26 research outputs found
PEGylated <i>N</i>âHeterocyclic Carbene Anchors Designed To Stabilize Gold Nanoparticles in Biologically Relevant Media
<i>N</i>-Heterocyclic carbenes
(NHCs) have emerged as
versatile ligands for surface functionalization. Their ease of synthesis
and ability to form strong bonds with a range of substrates provide
a unique complement to traditional surface modification methods. Gold
nanoparticles (NPs) are a particularly useful class of materials whose
applications intimately depend on surface functionalization. Here
we report the development of PEGylated-NHC ligands for Au-NP surfaces
and the first example of NHC-functionalized NPs that are compatible
with biologically relevant conditions. Our PEGylated-NHC-Au-NPs are
stable toward aggregation in aqueous solutions in the pH range of
3â14, in <250 mM electrolyte solutions, at high and low
temperatures (95 and â78 °C), in cell culture media, and
in aqueous H<sub>2</sub>O<sub>2</sub> solutions. This work demonstrates
for the first time that NHCs can serve as anchors for water-soluble
Au-NPs and opens the door to potential biomedical applications of
NHC surface anchors
Visible-Light-Controlled Living Radical Polymerization from a Trithiocarbonate Iniferter Mediated by an Organic Photoredox Catalyst
Living
radical polymerization of acrylates and acrylamides from
trithiocarbonate iniferters using a compact fluorescent lamp (CFL)
bulb and 10-phenylphenothiazine as an organic photoredox catalyst
is reported. With this system, chain growth can be efficiently switched
between âonâ and âoffâ in response to
visible light. Polymer molar masses increase linearly with conversion,
and narrow molar mass distributions are obtained. The excellent fidelity
of the trithiocarbonate-iniferter enables the preparation of triblock
copolymers from macro-iniferters under the same visible-light mediated
protocol, using UV light without a photoredox catalyst or under traditional
thermally induced RAFT conditions. We expect that the simplicity and
efficiency of this metal-free, visible-light-mediated polymerization
will enable the synthesis and modification of a range of materials
under mild conditions
Sub-10 nm Self-Assembly of Mesogen-Containing Grafted Macromonomers and Their Bottlebrush Polymers
We
explore the morphology and phase behavior of branched diblock
macromonomers and their polymers. A series of macromonomers was synthesized
based on a disubstituted norbornene. The first branch consists of
polydimethylsiloxane (PDMS) while the second branch is a quasi-mesogenic
structure incorporating one or more cyanobiphenyl (CB) moieties. Bottlebrush
polymers with varying degrees of polymerization were prepared by âgraft-throughâ
ring-opening metathesis of the macromonomers. The molecules in the
resulting library of macromonomers and bottlebrush polymers self-assemble
to form classically observed microphase-separated structures, including
spheres, hexagonally packed cylinders, bicontinuous gyroid, and lamellae.
The systematic variation of molecular structure, molecular weight
of each branch, and degree of polymerization of the polymers results
in a diverse set of structures and properties. We report the observation
of well-ordered lamellae and cylinders with <i>d</i>-spacings
as low as 6.1 and 8.0 nm, respectively. The system displays an asymmetric
phase diagram, with large deviations from the canonical phase behavior
of linear coilâcoil diblocks. Hexagonally packed cylinders
and lamellae are observed at remarkably small mass fractions of the
mesogen-containing block of 0.07 and 0.21, respectively. The samples
are highly birefringent, and polarized optical microscopy revealed
the formation of well-developed textures in microphase-separated states
formed by cooling samples through the orderâdisorder transition.
The textures are reminiscent of the classic fan-like or focal-conic
textures observed in small molecule liquid crystal mesophases, highlighting
the formation of unusually large and well-ordered grains of the microphase-separated
PDMS and CB microdomains. Apparent crystallization of the CB units in
systems with two or three CB moieties per monomer results in distortion
of the microphase-separated structure. The small <i>d</i>-spacings and large grain sizes observed here highlight the versatility
and potential utility of this molecular architecture for designing
and engineering new functional materials
Caspase-3-Responsive, Fluorogenic Bivalent Bottlebrush Polymers
Controlling the access of proteases
to cleavable peptides
placed
at specific locations within macromolecular architectures represents
a powerful strategy for biologically responsive materials design.
Here, we report the synthesis of peptide-containing bivalent bottlebrush (co)polymers (BBPs) featuring polyethylene glycol (PEG)
and 7-amino-4-methylcoumarin (AMC) pendants on each backbone repeat
unit. The AMCs are linked via caspase-3-cleavable peptides which,
upon enzymatic cleavage, provide a âturn-onâ fluorescence
signal due to the release of free AMC. Time-dependent fluorscence
measurements demonstrate that the caspase-3-induced peptide cleavage
and AMC release from BBPs is strongly dependent on the BBP backbone
length and the AMCâpeptide linker location within the BBP architecture,
revealing fundamental insights into the interactions of enzymes with
BBPs
Topological Structure of Networks Formed from Symmetric Four-Arm Precursors
Gels formed by coupling two different
four-arm star polymers lead
to polymer networks with high strength and low spatial heterogeneity.
However, like all real polymer networks, these materials contain topological
defects which affect their properties. In this study, kinetic graph
theory and Monte Carlo simulation are used to investigate the structure
and cyclic defects formed via AâB type end-linking of symmetric
tetra-arm star polymer precursors. While loops constituting of odd
number of junctions are forbidden by precursor chemistry, the amount
and the correlation of secondary loops are found to increase with
decreasing precursor concentration. This suppresses gelation, and
the delay of gel point is quantitatively predicted by the topological
simulations. Furthermore, comparison with network formed with asymmetric
bifunctionalâtetrafunctional precursors revealed that the behavior
of loops consisting of 2<i>n</i> junctions in the symmetric
system is analogous to the behavior of loops consisting of <i>n</i> junctions in the asymmetrical system, suggesting analogies
between chemically distinct networks
Addressable Carbene Anchors for Gold Surfaces
New
strategies to access functional monolayers could augment current
surface modification methods. Here we present addressable <i>N</i>-heterocyclic carbene (ANHC) anchors for gold surfaces.
