Dendronized Polymers with Peripheral Oligo(ethylene oxide) Chains: Thermoresponsive Behavior and Shape Anisotropy in Solution

Thermoresponsive dendritic oligoethylene glycols

Monodispersed molecules of low molar masses showing thermoresponsiveness are appealing both for mechanism investigation of the thermally-modulated dehydration and aggregation on molecular levels and for designing functional intelligent materials. In the present report, thermoresponsive properties of a homologous series of monodispersed dendritic macromolecules carrying three-, four- or six-fold dendritic oligoethylene glycol (OEG) segments were investigated. These dendritic macromolecules carry either methoxyl or ethoxyl terminals, and have different cores (alcohol, methyl ester or methacryloyl) to exhibit different overall hydrophilicity. They show characteristic thermoresponsive properties with sharp phase transitions when suitable structural units are combined. Three structural factors determine their phase transition temperatures, including the cores, branching density and peripheral terminals. Thermally-induced collapse and aggregation are monitored with temperature-varied NMR spectroscopy at the microscale level and optical microscopy at the macroscale level. At elevated temperature, these dendritic macromolecules undergo fast exchange between the dehydrated and the hydrated states. These dendritic macromolecules afford structure-dependent confinement to guest dyes through their multi-valent interactions.

Novel Amphiphilic Dendronized Copolymers Formed by Enzyme-Mediated "Green" Polymerization

This study reports the first enzyme-mediated polymerization of dendritic macromonomers. The enzyme substrates are prepared by "click" conjugation between tyrosine and hydrophilic triethylene glycol (TrEG)-based dendrons of three generations (G1, G2, and G3). The resulting enzyme-polymerizable dendrons are defect-free as revealed by mass spectrometry, size-exclusion chromatography, and spectroscopic techniques. The phenol-containing macromonomers are water soluble and their polymerizations into dendronized polymers (denpols) are catalyzed by laccase (an oxidoreductase) under benign conditions (45 °C and aqueous medium at pH = 4.0) with copolymer yields between 30 and 40%. The resulting denpols consist of unnatural poly(tyrosine) backbones and dendritic poly(ether-ester) side chains and have molecular masses up to ∼13 000 Da (generation 1), ∼20 000 Da (generation 2), and ∼36 000 Da (generation 3) determined by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analyses. They display amphiphilic properties and self-assemble in aqueous solutions to form aggregates with generation-dependent morphologies.

Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 2. Self-Organization in Aqueous Solutions on Heating

The behavior of amphiphilic molecular brushes in aqueous solutions on heating was studied by light scattering and turbidimetry. The main chain of the graft copolymers was polydimethylsiloxane, and the side chains were thermosensitive poly-2-isopropyl-2-oxazoline. The studied samples differed in the length of the grafted chains (polymerization degrees were 14 and 30) and, accordingly, in the molar fraction of the hydrophobic backbone. The grafting density of both samples was 0.6. At low temperatures, macromolecules and aggregates, which formed due to the interaction of main chains, were observed in solutions. At moderate temperatures, heating solutions of the sample with short side chains led to aggregation due to dehydration of poly-2-isopropyl-2-oxazoline and the formation of intermolecular hydrogen bonds. In the case of the brush with long grafted chains, dehydration caused the formation of intramolecular hydrogen bonds and the compaction of molecules and aggregates. The lower critical solution temperature for solutions of the sample with long side chains was higher than LCST for the sample with short side chains. It was shown that the molar fraction of the hydrophobic component and the intramolecular density are the important factors determining the LCST behavior of amphiphilic molecular brushes in aqueous solutions.

Confined Microenvironments from Thermoresponsive Dendronized Polymers

Confined microenvironments in biomacromolecules arising from molecular crowding account for their well-defined biofunctions and bioactivities. To mimick this, synthetic polymers to form confined structures or microenvironments are of key scientific value, which have received significant attention recently. To create synthetic confined microenvironments, molecular crowding effects and topological cooperative effects have been applied successfully, and the key is balance between self-association of structural units and self-repulsion from crowding-induced steric hindrance. In this article, formation of confined microenvironments from stimuli-responsive dendronized polymers carrying densely dendritic oligoethylene glycols (OEGs) moieties in their pendants is presented. These wormlike thick macromolecules exhibit characteristic thermoresponsive properties, which can provide constrained microenvironments to encapsulate effectively guest molecules including dyes, proteins, or nucleic acids to prevent their protonation or biodegradation. This efficient shielding effect can also mediate chemical reactions in aqueous phase, and even enhance chirality transferring efficiency. All of these can be switched off simply through the thermally-induced dehydration and collapse of OEG dendrons due to the amphiphilicity of OEG chains. Furthermore, the switchable encapsulation and release of guests can be greatly enhanced when these dendronized polymers are used as major constituents for fabricating bulk hydrogels or nanogels, which provide a higher-level confinement.

