Simple Monomers for Precise Polymer Functionalization During Ring-Opening Metathesis Polymerization

Controlling the monomer sequence of synthetic polymers is a grand challenge in polymer science. Conventional sequence control has been achieved in dispersed polymers by exploiting the kinetic tendencies of monomers and their order of addition. While the sequence of blocks in multiblock copolymers can be readily tuned using sequential addition of monomers (SAM), control over the sequence distribution is eroded as the targeted block size approaches a single monomer unit (i.e., Xn ∼ 1) due to the stochastic nature of chain-growth reactions. Thus, unique monomers are needed to ensure precise single additions. Herein, we investigate common classes of cyclic olefin monomers for ring-opening metathesis polymerization (ROMP) to identify monomers for single unit addition during sequential monomer addition synthesis. Through careful analysis of polymerization kinetics, we find that easily synthesized oxanorbornene imide monomers are suitable for single-addition reactions. With the identified monomers, we demonstrate the synthesis of multiblock copolymers containing up to three precise functionalization sites and singly cross-linked four-armed star copolymers. We envision that expanded kinetic analyses of monomer reactivities in ROMP reactions will enable novel polymer synthesis capabilities such as the autonomous synthesis of sequence-defined polymers or one-shot multiblock copolymer syntheses

Length Control of Biodegradable Fiber-Like Micelles via Tuning Solubility: A Self-Seeding Crystallization-Driven Self-Assembly of Poly(ε-caprolactone)-Containing Triblock Copolymers

The crystallization-driven self-assembly of polymers based on semicrystalline poly­(ε-caprolactone) cores is currently an area of high interest on account of their well-known biocompatibility and biodegradability, yet a comprehensive understanding of coil–crystalline–coil type triblock copolymer assembly behavior with respect to this core chemistry is yet to be realized. Herein, we demonstrate the simple preparation of well-defined tuneable 1D and 2D structures based on poly­(ε-caprolactone) (PCL) triblock copolymers of different block ratios synthesized by ring-opening polymerization (ROP) and reversible addition–fragmentation chain transfer (RAFT) polymerization. In this report, the assembly of PCL-based amphiphiles in various solvents was investigated to tune the morphology and size of the assemblies with well-defined 2D platelets and long cylinders produced when using long soluble coronal blocks or under good solvent conditions. By contrast, truncated short fibers were obtained for less soluble PCL-containing block copolymers or under poor solubility conditions for the core block as a consequence of the increasing amount of nuclei formed in the crystallization process. Furthermore, the length of PCL-based 1D nanostructures could be controlled by tuning self-assembly conditions where the micelles’ lengths varied from 93 to 1200 nm with narrow dispersities. This easy assembly methodology greatly simplifies the lengthy procedure required to prepare biodegradable 1D and 2D nanostructures from PCL with tuneable sizes, which demonstrate great potential as drug-delivery vehicles in the realm of biomedicine

Simulations of Glass Transition and Mechanical Behavior of Off-Stoichiometric Crosslinked Polymers

This work explores the influence of blend composition, network architecture, and hydrogen bonding on the material properties of crosslinked epoxy networks, focusing on the glass transition temperature (Tg) and Young’s modulus (Y). We used coarse-grained molecular dynamics simulations to simulate varying compositions of stiff and flexible components in epoxy monomer blends with varying excess of curative. We find that, without hydrogen bonding, networks of any composition show a monotonically increasing Tg with decreasing excess curative, consistent with theory. In contrast, we find that when hydrogen bonding is introduced, the binary blend networks show significant enhancement in Tg for lightly crosslinked systems. This result contributes to an explanation of the anomalous Tg behavior observed experimentally in these systems. We further find that Y is generally enhanced by hydrogen bonds, especially below Tg, demonstrating that hydrogen bonding has a significant influence on mechanical properties and can allow access to other desirable dynamic behavior, especially self-healing

Therapeutic Delivery of H₂S via COS: Small Molecule and Polymeric Donors with Benign Byproducts

Carbonyl sulfide (COS) is a gas that may play important roles in mammalian and bacterial biology, but its study is limited by a lack of suitable donor molecules. We report here the use of <i>N</i>-thiocarboxyanhydrides (NTAs) as COS donors that release the gas in a sustained manner under biologically relevant conditions with innocuous peptide byproducts. Carbonic anhydrase converts COS into H₂S, allowing NTAs to serve as either COS or H₂S donors, depending on the availability of the enzyme. Analysis of the pseudo-first-order H₂S release rate under biologically relevant conditions revealed a release half-life of 75 min for the small molecule NTA under investigation. A polynorbornene bearing pendant NTAs made by ring-opening metathesis polymerization was also synthesized to generate a polymeric COS/H₂S donor. A half-life of 280 min was measured for the polymeric donor. Endothelial cell proliferation studies revealed an enhanced rate of proliferation for cells treated with the NTA over untreated controls

