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. 2020 Feb 5;6(2):266–273. doi: 10.1021/acscentsci.9b01237

Triggered Transience of Plastic Materials by a Single Electron Transfer Mechanism

Adam M Feinberg †,‡, Oleg Davydovich †,‡, Evan M Lloyd †,§, Douglas G Ivanoff †,∥, Bethany Shiang ∥, Nancy R Sottos †,∥, Jeffrey S Moore †,‡,∥,*
PMCID: PMC7047432  PMID: 32123745

Abstract

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Transient polymers rapidly and controllably depolymerize in response to a specific trigger, typically by a chain-end unzipping mechanism. Triggers, such as heat, light, and chemical stimuli, are generally dependent on the chemistry of the polymer backbone or end groups. Single electron transfer (SET), in contrast to other triggering mechanisms, is achievable by various means including chemical, electrochemical, and photochemical oxidation or reduction. Here, we identify SET and subsequent mesolytic cleavage as the major thermal triggering mechanism of cyclic poly(phthalaldehyde) (cPPA) depolymerization. Multimodal SET triggering is demonstrated by both chemical and photoredox-triggered depolymerization of cPPA. Redox-active small molecules (p-chloranil and 1,3,5-trimethoxybenzene) were used to tune the depolymerization onset temperature of cPPA over the range 105–135 °C. Extending this mechanism to photoredox catalysis, N-methylacridinium hexafluorophosphate (NMAPF6) was used to photochemically degrade cPPA in solution and thin films. Finally, we fabricated photodegradable cPPA monoliths with a storage modulus of 1.8 GPa and demonstrated complete depolymerization within 25 min of sunlight exposure. Sunlight-triggered depolymerization of cPPA is demonstrated and potentially useful for the manufacture of transient devices that vanish leaving little or no trace. Most importantly, this new mechanism is likely to inspire other SET-triggered transient polymers, whose development may address the ongoing crisis of plastic pollution.

Short abstract

SET-induced triggered transience of cPPA is demonstrated by rapid photoredox-triggered depolymerization in sunlight.

Introduction

Synthetic polymers are ubiquitous in our day-to-day lives due to their ease of manufacture,1 wide range of mechanical properties,2 and resistance to corrosion and aging.3 In contrast to polymer production, plastic waste remediation has proven challenging. Synthetic polymers are largely unrecovered at the end of the material lifespan; notably, less than 10% of plastic waste in the U.S. was recycled in 2015, with the remainder largely being sent to landfills (ca. 75%) or burned (ca. 15%). This lack of end-of-life management has resulted in the global plastic pollution crisis.4 Transient materials may provide controlled end-of-life strategies for plastic waste mitigation.

The ideal transient material has good mechanical properties, is readily accessible from inexpensive feedstocks, is processable by conventional methods, and is easily tailored to respond to different and orthogonal triggering stimuli. Depolymerization reactions are typically triggered by light,5 acid,6 and specific ions,7 stimuli that are ubiquitous in everyday use. Single electron transfer (SET) triggering of transient polymers presents an alternative route to tunable transient materials. An SET-triggered polymer unzipping reaction offers versatility for materials formulation. Tailored additives would enable SET triggering from specific light,8 chemical,9,10 or electrical11 input to initiate depolymerization using a single, easily accessible pathway, shown schematically in Figure 1. In this work, we sought to demonstrate a transient material that is easily synthesized, has a storage modulus of at least 1 GPa, is processable, and is triggered to depolymerize by SET.

Figure 1.

Figure 1

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Putative single electron transfer (SET) induced depolymerization of cyclic poly(phthalaldehyde). One electron oxidation of cPPA via either chemical oxidation using p-chloranil (pCA) or photoredox catalysis using N-methylacridinium hexafluorophosphate (NMAPF6) results in the formation of the corresponding cation radical intermediate I. Following SET activation, mesolytic cleavage of I forms the distonic cation radical II, which is hypothesized to undergo cationic unzipping of the activated oxonium chain end, forming intermediate III, and ultimately the monomer oPA.