Preparation and characterization of a multistimuli-responsive photoluminescent monomer and its corresponding polymer

A novel multistimuli-responsive photo-luminescent monomer and its corresponding polymer were prepared.

Steric hindrance effect on the thermo- and photo-responsive properties of pyrene-based polymers

We synthesized a novel series of pyrene-based thermo- and photo-dual responsive polymers named poly(pyren-1-yl-o(m,p)-vinylbenzoate) (P(Py-o(m,p)-VB)). And the influence of the steric hindrance effect on LCST-type phase behaviors and photocleavage properties has been studied.

"Inverse" thermoresponse: heat-induced double-helix formation of an ethynylhelicene oligomer with tri(ethylene glycol) termini

Ethynylhelicene oligomers with TEG terminal groups showed a unique thermoresponse in aqueous solvents: double-helix formation upon heating and disaggregation upon cooling.

An ethynylhelicene oligomer [(M)-d-4]-C₁₂-TEG with six tri(ethylene glycol) (TEG) groups at the termini was synthesized, and double-helix formation was studied using CD, UV-Vis, vapor pressure osmometry, dynamic light scattering, and ¹H NMR. [(M)-d-4]-C₁₂-TEG reversibly changed its structure between a double helix and a random coil in response to heating and cooling in aromatic solvents, non-aromatic polar organic solvents, and aqueous solvent mixtures of acetone/water/triethylamine. Notably, [(M)-d-4]-C₁₂-TEG in acetone/water/triethylamine (1/2/1) formed a double helix upon heating and disaggregated into random coils upon cooling. The double helix/random coil ratio sharply changed in response to temperature changes. This is an unprecedented “inverse” thermoresponse, which is opposite to the “ordinary” thermoresponse in molecular dimeric aggregate formation. This phenomenon was explained by the dehydration of the terminal TEG groups and the formation of condensed triethylamine domains upon heating.

Preparation of water-soluble hyperbranched polymers with tunable thermosensitivity using chain-growth CuAAC copolymerization

Thermoresponsive hyperbranched polymers with dangling oligo(ethylene oxide) chain on every monomer unit were constructed using the chain-growth copper-catalyzed azide–alkyne cycloaddition (CuAAC) copolymerization of two AB₂-F monomers.

Reversible and contrasting changes of the cloud point temperature of pillar[5]arenes with one quinone unit and tri(ethylene oxide) chains induced by redox chemistry and host–guest complexation

A new water-soluble redox-active pillar[5]arene was synthesized by incorporation of one benzoquinone unit.

Thermoresponsive Homopolymer Tunable by pH and CO2

A new acrylamide monomer bearing isopropylamide and N,N-diethylamino ethyl groups in the side chain, i.e., N-(2-(diethylamino)ethyl)-N-(3-(isopropylamino)-3-oxopropyl)acrylamide (DEAE-NIPAM-AM), was synthesized through Aza-Michael addition reaction followed by amidation with acryloyl chloride. The homopolymers, poly(N-(2-(diethylamino)ethyl)-N-(3-(isopropylamino)-3-oxopropyl)acrylamide)s [poly(DEAE-NIPAM-AM)], with controlled molecular weights and relatively narrow molecular weight distributions were then prepared via RAFT polymerization. The lower critical solution temperature (LCST) of the homopolymer was examined to be influenced by molecular weight, salt concentration, and pH value of aqueous solution. The LCST of the homopolymer could be tuned in a wide temperature window by changing the pH value of aqueous solution, and it increased with the decrease of pH value. Particularly, CO₂ gas as a unique pH stimulus can also reversibly adjust the solubility of homopolymer without the addition of acids or bases.

Bulk and solution properties of a thermo-responsive rod–coil block polymer based on poly(N-isopropylacrylamide)

Molecular weight dependence on thermoresponsive behaviors of rod–coil diblock copolymers (x indicates the DP of rod PHIPPVTA blocks).

Mesogen-jacketed liquid crystalline polymers with peripheral oligo(ethylene oxide) chains: phase structure and thermoresponsive behavior

The influence of the number and length of EO terminal groups for MJLCPs on the phase structure and thermoresponsive behavior.

Thermoresponsive hydrogels from symmetrical triblock copolymers poly(styrene-block-(methoxy diethylene glycol acrylate)-block-styrene)

A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20-40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel-sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP(n) about 450), gels had already formed at 3.5 wt % at low temperatures. The gel-sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.