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ABSTRACT A new strategy is reported to accessα,ω‐dithiol polymer building blocks with tunable molecular weights and compositions for the preparation of random multiblock copolymers based on styrenic, acrylic, and siloxane repeat units. This facile synthetic approach provides access to dithiols through a two‐step process: (1) an initial copolymerization of vinyl monomers with ethyl lipoate followed by (2) disulfide bond reduction, producing dithiol terminated polymer products. Thiol‐terminated polymers are easily prepared over a wide range of molecular weights (2–32 kDa) by simply controlling the feed ratio of vinyl monomer to ethyl lipoate. Mixtures of these linear dithiol‐terminated building blocks were repolymerized via oxidative coupling to create random multiblock copolymers with high molecular weights (68–95 kDa) and controlled degradability. In summary, this approach for preparing and recombining telechelic dithiol polymers creates opportunities to manipulate the mechanical and physical properties of multiblock copolymers using a synthetically simple and versatile platform. 

Poly(acrylamide- co -acrylic acid) (P(AAm- co -AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm- co -AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength ( I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient ( µ ), which decreased with increasing AA concentration. P(AAm- co -AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity ( µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) ( µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm- co -AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface. 

Abstract A versatile synthetic platform is reported that affords high molecular weight graft copolymers containing polydimethylsiloxane (PDMS) backbones and vinyl‐based polymer side chains with excellent control over molecular weight and grafting density. The synthetic approach leverages thiol‐ene click chemistry to attach an atom‐transfer radical polymerization (ATRP) initiator to a variety of commercially available poly(dimethylsiloxane‐co‐methylvinylsiloxane) backbones (PDMS‐co‐PVMS), followed by controlled radical polymerization with a wide scope of vinyl monomers. Selective degradation of the siloxane backbone with tetrabutylammonium fluoride confirmed the controlled nature of side‐chain growth via ATRP, yielding targeted side‐chain lengths for copolymers containing up to 50% grafting density and overall molecular weights in excess of 1 MDa. In addition, by using a mixture of thiols, grafting density and functionality can be further controlled by tuning initiator loading along the backbone. For example, solid‐state fluorescence of the graft copolymers was achieved by incorporating a thiol‐containing fluorophore along the siloxane backbone during the thiol‐ene click reaction. This simple synthetic platform provides facile control over the properties of a wide variety of grafted copolymers containing flexible PDMS backbones and vinyl polymer side chains.