A promising transient polymer is cyclic poly(phthalaldehyde) (cPPA), which has a room temperature storage modulus of 1.5–2 GPa12 and is prepared in a scalable, one-step cationic polymerization reaction from ortho-phthalaldehyde (oPA), a readily available monomer.13,14 cPPA rapidly unzips to form oPA on thermal,15 acid,16 or mechanical17 triggering due to its low ceiling temperature (Tc = −36 °C18). The mechanisms of acid-triggered19 and mechanically triggered17 cPPA unzipping are well-known, but the mechanism of thermal triggering has not been established. While purely ionic and purely radical thermolytic depolymerization mechanisms have been previously proposed,20 we hypothesized that SET effectively initiates the unzipping of cPPA by mesolytic cleavage of the radical cation intermediate (Figure 1). Here, we present evidence in support of the SET-triggered depolymerization of cPPA and demonstrate the application of this mechanism in the manufacture of sunlight-degradable monolithic materials.

Results and Discussion

We hypothesized that, during thermolysis, SET oxidation of cPPA leads to the formation of a benzylic cation radical.21 Mesolytic fragmentation of the benzylic cation radical22,23 is followed by subsequent cationic chain unzipping throughout the polymer chain (Figure 1). To test the SET triggering hypothesis, we first examined the effect of a small molecule oxidant, p-chloranil (pCA, Ered= −0.005 V vs SCE in acetonitrile24), on cPPA thermal stability (Figure 2a). If SET triggers cPPA depolymerization, addition of an oxidant is expected to destabilize the polymer. The addition of pCA resulted in significantly reduced thermal stability, which decreased in a dose-dependent manner. This linear dependence enabled tuning of the thermal degradation onset temperatures over a ca. 15 °C range (0–2 phr (parts per hundred resin) pCA). These results demonstrate that SET is an effective method for triggering cPPA degradation but do not indicate whether the SET triggering mechanism is operative under typical thermolytic conditions, as SET requires a reductant/oxidant pair. A plausible thermo-oxidant is BF3-pyridine, known to be both a mild oxidant25 and a common impurity in cPPA which results in lowered polymer thermal stability.20

Figure 2.

Figure 2

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Thermolysis of cPPA in the presence of reductants and oxidants: (a) change in degradation onset temperature during 5 °C/min dynamic TGA scans of cPPA thin films as a function of 1,3,5-trimethoxybenzene (TMB) and p-chloranil (pCA) concentration, phr = parts per hundred resin. (b) Evolution of molecular weight (Mn) as a function of conversion during thermolysis of cPPA thin films containing 2 phr TEMPO. In each plot, error bands and error bars represent 95% confidence; plotted points in part a represent the average of three measurements.

Though it is not possible to determine if the BF3-pyridine in cPPA is the operative oxidant, we can conclusively establish whether SET is the primary mode of thermolytic cPPA depolymerization. The SET triggering mechanism further predicts that electron donors, such as 1,3,5-trimethoxybenzene (TMB, Eox= 1.539 V vs SCE),26 will inhibit the thermal degradation of cPPA. As shown in Figure 2a, addition of TMB to cPPA films results in a material with a higher degradation onset temperature. The onset of polymer degradation measured during dynamic TGA experiments varied linearly with TMB concentration. GPC analysis of the cPPA films during thermolysis in the presence and absence of TMB show no significant differences in Mn evolution (Supporting Information). This result indicates that the inhibitory action of TMB occurs before the activation of cPPA chains, consistent with the SET triggering mechanism and inconsistent with purely radical or purely ionic thermolysis mechanisms.

The addition of TEMPO to cPPA is known to stabilize the polymer toward thermolysis.20 This effect had previously been rationalized by trapping of radical chain ends by TEMPO. In contrast, the SET triggering hypothesis predicts that TEMPO inhibition is due to sacrificial oxidation of TEMPO (Eox = 0.50 V vs SCE27). To probe the nature of TEMPO inhibition, we monitored by GPC the evolution of polymer molecular weight during the thermolytic depolymerization of cPPA films in the presence and absence of added TEMPO. If the stabilizing effect of TEMPO was due to radical trapping, i.e., the deactivation of reactive cPPA termini after a scission event, the change in Mn during depolymerization would follow a nonlinear trend,28 generating low-molecular-weight chains at low conversion. Instead, Mn of cPPA samples without TEMPO and with 2 phr added TEMPO followed statistically similar linear trends during depolymerization, with no observable low Mn species at low conversion (Figure 2b). This trend indicates that the stabilizing effect of TEMPO is not due to radical trapping but rather suggests that TEMPO inhibits SET-activation of the polymer chain. These results exclude a homolytic thermal depolymerization mechanism and further support the SET triggering hypothesis.

