Free-Surface Frontal Ring-Opening Metathesis Polymerization

To depress the freezing point of DCPD, 5 wt % ENB was blended with DCPD at 35 °C. The liquid mixture was then degassed overnight at room temperature and 15 kPa with stirring. Unless otherwise noted, all references to DCPD refer to the 95:5 DCPD/ENB mixture. To enhance the fluorescence and aid in visualization of radial samples, 0.25 wt % TPE was dissolved in DCPD and degassed for an additional 12 h at room temperature and 15 kPa. In a typical experiment, 30 mg of GC2 (0.0353 mmol) was weighed out in a vial. In a separate container, TBP (9.6 μL, 0.0353 mmol) was added to DCPD (46.6 g, 10 000 mol equiv to GC2) or COD (38.4 g, 10 000 mol equiv to GC2). The monomer/inhibitor solution was then added to the vial containing GC2, and the resulting mixture was sonicated for 10 min to ensure complete dissolution of GC2. The solution was then filtered through a 0.22 μm PVDF membrane to remove any particulates. Filtered solutions were added to either a radial or channel mold to give a resin depth of 5 mm and incubated until the desired initial monomer temperature was achieved. Frontal polymerization was then initiated by local heating with a resistive wire. Polymerized samples were stored under a continuous flow of nitrogen gas to prevent surface oxidation.

Radial (D = 100 mm) and channel (150 mm × 25 mm) molds were fabricated by compressing an 8 mm thick polyurethane rubber gasket, cut into the desired geometry by waterjet, between a 1 mm thick laminated glass plate and a 3 mm thick aluminum plate. The top aluminum plate was machined to match the geometry of the rubber gasket and coated with Krylon ultraflat black spray paint prior to assembly to simulate a blackbody during thermal imaging. Initiation was achieved by heating a portion of the mold surface at 200 °C for 5 s with a resistive heater. Radial molds were heated in the center (D_initiation = 3.0 mm), and channel molds were heated along one 25 mm edge. Schematics of the molds and initiation schemes employed for individual experiments are summarized in Figure S2. Front temperatures were monitored from overhead with a FLIR T1020 thermal infrared camera focused at the liquid surface. The initial resin temperature and the ambient temperature were maintained in a custom-built environmental chamber equipped with an AC-162 Peltier module, TC-720 temperature controller, and PS-24–25 power supply from TE Technology, Inc. Resin temperature was monitored with a thermocouple and the overhead infrared camera.