Continuous Additive Manufacturing using Olefin Metathesis

Abstract The development of chemistry is reported to implement selective dual‐wavelength olefin metathesis polymerization for continuous additive manufacturing (AM). A resin formulation based on dicyclopentadiene is produced using a latent olefin metathesis catalyst, various photosensitizers (PSs) and photobase generators (PBGs) to achieve efficient initiation at one wavelength (e.g., blue light) and fast catalyst decomposition and polymerization deactivation at a second (e.g., UV‐light). This process enables 2D stereolithographic (SLA) printing, either using photomasks or patterned, collimated light. Importantly, the same process is readily adapted for 3D continuous AM, with printing rates of 36 mm h–1 for patterned light and up to 180 mm h–1 using un‐patterned, high intensity light.

Infrared spectroscopy. Mixtures of interest were introduced between NaCl salt plates (International Crystal Laboratories) separated by spacers (12.7 μm thick) to maintain constant sample thickness during polymerization. Each sample was placed in a Bruker-Tensor II FTIR spectrometer equipped with a horizontal transmission accessory (Pike) and spectra were collected from 400 to 4000 cm -1 with 1 scan per acquisition for rapid data collection. Samples were irradiated at the wavelength(s) of interest (365 and 405 nm at 10 mW⋅cm -2 and 475 nm at 20 mW⋅cm -2 ), and irradiation was continuous for the duration of the data collection period. Irradiation was performed with a Thorlabs CHROLIS 6-Wavelength High-Power LED Source. Samples used for calculating conversion were formulated and measured with 3 replications. Conversions were calculated by fitting Gaussians (OriginPro 2020b. OriginLab Corp., Northampton, MA.) to peaks centered at 1573 cm -1 in the FTIR spectra and monitoring the disappearance of these peaks over time.
Conversion at a given time, C(t), is calculated by the following equation: Where Ai is the area of the Gaussian fitted to the initial absorbance peak and At is the peak area at a given time (t).
Differential scanning calorimetry. DSC was performed using a TA Instruments Q200 calorimeter.
Approximately 5-10 mg of the sample was added to the bottom half of a standard DSC pan and the pan was then sealed. The sample was heated from room temperature to 250 °C, and this was followed by two additional cooling and heating cycles from 25-250 °C.
Thermogravimetric analysis. Thermal stability was evaluated using a TA Instruments Q5500 thermogravimetric analyzer. Samples were heated under nitrogen atmosphere (40 mL/min) at heating rate of 10 °C/min from room temperature to 600°C.
UV-Rheology. Rheology was performed on an ARES-G2 rheometer (TA Instruments) operating in small-amplitude oscillatory shear mode. Room temperature UV-rheology was performed using a Thorlabs CHROLIS 6-Wavelength High-Power LED Source and a UV accessory. Rheology was performed with a parallel plate assembly, 2 Hz oscillation frequency, and a 1.0% strain amplitude.
Exposure (60 mW⋅cm -2 ) began after 60 seconds and was continuous for all runs. An approximate gap size of 0.4 mm was generally used. Additionally, the glass transition temperature was measured using a similar set up with a 3 °C/min ramp from room temperature to 250 °C and back.
UV-visible spectrophotometry. UV-visible spectrophotometry was performed using an Agilent Technologies Cary 60 UV-Vis spectrophotometer. Spectra were collected from 200 to 800 nm with 1 nm spacing on solutions using a 10 mm path length quartz cuvette. Irradiation of samples was provided by a collimated, LED-based illumination source (Thorlabs M365LP1-C1) with an emittance centered at 365 nm, used in combination with a current-adjustable LED driver (Thorlabs LEDD1B) to produce an intensity of 30 mW⋅cm -2 .
Tensile testing Tensile testing was performed using an electromechanical load frame (Instron 5982). Dogbone specimens (ASTM type V) were printed between two glass slides using 0.5 mm spacers and the projector to pattern dogbone images. The photo-cured dogbones were post-cured in an oven initially for two hours at 75 °C and then at 230 °C for an additional 30 minutes.
Subsequently, the dogbones were subjected to uniaxial tension, and stress-strain curves were generated using low strain rates (3%⋅min -1 ) to give convenient total test times.
Optical microscopy and surface profilometry. Images of printed coupons were acquired using a Kyence VHX-7000 digital microscope. Serial recording at 40x magnification and a 5-micron vertical pitch was employed to capture the full lateral and vertical profile of printed specimens. Topographical contour mappings were produced using Keyence's 3D imaging and analysis software package. The accuracy of the mappings were verified using a Bruker Dektak kXT contact stylus profilometer.
The flask was placed in an ice bath. To the flask was added CDI (1.1 g, 6.5 mmol, 1 equiv) in one portion at 0 °C. Following this addition, the flask was removed from the ice bath and the reaction mixture was stirred for 1 h. TMG (0.82 mL, 6.5 mmol, 1 equiv) was then slowly added to the flask via syringe, and the reaction mixture was stirred at room temperature for 16 h. The reaction mixture was diluted with ~50 mL of CH2Cl2 and transferred to a separatory funnel. The organic mixture was washed with DI H2O (2 x 50 mL) and brine, dried over Na2SO4, and concentrated in vacuo.

