Efficient, one-pot preparation of synthetically challenging, high molecular weight (MW), narrowly dispersed brush block copolymers and random copolymers in high conversions was achieved by ring-opening metathesis (co)polymerization (ROMP) of various macromonomers (MMs) using the highly active, fast-initiating ruthenium olefin metathesis catalyst (H2IMes)(pyr)2(Cl)2RuCHPh. A series of random and block copolymers were prepared from a pair of MMs containing polylactide (PLA) and poly(n-butyl acrylate) (PnBA) side chains at similar MWs. Their self-assembly in the melt state was studied by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). In brush random copolymers containing approximately equal volume fractions of PLA and PnBA, the side chains segregate into lamellae with domain spacing of 14 nm as measured by SAXS, which was in good agreement with the lamellar thickness measured by AFM. The domain spacings and order−disorder transition temperatures of brush random copolymers were insensitive to the backbone length. In contrast, brush block copolymers containing approximately equal volume fractions of these MMs self-assembled into highly ordered lamellae with domain spacing over 100 nm. Their assemblies suggested that the brush block copolymer backbone adopted an extended conformation in the ordered state.

Room temperature atom transfer radical polymerizations of N-isopropylacrylamide (NIPAM) carried out in 2-propanol (i-PrOH) and tert-butyl alcohol (t-BuOH) resulted in PNIPAMs with polydispersities between 1.1 and 1.2 and degrees of polymerization of up to 300. Methyl 2-chloropropionate (MCP), copper(I) chloride, and tris[2-(dimethylamino)ethyl]amine (Me6TREN) were used as initiator, catalyst, and ligand in a 1:1:1 ratio. Conversions were as high as 91 and 79%, respectively, without the need for excess catalyst as was required in previous studies. Aqueous solutions of these narrow-disperse PNIPAMs showed a strong decrease of the phase transition temperature with increasing molecular weight, as measured by turbidimetry and differential scanning calorimetry. In low molecular weight samples, containing significant oligomeric fractions, the slightly hydrophobic methyl propionate end group becomes significant and further decreases the onset temperature of the phase transition.

Four series of narrow-disperse poly(N-isopropylacrylamide) (PNIPAM) with well-controlled molecular weights and with end groups of varying hydrophobicity were synthesized by room temperature atom transfer radical polymerization in 2-propanol using the corresponding chloropropionate and chloropropionamide initiators. The thermal phase transitions of aqueous solutions of these PNIPAMs were studied by turbidimetry and high-sensitivity differential scanning calorimetry (HS-DSC) and showed an inverse molecular weight (MW) dependence of their cloud points. The magnitude of the MW dependence decreases when using more hydrophobic end groups. The choice of end group further affected the shape of the cloud point curves and the enthalpy of the phase transition. Above the cloud point, narrow-disperse PNIPAM sedimented more rapidly than polydisperse PNIPAM produced by conventional free radical polymerization, especially at concentrations above 1%. Thus, multiple HS-DSC scans of PNIPAM prepared by ATRP typically gave repeatable results only at lower concentrations. IntroductionAqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) exhibit a reversible thermal phase separation above a critical temperature, known as the lower critical solution temperature (LCST). 1,2 On the molecular level, this involves a change from solvated random coils below the LCST to tightly packed globular particles above the LCST. 3-5 This thermoresponsiveness has led to applications in bioengineering 6-9 and nanotechnology 10-13 and promises exciting future applications in the area of biosensors and membranes. Much effort has also been invested in better understanding the phase transition behavior and the parameters affecting the phase transition temperature. Most often, this involved studying the cloud point of dilute aqueous PNIPAM solutions, rather than the actual LCST, i.e., the minimum of the two-phase curve in the PNIPAM-water phase diagram.The molecular weight (MW) dependence of the cloud point of such polymers has been an active yet controversial topic. The cloud points of PNIPAM and related thermoresponsive polymers have been reported to be inversely dependent, 14-20 directly dependent, 21,22 or independent 5,23,24 on the molecular weight. However, most of these studies involved conventionally prepared, polydisperse polymers, which may have precluded precise examination of MW effects. In addition, different initiators, terminators, or chain-transfer agents led to different polymer end groups, which can in turn affect the cloud points. 22,24-28 Hydrophobic end groups decrease cloud points while hydrophilic end groups tend to increase them, with the magnitude of the effect depending on the nature of the end group. Hydrophobic groups act by increasing the degree of ordering of solvating water while hydrophilic ones tend to decrease the ordering of solvating water. These effects are believed to be greater for hydrophobic/hydrophilic groups located at chain ends rather than midchain. 25 End group effects are most pronounced for low MW polymers bu...

Various macromonomers (MMs) were efficiently synthesized through the copper-catalyzed "click" coupling of a norbornene moiety to the chain end of poly(methylacrylate), poly(t-butylacrylate), and polystyrene that were prepared using atom transfer radical polymerization. Ring-opening metathesis polymerization (ROMP) of these MMs was carried out using the highly active, fast-initiating ruthenium catalyst (H 2 IMes)(pyr) 2 (Cl) 2 RuCHPh in THF at room temperature. ROMP of MMs was found to be living with almost quantitative conversions (>90%) of MMs, producing brush polymers with very low polydispersity indices of 1.01-1.07 and high M n 's of 200-2600 kDa. The efficient ROMP of such MMs provides facile access to a variety of brush polymers and overcomes previous difficulties in the controlled polymerization of MMs. Atomic force microscopy of the brush polymer products revealed extended, wormlike shapes as a result of significant steric repulsion of densely grafted side chains.

The reduced chain entanglement of brush polymers over their linear analogs drastically lowers the energetic barriers to reorganization. In this report, we demonstrate the rapid self-assembly of brush block copolymers to nanostructures with photonic bandgaps spanning the entire visible spectrum, from ultraviolet (UV) to near infrared (NIR). Linear relationships were observed between the peak wavelengths of reflection and polymer molecular weights. This work enables "bottom-up" fabrication of photonic crystals with application-tailored bandgaps, through synthetic control of the polymer molecular weight and the method of self-assembly. These polymers could be developed into NIR-reflective paints, to combat the "urban heat island effect" due to NIR photon thermalization.

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Fluorescence imaging of biological systems in the second near-infrared (NIR-II, 1000–1700 nm) window has shown promise of high spatial resolution, low background, and deep tissue penetration owing to low autofluorescence and suppressed scattering of long wavelength photons. Here we develop a bright organic nanofluorophore (named p-FE) for high-performance biological imaging in the NIR-II window. The bright NIR-II >1100 nm fluorescence emission from p-FE affords non-invasive in vivo tracking of blood flow in mouse brain vessels. Excitingly, p-FE enables one-photon based, three-dimensional (3D) confocal imaging of vasculatures in fixed mouse brain tissue with a layer-by-layer imaging depth up to ~1.3 mm and sub-10 µm high spatial resolution. We also perform in vivo two-color fluorescence imaging in the NIR-II window by utilizing p-FE as a vasculature imaging agent emitting between 1100 and 1300 nm and single-walled carbon nanotubes (CNTs) emitting above 1500 nm to highlight tumors in mice.

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.