Ring-Opening Metathesis Polymerization with the Second Generation Hoveyda–Grubbs Catalyst: An Efficient Approach toward High-Purity Functionalized Macrocyclic Oligo(cyclooctene)s

Herein, we present a facile and general strategy to prepare functionalized macrocyclic oligo­(cyclooctene)­s (cOCOEs) in high purity and high yield by exploiting the ring-opening metathesis polymerization (ROMP) intramolecular backbiting process with the commercially available second generation Hoveyda–Grubbs (<b>HG2</b>) catalyst. In the first instance, ROMP of 5-acetyloxycyclooct-1-ene (ACOE) followed by efficient quenching and removal of the catalyst using an isocyanide derivative afforded macrocyclic oligo­(5-acetyloxycyclooct-1-ene) (cOACOE) in high yield (95%), with a weight-average molecular weight (<i>M</i>_w) of 1.6 kDa and polydispersity index (PDI) of 1.6, as determined by gel permeation chromatography (GPC). The structure and purity of the macrocycles were confirmed by NMR spectroscopy and elemental analysis, which indicated the complete absence of end-groups. This was further supported by GPC-matrix assisted laser desorption ionization time-of-flight mass spectroscopy (GPC-MALDI ToF MS), which revealed the exclusive formation of macrocyclic derivatives composed of up to 45 repeat units. Complete removal of residual ruthenium from the macrocycles was confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The same methodology was subsequently extended to the ROMP of 5-bromocyclooct-1-ene and 1,5-cyclooctadiene to prepare their macrocyclic derivatives, which were further derivatized to produce a library of functionalized macrocyclic oligo­(cyclooctene)­s. A comparative study using the second and third generation Grubbs catalysts in place of the <b>HG2</b> catalyst for the polymerization of ACOE provided macrocycles contaminated with linear species, thus indicating that the bidendate benzylidene ligand of the Hoveyda–Grubbs catalyst plays an important role in the observed product distributions

Degradable Core Cross-Linked Star Polymers via Ring-Opening Polymerization

Degradable Core Cross-Linked Star Polymers via Ring-Opening Polymerizatio

Color-Switchable Polar Polymeric Materials

Spiropyran is an important mechanophore, which has rarely been incorporated as a cross-linker in polar polymer matrices, limiting its applications in innovative mechanochromic devices. Here, three spiropyrans with two- or three-attachment positions were synthesized and covalently bonded in polar poly­(hydroxyethyl acrylate) (PHEA), to achieve color-switchable materials, triggered by light and when swollen in water. The negative photochromism in the dark and mechanical activation by swelling in water were investigated. Measurements of negative photochromism were conducted in solution and cross-linked PHEA bulk polymers, with both showing color reversibility when stored in the dark or on exposure to visible light. The force of swelling in water was sufficient to induce the ring-opening reaction of spiropyran. It was found that tri-substituted spiropyran (SP3) was less influenced by the polar matrix but showed the fastest color activation during swelling. SP3 also showed accelerated ring opening to the colored state during the swelling process. Bleaching rates and color switchability were investigated under swollen and dehydrated conditions. The effect of cross-link density on the swelling activation was explored to better understand the interaction between the mechanophore and the polar environment. The results demonstrated that influences from both the polar environment and the mechanochromic nature of spiropyran had an impact on the absorption intensity, rate of change, and the decoloration rate of the materials. This study provides the opportunity to manipulate the properties of spiropyrans to afford materials with a range of color-switching properties under different stimuli

Enantioselective Adsorption of Surfactants Monitored by ATR-FTIR

The selectivity of adsorption of chiral surfactants to a chiral monolayer at the solid−liquid interface was studied using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). One enantiomer of the chiral surfactant was deuterated, which causes a change in the IR absorption frequency, and allows independent measurement of the adsorption of each molecule. Both the surfactant, N-lauroyl phenylalanine (NLP), and the chiral monolayer, N-l-phenylalaninoyl, 11-undecyl-silicon, were amino acid derivatives. An enantiomeric excess of 56 ± 22% of the l over d was observed for adsorption to the interface between a carbon tetrachloride solution containing a quasi-racemate of N-lauroyl phenylalanine and the N-l-phenylalaninoyl, 11-undecyl monolayer film on silicon. In contrast, equimolar adsorption occurred from an equimolar mixture of hydrogenated and deuterated forms of the l surfactant. The measured enantiomeric excess strongly depended on the density of chiral surface groups: the higher the density of chiral groups on the surface, the better the enantiodiscrimination, even though the total adsorption was roughly constant. This nonlinear behavior indicates that more than one chiral surface group is required for significant selectivity

Factors Influencing the Formation of Single-Chain Polymeric Nanoparticles Prepared via Ring-Opening Polymerization

The development of single-chain polymeric nanoparticles (SCNP) has been of great scientific interest in recent years. Recently, we have developed a robust system to form SCNP at high polymer concentration (ca. 100 mg mL^–1) via organocatalyzed ring-opening polymerization (ROP). In this approach, linear polymer precursors functionalized with pendent polymerizable caprolactone moieties undergo self-cross-linking in the presence of organocatalyst and alcohol initiator. Following on from our previous communication, we report in here a more in-depth fundamental investigation to better understand our system. For this, we have synthesized various linear random copolymer precursors (i.e., poly­(oligo­(ethylene glycol) acrylate) (<b>P1</b>), polystyrene (<b>P2</b>), and poly­(methyl acrylate) (<b>P3</b>)) by reversible addition–fragmentation chain transfer (RAFT) polymerization, and their abilities to form SCNP at high polymer concentration were evaluated. It was found that only <b>P1</b>, which contains oligo­(ethylene glycol) side chains, was able to successfully form SCNP while the other linear precursors resulted in multichain aggregates, indicating the importance of side-chain brushes in aiding SCNP formation at high polymer concentration. Furthermore, we tested several multifunctional alcohol initiators (mono-, di-, and tetrahydroxy) and found that the initiator structure has no effect on the SCNP formation process. In addition, we investigated the effect of initiator concentration and observed that the particle size can be reduced (from 7.6 to 6.6 nm) when the initiator and linear precursor are in equimolar concentration. It is anticipated that the information derived from this study may lead to the development of new SCNP for targeted (bio)­applications