A suite of experimental and theoretical methods was used to characterize
ANHC monolayers. We demonstrate grafting of highly fluorinated polymers
from surface-bound ANHCs. This work establishes ANHCs as viable anchors
for gold surfaces
Kinetic Monte Carlo Simulation for Quantification of the Gel Point of Polymer Networks
Accurate prediction of the gel point
for real polymer networks
is a long-standing challenge in polymer chemistry and physics that
is extremely important for applications of gels and elastomers. Here,
kinetic Monte Carlo simulation is applied to simultaneously describe
network topology and growth kinetics. By accounting for topological
defects in the polymer networks, the simulation can quantitatively
predict experimental gel point measurements without any fitting parameters.
Gel point suppression becomes more severe as the primary loop fraction
in the networks increases. A topological homomorphism theory mapping
defects onto effective junctions is developed to qualitatively explain
the origins of this effect, which accurately captures the gel point
suppression in the low loop limit where cooperative effects between
topological defects are small
Application of <sup>1</sup>H DOSY for Facile Measurement of Polymer Molecular Weights
To address the practical issues of polymer molecular
weight determination,
the first accurate polymer weight-average molecular weight determination
method in diverse living/controlled polymerization via DOSY (diffusion-ordered
NMR spectroscopy) is reported. Based on the linear correlation between
the logarithm of diffusion coefficient (log <i>D</i>) and
the molecular weights (log <i>M</i><sub>w</sub>), external
calibration curves were created to give predictions of molecular weights
of narrowly dispersed polymers. This method was successfully applied
to atom transfer radical polymerization (ATRP), reversible additionâfragmentation
chain transfer (RAFT), and ring-opening metathesis polymerization
(ROMP), with weight-average molecular weights given by this method
closely correlated to those obtained from GPC measurement
Main-Chain Zwitterionic Supramolecular Polymers Derived from <i>N</i>âHeterocyclic CarbeneâCarbodiimide (NHCâCDI) Adducts
Polyzwitterions have found extensive
applications in biological
and materials sciences. Despite this success, most polyzwitterions
have nondegradable polyolefin backbones with pendant zwitterionic
groups. Transcension of this structural paradigm via the formation
of main-chain zwitterionic supramolecular polymers could lead to readily
processable, as well as self-healing and/or degradable, polyzwitterions.
Herein, we report the synthesis and characterization of polyÂ(azolium
amidinate)Âs (PAzAms), which are a new class of supramolecular main-chain
polyzwitterions assembled via the formation of <i>N</i>-heterocyclic
carbeneâcarbodiimide (NHCâCDI) adducts. These polymers
exhibit a wide range of tunable dynamic properties due to the highly
structure-sensitive equilibrium between the NHCâCDI adduct
and its constituent NHCs and CDIs: e.g., PAzAms derived from <i>N</i>-aryl-<i>Nâ˛</i>-alkyl CDIs are dynamic
at lower temperatures than those derived from <i>N</i>,<i>Nâ˛</i>-diaryl CDIs. We develop a versatile synthetic
platform that provides access to PAzAms with control over the main-chain
charge sequence and molecular weight. In addition, block copolymers
incorporating PAzAm and polyÂ(ethylene glycol) (PEG) blocks are water
soluble (>30 mg mL<sup>â1</sup>) and self-assemble in aqueous
environments. This work defines structureâproperty relationships
for a new class of degradable main-chain zwitterionic supramolecular
polymers, setting the stage for the development of these polymers
in a range of applications
Logic-Controlled Radical Polymerization with Heat and Light: Multiple-Stimuli Switching of Polymer Chain Growth via a Recyclable, Thermally Responsive Gel Photoredox Catalyst
Strategies
for switching polymerizations between âONâ
and âOFFâ states offer new possibilities for materials
design and fabrication. While switching of controlled radical polymerization
has been achieve using light, applied voltage, allosteric effects,
chemical reagents, pH, and mechanical force, it is still challenging
to introduce multiple external switches using the same catalyst to
achieve logic gating of controlled polymerization reactions. Herein,
we report an easy-to-synthesize thermally responsive organo-/hydro-gel
that features covalently bound 10-phenylphenothiazine (PTH). With
this âGel-PTHâ, we demonstrate switching of controlled
radical polymerization reactions using temperature âLOWâ/âHIGHâ,
light âONâ/âOFFâ, and catalyst presence
âINâ/âOUTâ. Various iniferters/initiators
and a wide range of monomers including acrylates, methacrylates, acrylamides,
vinyl esters, and vinyl amides were polymerized by RAFT/iniferter
and ATRP methods using Gel-PTH and a readily available compact fluorescent
light (CFL) source. In all cases, polymer molar masses increased linearly
with conversion, and narrow molar mass distributions were obtained.
To further highlight the utility of Gel-PTH, we achieved âANDâ
gating of controlled radical polymerization wherein various combinations
of three stimuli were required to induce polymer chain growth. Finally,
block copolymer synthesis and catalyst recycling were demonstrated.
Logic-controlled polymerization with Gel-PTH offers a straightforward
approach to achieve multiplexed external switching of polymer chain
growth using a single catalyst without the need for addition of exogenous
reagents