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Abstract: Thermoplastic elastomers (TPEs) that are closed-loop recyclable are needed in a circular material economy, but many current materials degrade during recycling, and almost all are pervasive hydrocarbons. Here, well-controlled block polyester TPEs featuring regularly placed sodium/lithium carboxylate side chains are described. They show significantly higher tensile strengths than unfunctionalized analogues, with high elasticity and elastic recovery. The materials are prepared using controlled polymerizations, exploiting a single catalyst that switches between different polymerization cycles. ABA block polyesters of high molar mass (60–100 kg mol–1; 21 wt % A-block) are constructed using the ring-opening polymerization of ε-decalactone (derived from castor oil; B-block), followed by the alternating ring-opening copolymerization of phthalic anhydride with 4-vinyl-cyclohexene oxide (A-blocks). The polyesters undergo efficient functionalization to install regularly placed carboxylic acids onto the A blocks. Reacting the polymers with sodium or lithium hydroxide controls the extent of ionization (0–100%); ionized polymers show a higher tensile strength (20 MPa), elasticity (>2000%), and elastic recovery (>80%). In one case, sodium functionalization results in 35× higher stress at break than the carboxylic acid polymer; in all cases, changing the quantity of sodium tunes the properties. A leading sample, 2-COONa75 (Mn 100 kg mol–1, 75% sodium), shows a wide operating temperature range (−52 to 129 °C) and is recycled (×3) by hot-pressing at 200 °C, without the loss of mechanical properties. Both the efficient synthesis of ABA block polymers and precision ionization in perfectly alternating monomer sequences are concepts that can be generalized to many other monomers, functional groups, and metals. These materials are partly bioderived and have degradable ester backbone chemistries, deliver useful properties, and allow for thermal reprocessing; these features are attractive as future sustainable TPEs.

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Abstract: A series of ionic diblock copolymer nanoparticles was prepared in a typical nonpolar solvent (n-dodecane) via polymerization-induced self-assembly (PISA). A single cationic repeat unit was incorporated into the poly(stearyl methacrylate) (PSMA) stabilizer of otherwise uncharged poly(stearyl methacrylate)–poly(benzyl methacrylate) (PSMA–PBzMA) diblock copolymer nanoparticles. By using short PSMA stabilizer blocks, it was possible to obtain nanoparticles with the range of morphologies expected (spheres, worms, and vesicles). For nanoparticles where all stabilizer chains possessed an ionic group, higher-order morphologies were obtained at lower PBzMA degrees of polymerization than corresponding uncharged particles, and the particles were electrophoretic. For nanoparticles where only a fraction of the stabilizer chains contained an ionic group, higher-order morphologies were obtained at precisely the same PBzMA degrees of polymerization, and the electrophoretic response was greater than when the shell was fully ionic. These particles with a partially ionic shell are a fascinating system, providing morphologies that can be predicted from the existing knowledge of the diblock copolymer morphology yet with the highest possible electrophoretic mobility.

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Abstract: Reversible addition–fragmentation chain transfer (RAFT) solution polymerization of 3-[tris(trimethylsiloxy)silyl] propyl methacrylate (SiMA) was conducted in toluene to prepare three PSiMA precursors with mean degrees of polymerization (DP) of 12, 13, or 15. Each precursor was then chain-extended in turn via RAFT dispersion polymerization of benzyl methacrylate (BzMA) in a low-viscosity silicone oil (decamethylcyclopentasiloxane, D5). 1H NMR studies confirmed that such polymerizations were relatively fast, with more than 99% BzMA conversion being achieved within 100 min at 90 °C. Moreover, gel permeation chromatography analysis indicated that these polymerizations were well controlled, with dispersities remaining below 1.25 when targeting PBzMA DPs up to 200. A phase diagram was constructed at a constant copolymer concentration of 20% w/w. Only spherical micelles were accessible when the PSiMA15 stabilizer was utilized, as determined by transmission electron microscopy and small-angle X-ray scattering (SAXS) studies. Nevertheless, these spheres exhibited narrow size distributions and tunable z-average diameters ranging between 19 and 49 nm, as determined by dynamic light scattering. In contrast, spheres, worms, or vesicles could be prepared depending on the target PBzMA DP when utilizing the relatively short PSiMA12 precursor. Moreover, each of these nano-objects could be obtained at copolymer concentrations as low as 5% w/w. To obtain more detailed structural information, these spheres, worms and vesicles were further characterized by SAXS. PSiMA12-PBzMA55 worms formed reasonably transparent free-standing gels when prepared at copolymer concentrations as low as 5% w/w and exhibited an elastic modulus (G′) of 90 Pa at 25 °C, as judged by oscillatory rheology studies. Finally, broadening of the molecular weight distribution was observed during the long-term storage of PSiMA-PBzMA dispersions at ambient temperature. We tentatively suggest that this instability is related to hydroxyl impurities in the SiMA, which leads to cross-linking side reactions. This problem also causes incipient flocculation of the spheres and worms during the long-term storage of such dispersions at 20 °C.

