Particles without a Box: Brush-first Synthesis of Photodegradable PEG Star Polymers under Ambient Conditions

Convenient methods for the rapid, parallel synthesis of diversely functionalized nanoparticles will enable discovery of novel formulations for drug ...

Massachusetts Institute of Technology

A simple scheme involving atom transfer radical polymerization (ATRP) from a bifunctional initiator, conversion of the bromine end groups of the resulting telechelic polymer to azides, and cross-linking of this azido-telechelic macromonomer with multi-acetylene functionalized small molecules via copper-catalyzed azide-alkyne cycloaddition was employed to prepare the first tert-butyl acrylate model networks. This general scheme is wide in scope, enabling synthesis of model networks possessing defined pore size from any monomer polymerizable by ATRP. Introduction of an olefin moiety into the ATRP initiator enabled degradation of the materials by ozonolysis to yield star polymer products bearing three or four arms depending on which cross-linker was employed in the parent network. Size-exclusion chromatography of the ozonolysis products confirmed the pore size of the parent network and yielded insight into the number of unreacted functionalities. Model networks derived from a trifunctional alkyne were found to be more completely cross-linked than those derived from a tetrafunctional alkyne, presumably due to less steric hindrance in the former system.

Synthesis of degradable model networks via ATRP and click chemistry.

A new means for functionalizing metal oxide surfaces, specifically nanoparticles, is demostrated. This process involves the design of stable ligands that bind strongly to the surface of metal oxides and can undergo further chemical modification via click chemistry, with both small molecules as well as polymers, to yield metal oxide surfaces with tailored functionality. The nanoparticles are stable and easily dispersed in both polar and nonpolar solvents, a property that is controlled by the ligand. The resultant nanoparticles were characterized by TEM, TGA, FTIR, and NMR.

Toward the syntheses of universal ligands for metal oxide surfaces: controlling surface functionality through click chemistry.

Bifunctional, fluorinated cyclooctynes were used for the in situ "click" crosslinking of azide-terminated photodegradable star polymers, yielding photodegradable polymeric model networks with well-defined structures and tunable gelation times.

Copper-free click chemistry for the in situ crosslinking of photodegradable star polymers.

The interconversion of ortho-hydrogen (oH(2)) and para-hydrogen (pH(2)), the two nuclear spin isomers of dihydrogen, requires a paramagnetic spin catalyst such as a nitroxide. We report the design and demonstration of spin catalysis of the interconversion of oH(2) and pH(2) incarcerated in an endofullerene based on a reversible nitroxide/hydroxylamine system. The system is an example of a reversible magnetic spin catalysis switch that can increase the rate of interconversion of the nuclear spin isomers of H(2) by a factor of approximately 10(4).

A magnetic switch for spin-catalyzed interconversion of nuclear spin isomers.

Residue-specific incorporation of non-canonical amino acids into proteins allows facile alteration and enhancement of protein properties. In this review, we describe recent technical developments and applications of residue-specific incorporation to problems ranging from elucidation of biochemical mechanisms to engineering of protein-based biomaterials. We hope to inform the reader of the ease and broad utility of residue-specific non-canonical amino acid incorporation with the goal of inspiring investigators outside the field to consider applying this tool to their own research.

Residue-specific incorporation of non-canonical amino acids into proteins: recent developments and applications.

The combination of highly efficient polymerizations with modular "click" coupling reactions has enabled the synthesis of a wide variety of novel nanoscopic structures. Here we demonstrate the facile synthesis of a new class of clickable, branched nanostructures, polyethylene glycol (PEG)-branch-azide bivalent-brush polymers, facilitated by "graft-through" ring-opening metathesis polymerization of a branched norbornene-PEG-chloride macromonomer followed by halide-azide exchange. The resulting bivalent-brush polymers possess azide groups at the core near a polynorbornene backbone with PEG chains extended into solution; the structure resembles a unimolecular micelle. We demonstrate copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click-to" coupling of a photocleavable doxorubicin (DOX)-alkyne derivative to the azide core. The CuAAC coupling was quantitative across a wide range of nanoscopic sizes (âˆ¼6-âˆ¼50 nm); UV photolysis of the resulting DOX-loaded materials yielded free DOX that was therapeutically effective against human cancer cells.