Log <i>P</i>_oct/SA Predicts the Thermoresponsive Behavior of P(DMA-<i>co</i>-RA) Statistical Copolymers

Polymers that exhibit a lower critical solution temperature (LCST) have been of great interest for various biological applications such as drug or gene delivery, controlled release systems, and biosensing. Tuning the LCST behavior through control over polymer composition (e.g., upon copolymerization of monomers with different hydrophobicity) is a widely used method, as the phase transition is greatly affected by the hydrophilic/hydrophobic balance of the copolymers. However, the lack of a general method that relates copolymer hydrophobicity to their temperature response leads to exhaustive experiments when seeking to obtain polymers with desired properties. This is particularly challenging when the target copolymers are comprised of monomers that individually form nonresponsive homopolymers, that is, only when copolymerized do they display thermoresponsive behavior. In this study, we sought to develop a predictive relationship between polymer hydrophobicity and cloud point temperature (TCP). A series of statistical copolymers were synthesized based on hydrophilic N,N-dimethyl acrylamide (DMA) and hydrophobic alkyl acrylate monomers, and their hydrophobicity was compared using surface area-normalized octanol/water partition coefficients (Log Poct/SA). Interestingly, a correlation between the Log Poct/SA of the copolymers and their TCPs was observed for the P­(DMA-co-RA) copolymers, which allowed TCP prediction of a demonstrative copolymer P­(DMA-co-MMA). These results highlight the strong potential of this computational tool to improve the rational design of copolymers with desired temperature responses prior to synthesis

Additive Manufacturing of Degradable Materials via Ring-Opening Metathesis Polymerization (ROMP)

Thermoset materials comprise a significant proportion of high-performance plastics due to their shape permanence and excellent thermal and mechanical properties. However, these properties come at the expense of degradability. Here, we show for the first time that the industrial thermoset polydicyclopentadiene (PDCPD) can be additively manufactured (AM) with degradable 2,3-dihydrofuran (DHF) linkages using a photochemical approach. Treatment of the manufactured objects with acid results in rapid degradation to soluble byproducts. This work highlights the potential of ring-opening metathesis polymerization (ROMP) chemistry to create degradable materials amenable to advanced manufacturing processes

Photoinitiated Olefin Metathesis and Stereolithographic Printing of Polydicyclopentadiene

Recent progress in photoinitiated ring-opening metathesis polymerization (photoROMP) has enabled the lithographic production of patterned films from olefinic resins. Recently, we reported the use of a latent ruthenium catalyst (HeatMet) in combination with a photosensitizer (2-isopropylthioxanthone) to rapidly photopolymerize dicyclopentadiene (DCPD) formulations upon irradiation with UV light. While this prior work was limited in terms of catalyst and photosensitizer scope, a variety of alternative catalysts and photosensitizers are commercially available that could allow for tuning of thermomechanical properties, potlifes, activation rates, and irradiation wavelengths. Herein, 14 catalysts and 8 photosensitizers are surveyed for the photoROMP of DCPD and the structure–activity relationships of the catalysts examined. Properties relevant to stereolithography additive manufacturing (SLA AM)potlife, irradiation dose required to gel, conversionare characterized to develop catalyst and photosensitizer libraries to inform development of SLA AM resin systems. Two optimized catalyst/photosensitizer systems are demonstrated in the rapid SLA printing of complex, multidimensional pDCPD structures with microscale features under ambient conditions

Unexpected Thermomechanical Behavior of Off-Stoichiometry Epoxy/Amine Materials

Recent studies on off-stoichiometric thermosets reveal unique viscoelastic behavior derived from increased free volume and physical interactions between chain ends. To understand structural characteristics arising from cure and its effect on properties, we developed a Monte Carlo model based on step-growth polymerization. Our model accurately predicted structure–property trends for a two-component system of EPON 828 (EPON) and ethylenediamine. A second epoxy monomer, D.E.R. 732 (DER), was investigated to modulate Tg. Binary mixtures of EPON and DER in off-stoichiometric, amine-rich formulations resulted in nonlinear evolution of thermomechanical properties with respect to initial formulation stoichiometry. Modifying our model with kinetic parameters allowing for differential epoxide/amine reaction kinetics only partially accounted for trends in Tg, suggesting that spatiotemporal contributionsnot captured by our modelwere significant determinants of material properties compared to polymer architecture for three-component systems. These findings underpin the importance of spatial awareness in modeling to inform the development of dynamic thermosets