Telechelic dithiol copolymers as tunable building blocks for synthesizing multiblock materials

A new strategy is reported to access α,ω‐dithiol polymer building blocks with tunable molecular weights and compositions for the preparation of random multiblock copolymers based on styrenic, acrylic, and siloxanes. 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.

Methods
Characterization methods

1H nuclear magnetic resonance (NMR) spectroscopy

 1H-NMR spectra were collected on a Bruker Avance NEO 500 MHz at 25 °C using chloroform-d as a deuterated solvent.

Size-exclusion chromatography (SEC) instrumentation

Size-exclusion chromatography (SEC) was performed on a Waters instrument using a differential refractive index detector and two Tosoh columns (TSKgel SuperHZM-N, 3 μm polymer, 150 × 4.6 mm) with THF at 35 °C or chloroform containing 0.25% TEA at 35 °C for the mobile phase. Molar mass and molar mass dispersities (Đ) were determined against narrow PS standards (Agilent).

Differential scanning calorimetry (DSC)

DSC was performed on a TA Instruments Q2000 DSC with three repeated cycles between −150 and +150 °C at a constant rate of 10 °C/min. Only the second heating scan is shown.

Small-angle X-ray scattering (SAXS)

SAXS measurements were performed on a custom-built SAXS diffractometer housed at the University of California, Santa Barbara. The sample-to-detector distance was 1.7 m with a fixed wavelength of 1.54 Å. For all SAXS experiments, a silver behenate standard was used to calibrate the scattering angles. Two-dimensional (2D) data were reduced by azimuthal averaging to give I(q), where I is the intensity in arbitrary units, q = |q| =4πλ–1 sin(θ/2) is the magnitude of the scattering wave vector, λ is the wavelength of the incident beam, and θ is the scattering angle. Two data points (at q = 0.15438 Å and q = 0.16026 Å), which correspond to two known dead pixels on the detector, were excised from the resulting data set for clarity.

Mechanical tensile testing

For uniaxial extension tensile tests, a Teflon dog bone mold was used to shape samples on a Carver press (Wabash, IN) 10 kPA at 160 °C for 10 min and then cooled (gauge depth = 0.5 mm, gauge width = 1.5 mm, gauge length = 10 mm, transition zone radius = 2.5 mm). Uniaxial tensile testing was performed on a TA.XT Plus Connect Texture Analyzer with a 50N load cell using the Miniature Tensile grips (TA-96B) from the manufacturer. Grip surfaces were covered with adhesive backed foam padding to avoid damage to tensile bars and slip of samples from the grips. Samples were deformed at 0.1 mm/s until failure. Mechanical testing of polymer samples was performed in triplicate and representative data is included in the main text.