3 research outputs found
“Brush-First” Method for the Parallel Synthesis of Photocleavable, Nitroxide-Labeled Poly(ethylene glycol) Star Polymers
We describe the parallel, one-pot synthesis of core-photocleavable,
polyÂ(norbornene)-<i>co</i>-polyÂ(ethylene glycol) (PEG) brush-arm
star polymers (BASPs) via a route that combines the “graft-through”
and “arm-first” methodologies for brush polymer and
star polymer synthesis, respectively. In this method, ring-opening
metathesis polymerization of a norbornene–PEG macromonomer
generates small living brush initiators. Transfer of various amounts
of this brush initiator to vials containing a photocleavable bis-norbornene
cross-linker yielded a series of water-soluble BASPs with low polydispersities
and molecular weights that increased geometrically as a function of
the amount of bis-norbornene added. The BASP cores were cleaved upon
exposure to UV light; the extent of photo-disassembly depended on
the amount of cross-linker. EPR spectroscopy of nitroxide-labeled
BASPs was used to probe differences between the BASP core and surface
environments. We expect that BASPs will find applications as easy-to-synthesize,
stimuli-responsive core–shell nanostructures
EPR Study of Spin Labeled Brush Polymers in Organic Solvents
Spin-labeled polylactide brush polymers were synthesized via ring-opening metathesis polymerization (ROMP), and nitroxide radicals were incorporated at three different locations of brush polymers: the end and the middle of the backbone, and the end of the side chains (periphery). Electron paramagnetic resonance (EPR) was used to quantitatively probe the macromolecular structure of brush polymers in dilute solutions. The peripheral spin-labels showed significantly higher mobility than the backbone labels, and in dimethylsulfoxide (DMSO), the backbone end labels were shown to be more mobile than the middle labels. Reduction of the nitroxide labels by a polymeric reductant revealed location-dependent reactivity of the nitroxide labels: peripheral nitroxides were much more reactive than the backbone nitroxides. In contrast, almost no difference was observed when a small molecule reductant was used. These results reveal that the dense side chains of brush polymers significantly reduce the interaction of the backbone region with external macromolecules, but allow free diffusion of small molecules
Using EPR To Compare PEG-<i>branch</i>-nitroxide “Bivalent-Brush Polymers” and Traditional PEG Bottle–Brush Polymers: Branching Makes a Difference
Attachment of polyÂ(ethylene glycol) (PEG) to polymeric
nanostructures
is a general strategy for sterically shielding and imparting water
solubility to hydrophobic payloads. In this report, we describe direct
graft-through polymerization of branched, multifunctional macromonomers
that possess a PEG domain and a hydrophobic nitroxide domain. Electron
paramagnetic resonance (EPR) spectroscopy was used to characterize
microenvironments within these novel nanostructures. Comparisons were
made to nitroxide-labeled, traditional bottle-brush random and block
copolymers. Our results demonstrate that bivalent bottle-brush polymers
have greater microstructural homogeneity compared to random copolymers
of similar composition. Furthermore, we found that compared to a traditional
brush polymer, the branched-brush, “pseudo-alternating”
microstructure provided more rotational freedom to core-bound nitroxides,
and greater steric shielding from external reagents. The results will
impact further development of multivalent bottle-brush materials as
nanoscaffolds for biological applications