Carbosiloxane Bottlebrush Networks for Enhanced Performance and Recyclability

Silicone bottlebrush copolymers and networks derived from cyclic carbosiloxanes are reported and shown to have enhanced properties and recyclability compared to traditional dimethylsilxoane-based materials. The preparation of these materials is enabled by the synthesis of well-defined heterotelechelic macromonomers with Si–H and norbornene chain ends via anionic ring-opening polymerization of the hybrid carbosiloxane monomer 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane. These novel heterotelechelic α-Si–H/ω-norbornene macromonomers undergo efficient ring-opening metathesis copolymerization to yield functional bottlebrush polymers with accurate control over molecular weight and functional-group density. Si–H groups retained at the ends of side-chains after ring-opening metathesis copolymerization allow for the preparation of super-soft networks via hydrosilylation with crosslinkers such as tetrakis[dimethyl(vinyl)silyl]orthosilicate. In contrast to traditional PDMS systems, the incorporation of poly(carbosiloxane) side chains allows the resulting networks to be recycled back to the original monomer (>85% recovery) via depolymerization at elevated temperatures (250 °C) in the presence of base catalysts (potassium hydroxide and tetramethylammonium hydroxide). Recovered monomer was successfully repolymerized through anionic ring-opening polymerization with no decrease in structural fidelity or activity. In summary, this combination of unique (macro)monomer design and bottlebrush architecture creates new opportunities in sustainable practices by offering a robust, recyclable alternative to commercial silicone-based materials.

Methods
Chemical Characterization

1H, 13C and 29Si nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance NEO 500 MHz spectrometer, using CDCl3 as the solvents. Size-exclusion chromatography (SEC) was measured using Waters e2695 separation module with a Waters 2414 differential refractive index detector equipped with two columns (Tosoh TSKgel SuperHZM-N 3 μm polymer, 150 × 4.6 mm) with THF at 35 °C as the mobile phase. Molecular weights and dispersities (Đ) were determined against narrow polystyrene standards (Agilent). Bruker Microflex LRF MALDI TOF mass spectrometer in positive reflection mode; the analyte, matrix (DCTB) were dissolved in THF at the concentration of 2.5 and 10 mg/mL respectively, and cationization agent (NaI) was dissolved in THF at concentrations of 1 mg/mL, then mixed in a volume ratio of 20 : 1 : 1 (DCTB : NaI : sample). 0.5 μL of this mixed solution was spotted onto a ground steel target plate and the solvent was allowed to evaporate prior to analysis. Gas chromatography–high-resolution mass spectrometry (GC-HRMS) was carried out on a JMS-700 from JEOL in FAB mode. Thermogravimetric analysis (TGA) was performed under N2 on a TA Instruments Q500 at a heating rate of 10 °C/min. Differential Scanning Calorimetry (DSC) was performed using a TA Instruments DSC 2500 at a heating/cooling rate of 10 °C/min with a sealed aluminum pan.

Rheology

Rheology experiments were performed on a strain-controlled TA Instruments ARES-G2 rheometer affixed with a forced convection oven (FCO) in a nitrogen atmosphere. 8 mm sample pucks were punched out of square molds. Frequency sweeps were performed from high to low frequency at room temperature and 3% strain, which was confirmed to be in the linear viscoelastic regime via amplitude sweeps. The curing profile (Figure S16) was obtained by performing a time sweep for 10 minutes at room temperature, 1000% strain, and a frequency of 1 rad/s; a temperature sweep from room temperature to 100 °C at a ramp rate of 30 °C/min, 1000% strain, and a frequency of 1 rad/s; and, finally, a time sweep for approximately 6 hours at 100 °C, 1% strain, and a frequency of 1 rad/s.