We report facile synthesis of 3-trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione and ring opening polymerization of the fluoro-lactide monomer to prepare polylactides composed of trifluoromethyl and methyl pendent groups on each repeat unit (FPLA). Molecular weights of the prepared polymers correlated well with the initial molar ratio of monomer to initiator, and were found to range from 6.6 to 22.5 kDa as determined by 1H NMR spectroscopy. GPC analysis revealed an Mn of up to 16.5 kDa. 1H, 13C, and 19F NMR spectroscopy were consistent with the structures of the lactide monomer isomers, and 1H NMR analysis was consistent with polymer backbones of alternating trifluoromethyl- and methyl-substituted lactate constituents. Glass transition temperature (Tg) and decomposition temperature (Td) of the new FPLA were found to be 39 °C and 225 °C by DSC and TGA, respectively. Additionally, we prepared amphiphilic block copolymers of FPLA and polyethylene glycol (PEG). Specifically, FPLA-b-PEG diblocks and FPLA-PEG-FPLA triblocks were synthesized by using PEG monomethyl ether (mPEG) or PEG as alcohol initiators, respectively. We observed the formation of vesicles or worm-like micelles from the particles of FPLA-PEG-FPLA in dilute aqueous solution by transmission electron microscopy (TEM), suggesting potential applications for drug delivery.
Teaching Problem-based Learning at Sammamish High School
The Boydston Group has been involved in discussions regarding lesson plans and lab planning, and has helped develop hands-on lab experiences in which SHS students work with UW graduate researchers to address a specific scientific problem. The environment provides an opportunity for young students to get a glimpse of how researchers approach scientific problems, and for graduate researchers to explain the impacts of their research and how it builds from key fundamental principles in chemistry.
Modular Elastomer Photoresins for Digital Light Processing Additive Manufacturing
A series of photoresins suitable for production of elastomeric objects via digital light processing additive manufacturing are reported. Notably, the printing procedure is readily accessible using only entry-level equipment under ambient conditions using visible light projection. The photoresin formulations were found to be modular in nature and straightforward adjustments to the resin components enabled access to a range of compositions and mechanical properties. Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens displayed maximum elongations of up to 472% under tensile load, tunable swelling behavior in water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to print a functional multi-material three-armed pneumatic gripper. These photoresins could be transformative to advanced prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable devices, and soft robotics.
Bidirectional Metal-Free ROMP from Difunctional Organic Initiators
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.