Core-clickable PEG-branch-azide bivalent-bottle-brush polymers by ROMP: grafting-through and clicking-to.

Graft-through ring-opening metathesis polymerization (ROMP) using ruthenium N-heterocyclic carbene catalysts has enabled the synthesis of bottle-brush polymers with unprecedented ease and control. Here we report the first bivalent-brush polymers; these materials were prepared by graft-through ROMP of drug-loaded polyethylene-glycol (PEG) based macromonomers (MMs). Anticancer drugs doxorubicin (DOX) and camptothecin (CT) were attached to a norbornene-alkyne-PEG MM via a photocleavable linker. ROMP of either or both drug-loaded MMs generated brush homo- and co-polymers with low polydispersities and defined molecular weights. Release of free DOX and CT from these materials was initiated by exposure to 365 nm light. All of the CT and DOX polymers were at least 10-fold more toxic to human cancer cells after photoinitiated drug release while a copolymer carrying both CT and DOX displayed 30-fold increased toxicity upon irradiation. Graft-through ROMP of drug-loaded macromonomers provides a general method for the systematic study of structure-function relationships for stimuli-responsive polymers in biological systems.

Drug-loaded, bivalent-bottle-brush polymers by graft-through ROMP.

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.

EPR study of spin labeled brush polymers in organic solvents.

We describe the parallel, one-pot synthesis of core-photocleavable, poly(norbornene)-co-poly(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.

"Brush-first" method for the parallel synthesis of photocleavable, nitroxide-labeled poly(ethylene glycol) star polymers.

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Much of our fundamental knowledge related to polymer networks is built on an assumption of ideal end-linked network structure. Real networks invariably possess topological imperfections that negatively affect mechanical properties; modifications of classical network theories have been developed to account for these defects. Despite decades of effort, there are no known experimental protocols for precise quantification of even the simplest topological network imperfections: primary loops. Here we present a simple conceptual framework that enables primary loop quantification in polymeric materials. We apply this framework to measure the fraction of primary loop junctions in trifunctional PEG-based hydrogels. We anticipate that the concepts described here will open new avenues of theoretical and experimental research related to polymer network structure.

Counting primary loops in polymer gels.

Une grande partie de nos connaissances fondamentales li&eacute;es aux r&eacute;seaux de polym&egrave;re repose sur l&#39;hypoth&egrave;se d&#39;une structure de r&eacute;seau fin id&eacute;ale. Real networks a invariablement poss&egrave;dent des d&eacute;fauts topologiques qui affectent n&eacute;gativement les propri&eacute;t&eacute;s m&eacute;caniques ; modifications des th&eacute;ories classiques de r&eacute;seau ont &eacute;t&eacute; d&eacute;velopp&eacute;es pour tenir compte de ces d&eacute;fauts. Malgr&eacute; des d&eacute;cennies d&#39;efforts, il n&#39;y a aucun protocole exp&eacute;rimental connu pour une quantification pr&eacute;cise des imperfections topologique r&eacute;seau encore plus simples : les boucles primaires. Nous pr&eacute;sentons un cadre conceptuel simple qui permet la quantification de la boucle primaire dans des polym&egrave;res. Nous appliquons ce cadre pour mesurer la fraction des jonctions de la boucle primaire dans trifonctionnelle hydrogels ax&eacute;e sur la cheville. Nous pr&eacute;voyons que les concepts d&eacute;crits ici ouvrira de nouvelles voies de recherches th&eacute;oriques et exp&eacute;rimentales li&eacute;es &agrave; la structure du r&eacute;seau polym&egrave;re.

Comptage des boucles primaires dans des gels de polym&egrave;res.