Open Access

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Abstract: The influence of reaction rate on the evolving polymer structure of photo-activated dimethacrylate biomedical resins was investigated using neutron and in situ synchrotron X-ray scattering with simultaneous Fourier-transform-near-infrared spectroscopy. Previous studies have correlated the degree of reactive group conversion with mechanical properties, but the impact of polymerization rate on the resultant polymer structure is unknown. Here, we demonstrate that the medium-range structural order at the functional end groups of these materials is dependent on the reaction rate. Accelerating polymerization increases correlation lengths in the methacrylate end groups but reduces the medium-range structural order per converted vinyl bond when compared with more slowly polymerized systems. At faster rates of polymerization, the conformation of atoms at the reacting end group can become fixed into the polymer structure at the onset of autodeceleration, storing residual strain. Neutron scattering confirms that the structural differences observed are reproduced at longer length scales. This effect is not as prominent in systems polymerized at slower rates despite similar final degrees of reactive group conversion. Results suggest that current interpretations of these materials, which extrapolate mechanical properties from conversion, may be incomplete. Accelerating polymerization can introduce structural differences, which will dictate residual strain and may ultimately explain the discrepancies in the predictive modeling of the mechanical behavior of these materials using conventional techniques.

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Abstract: A near-monodisperse monohydroxy-terminated polydimethylsiloxane (PDMS; mean degree of polymerization = 66) was esterified using a carboxylic acid-functionalized trithiocarbonate to yield a PDMS66 precursor with a mean degree of functionality of 92 ± 5% as determined by 1H NMR spectroscopy. This PDMS66 precursor was then chain-extended in turn using eight different methacrylic monomers in a low-viscosity silicone oil (decamethylcyclopentasiloxane, D5). Depending on the monomer type, such syntheses proceeded via either RAFT dispersion polymerization or RAFT emulsion polymerization. In each case the target DP of the core-forming block was fixed at 200, and the copolymer concentration was 25% w/w. Transmission electron microscopy studies indicated that kinetically trapped spheres were obtained in almost all cases. The only exception was 2-(dimethylamino)ethyl methacrylate (DMA), which enabled access to spheres, worms, or vesicles. This striking difference is attributed to the relatively low glass transition temperature for this latter block. A phase diagram was constructed for a series of PDMS66–PDMAx nano-objects by systematically increasing the PDMA target DP from 20 to 220 and varying the copolymer concentration between 10 and 30% w/w. Higher copolymer concentrations were required to access a pure worm phase, while only spheres, vesicles, or mixed phases were accessible at lower copolymer concentrations. Gel permeation chromatography studies indicated a linear evolution of number-average molecular weight (Mn) with PDMA DP while dispersities remained below 1.40, suggesting relatively well-controlled RAFT polymerizations. Small-angle X-ray scattering (SAXS) was used to characterize selected examples of spheres, worms, and vesicles. PDMS66–PDMA100–112 worms synthesized at 25–30% w/w formed free-standing gels at 20 °C. Oscillatory rheology studies performed on a 30% w/w PDMS66–PDMA105 worm dispersion indicated a storage modulus (gel strength) of 1057 Pa and a critical gelation concentration (CGC) of approximately 12% w/w. Finally, PDMS66–PDMAx worms could also be prepared in n-dodecane, hexamethyldisiloxane, or octamethylcyclotetrasiloxane. Rotational rheometry studies indicate that such worms are efficient viscosity modifiers for these nonpolar oils.