Andrew P. Vogt

Constructing star polymersvia modular ligation strategies

Branched polymers result in a more compact structure in comparison to linear polymers of identical molecular weight, due to their high segment density which affects the crystalline, mechanical, and viscoelastic properties of the polymer. Star polymers constitute the simplest form of branched macromolecules where all of the chains—or arm segments—of one macromolecule are linked to a centre defined as the core. Over recent years, modular ligation reactions—some of which adhere to click criteria—have enabled the synthesis of a variety of star polymersvia efficient polymer–polymer conjugations. While the modified Huisgen [3 + 2] dipolar copper catalyzed azide and alkyne cycloaddition (CuAAC) has been widely employed for macromolecular star synthesis, Diels–Alder and hetero Diels–Alder reactions offer alternative pathways which allow for similarly efficient macromolecular conjugations. Moreover, combinations of these protocols afford the synthesis of more complex star polymer structures which previously had not been achievable.

Temperature and redox responsive hydrogels from ABA triblock copolymers prepared by RAFT polymerization

Triblock copolymers prepared by reversible addition–fragmentation chain transfer polymerization were capable of forming hydrogels that were both temperature- and redox-responsive, as a result of thiol-disulfide chemistry provided by aminolysis of an internal trithiocarbonate linkage.

Tuning the Magnetic Resonance Imaging Properties of Positive Contrast Agent Nanoparticles by Surface Modification with RAFT Polymers

A novel surface modification technique was employed to produce a polymer modified positive contrast agent nanoparticle through attachment of well-defined homopolymers synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. A range of RAFT homopolymers including poly[N-(2-hydroxypropyl)methacrylamide], poly(N-isopropylacrylamide), polystyrene, poly(2-(dimethylamino)ethyl acrylate), poly(((poly)ethylene glycol) methyl ether acrylate), and poly(acrylic acid) were synthesized and subsequently used to modify the surface of gadolinium (Gd) metal-organic framework (MOF) nanoparticles. Employment of a trithiocarbonate RAFT agent allowed for reduction of the polymer end groups under basic conditions to thiolates, providing a means of homopolymer attachment through vacant orbitals on the Gd3+ ions at the surface of the Gd MOF nanoparticles. Magnetic resonance imaging (MRI) confirmed the relaxivity rates of these novel polymer modified structures were easily tuned by changes in the molecular weight and chemical structures of the polymers. When a hydrophilic polymer was used for modification of the Gd MOF nanoparticles, an increase in molecular weight of the polymer provided a respective increase in the longitudinal relaxivity. These relaxivity values were significantly higher than both the unmodified Gd MOF nanoparticles and the clinically employed contrast agents, Magnevist and Multihance, which confirmed the constructs ability to be utilized as a positive contrast nanoparticle agent in MRI. Further characterization confirmed that increased hydrophobicity of the polymer coating on the Gd MOF nanoparticles yielded minimal changes in the longitudinal relaxivity properties but large increases in the transverse relaxivity properties in the MRI.

Hyperbranched Polymers via RAFT Copolymerization of an Acryloyl Trithiocarbonate

Hyperbranched copolymers of N-isopropylacrylamide (NIPAM) and styrene were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of a novel acryloyl trithiocarbonate, namely 1-[3-(2-methyl-2-dodecylsulfanylthiocarbonylsulfanylpropionyloxy)propyl]-1H-[1,2,3]triazol-4-ylmethyl acrylate. By employing an example of ‘click chemistry’, we were able to prepare the vinyl RAFT chain transfer agent (CTA) by copper-catalyzed 1,3-dipolar cycloaddition of an azido-functionalized trithiocarbonate and propargyl acrylate. The resulting CTA facilitated the preparation of highly branched poly(N-isopropylacrylamide) (PNIPAM) and polystyrene. Interestingly, the branched PNIPAM demonstrated a reduced lower critical solution temperature (LCST) of 25°C as opposed to the conventional value of 32°C expected for linear PNIPAM, an effect attributed to increased contribution of hydrophobic dodecyl trithiocarbonate end groups.