Ein Gro&szlig;teil unserer grundlegenden Wissen zu Polymer-Netze baut auf eine Annahme der ideale Ende legata Netzwerkstruktur. Reale Netzwerke besitzen ausnahmslos topologische Unvollkommenheiten, die mechanische Eigenschaften negativ beeinflussen; &Auml;nderungen der klassischen Netzwerk-Theorien wurden entwickelt, um diese M&auml;ngel zu ber&uuml;cksichtigen. Trotz jahrzehntelanger Bem&uuml;hungen, es gibt keine bekannte experimentelle Protokolle f&uuml;r pr&auml;zise Quantifizierung der selbst die einfachsten topologische Netzwerk Unvollkommenheiten: prim&auml;re Schleifen. Hier pr&auml;sentieren wir einen einfachen konzeptionellen Rahmen, der prim&auml;re Schleife Quantifizierung in polymeren Werkstoffen erm&ouml;glicht. Wir wenden dieses Rahmens um den Bruchteil der prim&auml;ren Schleife Kreuzungen in trifunktionalen PEG-basierte Hydrogele zu messen. Wir erwarten, dass die hier beschriebenen Konzepte neue Wege der theoretische und experimentelle Forschung im Zusammenhang mit Polymer-Netzwerkstruktur er&ouml;ffnen.

Z&auml;hlen die prim&auml;re Schleifen in Polymer-Gele.

Gran parte delle nostre conoscenze fondamentali relative alle reti di polimero &egrave; costruito su un presupposto della struttura di rete collegato a fine ideale. Real networks possiedono invariabilmente topologiche imperfezioni che influiscono negativamente sulla propriet&agrave; meccaniche; modifiche delle teorie classiche della rete sono state sviluppate per conto di questi difetti. Nonostante decenni di sforzi, non ci sono nessun conosciuti protocolli sperimentali per la precisa quantificazione delle imperfezioni rete topologica ancora pi&ugrave; semplice: loop primario. Qui vi presentiamo un quadro concettuale semplice che consente la quantificazione del ciclo primario in materiali polimerici. Applichiamo questo quadro per misurare la frazione di giunzioni ciclo primario in trifunzionale idrogel a base di PEG. Anticipiamo che i concetti qui descritti si apriranno nuove vie di ricerca teorica e sperimentale relativi alla struttura di rete del polimero.

Conteggio cicli primari in gel polimero.

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Gran parte de nuestros conocimientos fundamentales relacionados con redes de pol&iacute;mero se basa en una hip&oacute;tesis de la estructura de la red ligada final ideal. Reales redes invariablemente poseen imperfecciones topol&oacute;gicas que afectan negativamente las propiedades mec&aacute;nicas; modificaciones de las teor&iacute;as cl&aacute;sicas de la red han sido desarrollados para tener en cuenta estos defectos. A pesar de d&eacute;cadas de esfuerzo, no hay ninguna conocidos protocolos experimentales para la cuantificaci&oacute;n precisa de incluso las imperfecciones m&aacute;s simples de la red topol&oacute;gica: lazos primarios. Aqu&iacute; presentamos un marco conceptual simple que permite la cuantificaci&oacute;n de bucle primario en materiales polim&eacute;ricos. Aplicamos este marco para medir la fracci&oacute;n de ensambladuras del circuito primario en trifuncional Hidrogeles basados en PEG. Anticipamos que los conceptos descritos aqu&iacute; abrir&aacute;n nuevas v&iacute;as de investigaci&oacute;n te&oacute;rica y experimental relacionados con la estructura de la red del pol&iacute;mero.

Cuenta primarios bucles en geles de pol&iacute;mero.

Photo-controlled growth of telechelic polymers and end-linked polymer gels.

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 D) and the molecular weights (log Mw), 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 addition-fragmentation 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.

Application of (1)H DOSY for Facile Measurement of Polymer Molecular Weights.

New strategies to access functional monolayers could augment current surface modification methods. Here we present addressable N-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.

Jeremiah A. Johnson

Department of Chemistry

Massachusetts Institute of Technology

Particles without a Box: Brush-first Synthesis of Photodegradable PEG Star Polymers under Ambient Conditions

Jeremiah A. Johnson

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Chemistry

Massachusetts Institute of Technology

Particles without a Box: Brush-first Synthesis of Photodegradable PEG Star Polymers under Ambient Conditions

Department of Chemistry