Synthesis of 4,5-dimethoxy-2-nitrobenzyl TMG carbamate (NVOC-TMG)
NVOC-Cl (1.1 g, 4.1 mmol, 1 equiv) was dissolved in 20 mL of CH2Cl2 in a 100 mL round bottom flask. The flask was placed in an ice bath. To the flask was added TMG (1.0 mL, 8.2 mmol, 2 equiv) dropwise via syringe at 0 °C. Following this addition, the flask was removed from the ice bath and the reaction mixture was stirred for 4 h. The reaction mixture was diluted with ~50 mL of CH2Cl2 and transferred to a separatory funnel. The organic mixture was washed with DI H2O (2 x 50 mL) and brine, dried over Na2SO4, and concentrated in vacuo. The residue was further purified on a silica gel column, eluting with a gradient from 0 -10% MeOH in CH2Cl2. The purified product was isolated as a light-yellow oil that crystallized upon standing (0.90 g, 62% yield

Synthesis of 2-(2-nitrophenyl)propyl TMG carbamate (NPPOC-TMG)
NPPOC-Cl (0.62 g, 2.5 mmol, 1 equiv) was dissolved in 20 mL of CH2Cl2 in a 100 mL round bottom flask. The flask was placed in an ice bath. To the flask was added TMG (0.64 mL, 5.1 mmol, 2 equiv) dropwise via syringe at 0 °C. Following this addition, the flask was removed from the ice bath and the reaction mixture was stirred for 16 h. The reaction mixture was diluted with ~50 mL of CH2Cl2 and transferred to a separatory funnel. The organic mixture was washed with brine (3x), dried over Na2SO4, and concentrated in vacuo. The residue was further purified on a silica gel column, eluting with a gradient from 0 -5% MeOH in CH 2 Cl 2 . The purified product was isolated as a light-yellow oil that crystallized upon standing (0.52 g, 64% yield

Representative DCPD resin formulation
DCPD/ENB mixtures were first prepared at 5 wt% ENB by adding DCPD melted at 40-50 ᵒC to a glass jar containing ENB and agitating until fully mixed. Photoresin was then formulated using the DCPD/ENB mixture as follows: to a 125 mL Thinky TM cup was added 20 mg of HM (0.030 mmol, 1 equiv), 200 mg of CQ (1.2 mmol, 40 equiv), 400 mg of EDAB (2.1 mmol, 70 equiv), and 140 mg of NPPOC-TMG (0.45 mmol, 15 equiv). CH2Cl2 was added in portions (~ 1 mL total volume) to fully homogenize these components, consistent with established literature procedures. [1,2] 20 g of DCPD/ENB mixture was then added, and the resin was agitated to homogenize. The photoresin was used immediately after preparation.

UV-Vis spectroscopic analysis of HM in the presence of Bronsted base.
Stock solutions of HM, norbornene monomer, and various Bronsted bases were prepared in 1,2dichloroethane (DCE). For each experiment, these stock solutions were mixed immediately before measurements in 4 mL glass vials and diluted to 3 mL total volume with DCE. These mixtures were transferred to quartz cuvettes, and UV-Vis spectra were collected from 230-700 nm at 1 min intervals for a period of 30 min to monitor changes in electronic signatures associated with the HM catalyst.

Selective dual-wavelength olefin metathesis polymerization (SWOMP)
Cure          1. Note that a dramatic increase in absorbance was observed for the HM+NB sample under 365 irradiation due to P(NB) precipitation, signifying that polymerization was occurring.
Scheme S1. Proposed mechanism of catalyst deactivation by TMG.
Scheme S2. Photolysis of the PBGs generates TMG along with other decomposition products.      Figure S17. FT-IR spectra of a part produced via SWOMP immediately after photopolymerization (black trace) and following a thermal post cure at 250 °C for 30 min (orange trace). The signal at 1573 cm -1 corresponds to residual unreacted alkenes derived from residual DCPD monomer that become fully consumed during the post-curing process. Figure S18. Thermal analysis of PDCPD films prepared by photo-activated ROMP using the optimized resin formulation under 475 nm irradiation. A) DSC of PDCPD films prepared using resins with (blue trace) and without (black trace) NPPOC-TMG. The calculated Tg values were 164 °C and 153 °C, respectively. Shown is the 2 nd heating cycle, wherein the temperature was increased from 50-250 °C at 10 °C min -1 under N2. B) TGA of the same films prepared using resins with (blue trace) and without (black trace) NPPOC-TMG. Note that the traces are overlapped. A heating rate of 20 °C min -1 was used. Complete DMA data for the same films prepared using resins with (C) and without (D) NPPOC-TMG. Shown are storage modulus (closed circles, darker traces), loss modulus (closed circles, lighter traces) and tan delta (open circles).
[DCPD]/[HM] = 5000 : 1 was used for these experiments, with 2 wt% EDAB and 1 wt% CQ. Figure S20. Stereolithography using dual-wavelength approach. A) Schematic of stereolithographic setup, wherein the resin is illuminated with a constant background of blue light from below and patterned UV light from above. Resin curing is inhibited in the regions where the UV light is present. B) Optical photographs of photomasks and corresponding cured resins obtained by this process. Images from different stages of the printing and post cure process were included to highlight the changes in appearance of the polymer films. Figure S21. Additional 2D objects produced using a photomask and 475 nm irradiation from a high-intensity light source (two leftmost images), an image projected with a blue light emitting projector (second to last image), and 475 nm irradiation from a high-intensity light source and a 365 nm light engine to pattern UV light similar to the setup shown in Figure S20 (rightmost image). These objects showcase sub-mm x,y resolution can be achieved using either a single-wavelength or a dual-wavelength approach. Grids represent 1 mm × 1 mm squares.
[DCPD]/[NPPOC-TMG]/[HM] = 5000 : 15 : 1 was used for these experiments with 1 wt% CQ and 2 wt% EDAB. Figure S22. Stereolithography using a dual-wavelength approach enables facile translation of digital images to cured parts. White regions on the grayscale images yield the shallowest deactivation depths, while black one result in the largest deactivation depths (and thus no cure). This experiment highlights the ability to produce complex topological objects from a single exposure. [