Modular ambient temperature functionalization of carbon nanotubes with stimuli-responsive polymer strands

Cyclopentadienyl end-capped poly(N-isopropylacrylamide) (PNIPAM-Cp, Mn = 5400 g mol−1, PDI = 1.13) was synthesized via a combination of RAFT (Reversible Addition–Fragmentation Chain Transfer) polymerization and modular conjugation (characterized via Nuclear Magnetic Resonance (NMR) as well as Electrospray Ionization-Mass Spectrometry (ESI-MS)), and reacted with untreated Single Walled Carbon Nanotubes (SWCNTs) as dienophiles in a Diels–Alder reaction with PNIPAM-Cp (diene) at ambient temperature in the absence of any catalyst. The obtained stimuli-responsive hybrid materials display thermo-responsive behaviour evidenced via UV-VIS-spectroscopy and Dynamic Light Scattering (DLS). The grafting density of the PNIPAM chains at the surface of the SWCNTs was determined via Thermogravimetric Analysis (TGA), Elemental Analysis (EA) and X-ray Photoelectron Spectroscopy (XPS), to be close to 0.0288 chains per nm2.

Modular design of glyco-microspheres via mild pericyclic reactions and their quantitative analysis

The facile and efficient functionalization of porous poly(glycidyl methacrylate) (pGMA) microspheres via hetero Diels–Alder (HDA) chemistry with poly(3-O-acryloyl-1,2:5,6-di-O-isopropylidene-α-D-glucofuranoside) (pAIpGlc) prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization employing electron deficient thiocarbonylthio compounds (benzyl pyridin-2-yldithioformate (BPDF)) is described in detail. The efficiency of the employed ‘grafting to’ approach is qualitatively and quantitatively analyzed. Initially the microspheres are functionalized with a highly reactive diene – cyclopentadiene (Cp) – in one step with sodium cyclopentadienide, and subsequently reacted with a protected glycopolymer (number-average molecular weight, Mn = 4200 g mol−1; polydispersity index, PDI = 1.2) that carries a thiocarbonyl moiety functioning as a dienophile. The functionalization of the microspheres is achieved under mild conditions (T = 50 °C) with trifluoroacetic acid (TFA) as a readily removable catalyst. Deprotection of the grafted pAIpGlc to poly(3-O-acryloyl-α,β-D-glucopyranoside) (pAGlc) can be performed after functionalization in one pot with formic acid at ambient temperature. The obtained loading capacity is 2.63 × 1019 chains per g and the grafting density is close to 0.16 chains per nm2. Quantitative analysis of the grafting densities is achieved via elemental analysis; the pore size distribution before functionalization was analyzed by inverse size exclusion chromatography (iSEC). Further employed characterization techniques include scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and high resolution attenuated total reflectance (ATR) FT-IR microscopy supporting the successful modification of the microspheres.

A Facile Route to Boronic Acid Functional Polymeric Microspheres via Epoxide Ring Opening

Boronic acid-functionalized microspheres are prepared for the first time via mild epoxide ring opening based on porous cross-linked polymeric microspheres (diameter ≈ 10 μm, porosity ≈ 1000 Å). Quantitative chemical analysis by XPS and EA evidences that there is a greater functionalization with boronic acid when employing a sequential synthetic method [1.7 atom% boron (XPS); 1.12 wt% nitrogen (EA)] versus a one-pot synthetic method [0.2 atom% boron (XPS); 0.60 wt% nitrogen (EA)] yielding grafting densities ranging from approximately 2.5 molecules of boronic acid per nm(2) to 1 molecule of boronic acid per nm(2), respectively. Furthermore, the boronic acid-functionalized microspheres are conjugated with a novel fluorescent glucose molecule demonstrating a homogeneous spatial distribution of boronic acid.

Polyphthalaldehyde-block-polystyrene as a nanochannel template

A degradable polyphthalaldehyde-polystyrene block copolymer generated by modular ligation is reported for the first time serving as a nanochannel template for the formation of nanostructured materials. The polyphthalaldehyde-b-polystyrene copolymer was spin-coated onto a surface with subsequent polyphthalaldehyde block removal. Block conjugation and block removal were confirmed by H-NMR, SEC, AFM, and SEM.

Clicking dendritic peptides onto single walled carbon nanotubes

Here, we demonstrate a straightforward click-chemistry-based approach for the functionalisation of single walled carbon nanotubes with oligo-lysine dendrons. Azide-functionalised nanotubes were reacted with alkyne-focal dendrons using the 1,3 dipolar Cu-catalysed azide alkyne cycloaddition reaction. Peptide dendron functionalised nanotubes showed significantly increased biocompatibility in rat mesenchymal stem cell culture.

High-order graphene oxide nanoarchitectures

We fabricate unique photoluminescent three dimensional graphene oxide (GO) architectures, so-called GO flowers, by self-assembly onto silicon substrates via solvent-mediated volume-controlled growth. The GO flowers exhibited bright photoluminescence and a photoresponse demonstrating their potential for advanced optical and electronic applications, such as advanced photovoltaic devices and organic light emitting diodes.