Ditopic initiators were evaluated for bidirectional organocatalyzed ROMP. Incorporation of monomer was found to be successful for both inward and outward polymer growth, stemming from divinyl ethers with different relative orientation of alkoxy moieties. Macroinitiators were also used to prepare triblock and graft copolymers that were found to be easily cleaved with acid catalyst.
Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization
We have investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization (ROMP). Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative 13C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, we also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, we observed results consistent with chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) when reaction times were extended.
Amphiphilic Copolymers Capable of Concomitant Release of HNO and Small Molecule Organics
We demonstrate concomitant release of HNO and small molecule organics from amphiphilic poly(norbornene)-based copolymers. This key function was achieved by incorporation of thermally-labile oxazine units within random and block copolymer architectures. Upon thermolysis, we observed generation of HNO and release of a small molecule conjugate. Importantly, the release kinetics of HNO and a UV-active small molecule (4-nitroaniline) were found to be 1:1, signifying an ability to monitor HNO production indirectly, or to simultaneously release organic therapeutics (e.g., nonsteroidal anti-inflammatory drugs)) along with HNO. To our knowledge, these are the first reported polymeric materials demonstrating HNO release from covalently attached HNO donors.
Production of Materials with Spatially-Controlled Crosslink Density via Vat Photopolymerization
We describe an efficient method to produce objects comprising spatially controlled and graded crosslink densities using vat photopolymerization additive manufacturing (AM). Using a commercially available diacrylate-based photoresin, 3D printer, and digital light processing (DLP) projector, we projected grayscale images to print objects in which the varied light intensity was correlated to controlled crosslink densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and crosslink density in the resulting specimens. Mechanical testing of octet truss unit cells in which the properties of the crossbars and vertices were independently modified, revealed unique mechanical responses from the different compositions. From the various test geometries, we measured changes in mechanical properties such as increased strain-to-break in inhomogeneous structures in comparison with homogeneous variants.
Investigation of the Dynamic Nature of 1,2-Oxazines Derived from Peralkylcyclopentadiene and Nitrosocarbonyl Species
We have investigated the reversible hetero-Diels–Alder reaction of 1,2-oxazines derived from a peralkylcyclopentadiene and a series of nitrosocarbonyl dienophiles. The nature of the dienophile was found to impart broad tunability to the dynamic character of the oxazine adducts. The reversibility was also observed in polymeric systems. The fidelity of the reaction and tunable sensitivity toward elevated temperature and water signify potential applications in the development of dynamic covalent materials or delivery systems for small molecule payloads.
Expanded Functionality of Polymers Prepared Using Metal-Free Ring-Opening Metathesis Polymerization
Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.
Comparison of Pyrylium and Thiopyryium Photo-oxidants in Metal-Free Ring-Opening Metathesis Polymerization
Systematically varied pyrylium and thiopyrylium photo-oxidants have been evaluated in the metal-free ring-opening-metathesis polymerization (MF-ROMP) of norbornene. Across the series, we observed higher conversion into polynorbornene from thiopyrylium species in comparison with pyrylium salts that were otherwise similarly functionalized. Additionally, more electron-rich photo-oxidants (i.e., weaker oxidants) correlated with higher conversions.
Investigations in Fundamental and Applied Polymer Mechanochemistry
The field of polymer mechanochemistry has experienced rapid growth over the past decade, propelled largely by the development of force-activated functional groups (mechanophores) and polymer structure-reactivity principles for mechanochemical transduction. In addition to fundamental guidelines for converting mechanical input into chemical output, there has also been increasing focus toward the application of polymer mechanochemistry for specific functions, materials, and devices. These endeavors are made possible by multidisciplinary approaches involving designer polymer synthesis, computational modeling and design, and different fields of engineering. Described herein are contributions from our group on the development of flex activated mechanophores for small molecule release and star polymer mechanochemistry, as well as collaborative efforts toward mechanochemically triggered depolymerizations and 3D printed mechanochromic materials.
Sunflower Polymers for Folate-Mediated Drug Delivery
Polymeric delivery vehicles can improve the safety and efficacy of chemotherapy drugs by facilitating preferential tumor delivery. Polymer–drug conjugates are especially attractive carriers because additional formulation steps are not required during manufacturing, and drug release profiles can be altered based on linker choice. For clinical translation, these vehicles should also be reproducibly and controllably synthesized. Recently, we reported the development of a class of materials called “sunflower polymers,” synthesized by controlled radical polymerization of hydrophilic “petals” from a cyclic multimacroinitiator “core”. This synthesis strategy afforded control over the size of the polymer nanostructures based on their petal polymerization time. In this work, we demonstrate that particle size can be further tuned by varying the degree of polymerization of the cyclic core in addition to that of the petals. Additionally, we investigate the application of these materials for tumor-targeted drug delivery. We demonstrate that folate-targeted, doxorubicin-conjugated sunflower polymers undergo receptor-mediated uptake into cancer cells and pH-triggered drug release leading to cytotoxicity. These materials are attractive as drug carriers due to their discrete and small size, shielded drug cargo that can be triggered for release, and relative ease of synthesis.
Developments in Externally Regulated Ring-Opening Metathesis Polymerization
This account details externally regulated ring-opening metathesis polymerization (ROMP) methods. Various external stimuli are discussed which collectively span chemical, thermal, photochemical, electrochemical, and mechanical modes of catalyst activation. Specific attention is also given to the recent development of a metal-free approach to ROMP that includes electro-organic and photoredox-mediated systems.