomers was examined, allowing the synthesis of vinyl polymers while controlling multiple factors, including molecular weight, dispersity, alternating sequence, head-to-head regioregularity, and end-group functionality. The living characteristics of the developed method were further demonstrated by block copolymerization. The synthesized vinyl polymers exhibited unique thermal properties and underwent fast photodegradation even under sunlight.

Polymeric Heart Valves Will Displace Mechanical and Tissue Heart Valves: A New Era for the Medical Devices

The development of a novel artificial heart valve with outstanding durability and safety has remained a challenge since the first mechanical heart valve entered the market 65 years ago. Recent progress in high-molecular compounds opened new horizons in overcoming major drawbacks of mechanical and tissue heart valves (dysfunction and failure, tissue degradation, calcification, high immunogenic potential, and high risk of thrombosis), providing new insights into the development of an ideal artificial heart valve. Polymeric heart valves can best mimic the tissue-level mechanical behavior of the native valves. This review summarizes the evolution of polymeric heart valves and the state-of-the-art approaches to their development, fabrication, and manufacturing. The review discusses the biocompatibility and durability testing of previously investigated polymeric materials and presents the most recent developments, including the first human clinical trials of LifePolymer. New promising functional polymers, nanocomposite biomaterials, and valve designs are discussed in terms of their potential application in the development of an ideal polymeric heart valve. The superiority and inferiority of nanocomposite and hybrid materials to non-modified polymers are reported. The review proposes several concepts potentially suitable to address the above-mentioned challenges arising in the R&D of polymeric heart valves from the properties, structure, and surface of polymeric materials. Additive manufacturing, nanotechnology, anisotropy control, machine learning, and advanced modeling tools have given the green light to set new directions for polymeric heart valves.

Designing Nanostructured 3D Printed Materials by Controlling Macromolecular Architecture

Nanostructured polymeric materials play important roles in many advanced applications, however, controlling the morphologies of polymeric thermosets remains a challenge. This work uses multi-arm macroCTAs to mediate polymerization-induced microphase separation (PIMS) and prepare nanostructured materials via photoinduced 3D printing. The characteristic length scale of microphase-separated domains is determined by the macroCTA arm length, while nanoscale morphologies are controlled by the macroCTA architecture. Specifically, using 2- and 4- arm macroCTAs provides materials with different morphologies compared to analogous monofunctional linear macroCTAs at similar compositions. The mechanical properties of these nanostructured thermosets can also be tuned while maintaining the desired morphologies. Using multi-arm macroCTAs can thus broaden the scope of accessible nanostructures for extended applications, including the fabrication of actuators and potential drug delivery devices.