Photoiniferter RAFT Accelerated by Ionic Liquids in Organic Solvent Systems

Ionic liquids provide an environmentally friendly alternative to organic solvents that have previously been shown to greatly improve the kinetics of photoiniferter reversible addition–fragmentation chain-transfer polymerization reactions. Here, we expand on previous work in our group by mixing common organic solvents and monomers such as methyl methacrylate with hydrophobic ionic liquids. We demonstrate that it greatly improves the reaction rate by up to 3.3-fold compared to only an organic solvent while maintaining a narrow polydispersity. It was hypothesized that this is due to the viscosity and polarity of the ionic liquids disfavoring the biomolecular termination of propagating chains. Chain-end fidelity achieved by the photoiniferter mechanism was demonstrated by chain extension experiments. The system exhibited temporal control and oxygen tolerance

Shear Induced Alignment of Low Aspect Ratio Gold Nanorods in Newtonian Fluids

The flow-induced alignment of small gold nanorods ranging in aspect ratio from 2.4 to 4.2 in aqueous sucrose solutions is reported. Optical absorption spectra have been measured over a range of shear rates using polarized incident light in an optically transparent quartz Couette cell. The measured spectral changes are directly attributed to the shear-induced anisotropy in the suspension due to particle alignment that saturates at Péclet number of around 200. The measured optical changes are reversible, indicating that the nanorods do not undergo aggregation during measurement. Numerical simulations show that the spectral shifts are consistent with the rods flipping between extreme orientations of the Jeffery’s orbits and that the effect of the Brownian motion on the gold nanorods cannot be ignored even at large Péclet number

On-Demand Cascade Release of Hydrophobic Chemotherapeutics from a Multicomponent Hydrogel System

A multicomponent hydrogel drug-delivery platform with covalent incorporation of core-cross-linked star (CCS) polymers has been fabricated through dual cross-linking reactions. The presence of amphiphilic CCS polymers enhances the mechanical stability of the network and allows for immobilization of hydrophobic chemotherapeutics (e.g., doxorubicin: DOX) within a hydrophilic matrix. Thanks to thiol-responsive disulfide segments, the hybrid network is selectively degradable upon exposure to the glutathione-rich tumor environment, leading to an on-demand cascade release of payloads in a prolonged manner. After network degradation, the integrity of the therapeutic cargos is still well-protected by the CCS polymeric carriers. A cellular study demonstrated efficient internalization of released drug-containing CCS polymers by HeLa cells, intracellular delivery of encapsulated drugs following pH-mediated erosion of the CCS containers, and excellent cytocompatibility of this hydrogel system

Alignment of Red Poly[dodecadyin-1,12-diol-bis(4-butoxycarbonyl-methyl-urethane)] in Couette Flow

The flow-induced alignment of red poly­[dodecadyin-1,12-diol-bis­(4-butoxycarbonyl-methyl-urethane)] (poly-4BCMU) in chloroform/toluene solution is reported. Absorption spectra have been measured over a range of shear rates in an optically transparent quartz Couette cell. The measured spectra show that the poly-4BCMU structure stays the same in flow, while the measured absorbance anisotropy is attributed to the flow-induced particle alignment in the red form poly-4BCMU solutions. A limiting orientation at shear rates >50 s^–1 is observed. Numerical simulations show that the spectral changes are consistent with the rodlike poly-4BCMU particle having an aspect ratio of 2.9. The dichroic ratio of 1.9 interpreted from the data indicates that the individual poly-4BCMU chains do not aggregate amorphously in the rodlike conformation, rather they show a preferred orientation along the long axis of the prolate aggregates

Organic Catalyst-Mediated Ring-Opening Polymerization for the Highly Efficient Synthesis of Polyester-Based Star Polymers

A facile, highly efficient, and metal-free synthesis of well-defined polyester-based core cross-linked star (CCS) polymers with yields of up to 96 % was achieved via an organic catalyst (i.e., methanesulfonic acid) mediated ring-opening polymerization (ROP) at room temperature, through either a two-pot or a one-pot, two-step strategy. CCS polymers with narrow molecular weight distributions (PDI ≤ 1.3) and macroinitiator (MI) conversions of 90–96% were prepared using poly­(ε-caprolactone) (PCL) MIs with molecular weights ranging from 9.9 to 36.2 kDa and [4,4′-bioxepane]-7,7′-dione (BOD) as the cross-linker. Furthermore, transesterification was identified as being responsible for the small percentage of unincorporated low molecular weight polymer remaining and star–star couplings in the star formation. Compared to CCS polymers synthesized via the methanesulfonic acid-mediated ROP, CCS polymers prepared via ROP mediated by high transesterification rate catalysts (i.e., stannous octoate (Sn­(Oct)₂)) suffer from much lower star purity (ca. 70%) and star–star coupled products due to more prominent transesterification side-reactions