Having established a novel SET triggering mechanism as the primary thermal depolymerization pathway for cPPA, we were interested in the application of SET chemistry to develop photodegradable monolithic materials using photoinduced single electron transfer. Toward this end, we first investigated the photo-depolymerization of cPPA by the photo-oxidant NMAPF629 in solution (Figure 3a). A solution of NMAPF6 (0.35 mM) and cPPA (20 mg/mL) in dichloromethane-d2 was prepared. Photolysis at 375 nm (0.1 W/cm2) resulted in the complete depolymerization of cPPA within 4 min. Depolymerization was confirmed by both GPC and 1H NMR (Figure 3b,c, respectively). The high-molecular-weight polymer peak at a retention time of 25 min in the prephotolysis GPC trace is completely absent after UV exposure. Additionally, only resonances corresponding to cPPA and residual dichloromethane are visible by 1H NMR (CD2Cl2, 60 MHz) before photolysis, while only oPA resonances (in addition to residual dichloromethane) are observed postphotolysis. Control samples that were kept in the dark and samples that were exposed to 0.1 W/cm2 375 nm light in the absence of NMAPF6 did not degrade appreciably over the same time period (Supporting Information). These results clearly demonstrate that photoinduced SET is an effective depolymerization trigger for cPPA.

Figure 3.

Figure 3

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Photo-oxidative depolymerization of cPPA (20 mg/mL) by NMAPF6 in dichloromethane solution: (a) reaction scheme; (b) gel permeation chromatography refractive index detection traces showing the presence and absence of high-molecular-weight polymer before and after photolysis, respectively; and (c) 1H NMR of the reaction mixture before and after photolysis, showing complete conversion to the monomer, oPA. 1H NMR was collected at 60 MHz in dichloromethane-d2.

Having demonstrated that photoinduced SET triggers cPPA depolymerization in the solution state, we sought to investigate its application to solid-state depolymerization. To probe the utility of SET triggering in the solid state, we blended cPPA with NMAPF6 in dichloromethane solution and drop cast to produce 100 μm thick films. Thin films with no added NMAPF6 were clear and colorless and did not visibly degrade upon exposure to UV light (Figure 4a,b). Thin films doped with NMAPF6, in contrast, were vibrant yellow and visibly degraded, forming a purple gel, when exposed to UV light (Figure 4c,d).

Figure 4.

Figure 4

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Controlled depolymerization of cPPA thin films by photochemical SET triggering at ambient temperature: micrographs of control cPPA films before and after exposure to UV light (a, b) and cPPA films with 2 mol % NMAPF6 before and after exposure to UV light (c, d). (e) Depolymerization of cPPA thin films as monitored by 1H NMR (60 MHz, CD2Cl2) during 375 nm UV irradiation at 0.1 W/cm2 with curves fitted to an exponential function. (f) Depolymerization of cPPA thin films in ambient room light as monitored by 1H NMR (60 MHz, CD2Cl2) with curves fitted to a logistic function. Each plotted point is the average of three measurements, and error bars represent 95% confidence.

The kinetics of film depolymerization were monitored by 1H NMR during irradiation of thin films at 375 nm (0.1 W/cm2) (see the Supporting Information for data and analysis). Upon UV excitation, films rapidly depolymerized, with oPA as the only product visible by 1H NMR. The rate of depolymerization was highly dependent on the loading of NMAPF6 in the thin film. Figure 4e demonstrates the NMAPF6 dose dependence of cPPA depolymerization kinetics in samples with 0.5, 1.0, and 2.0 mol % NMAPF6. Samples with no added NMAPF6 did not degrade upon UV exposure, while those with 0.5, 1.0, and 2.0 mol % NMAPF6 completely depolymerized within 6.5, 5.5, and 3.5 min, respectively. Importantly, NMAPF6-doped samples that were not exposed to UV light did not depolymerize to any observable degree over the course of 1 week (Supporting Information). These results indicate that the SET triggering process is facile in solid cPPA matrices.

Ambient room lighting was also sufficient to degrade the NMAPF6-doped cPPA films. Figure 4f shows the depolymerization of thin films as a function of time at various photocatalyst loadings. At 2.0 mol % NMAPF6, nearly quantitative conversion to monomer was observed after 1 week in ambient room lighting. Control samples without added NMAPF6 did not degrade during the course of the experiment (Figure 5f), nor did NMAPF6-doped cPPA films which were kept in the dark during the same period of time (Supporting Information). cPPA samples with lower loadings of NMAPF6 (i.e., 0.5, 1.0 mol %) are expected to continue degrading if left under ambient lighting. This NMAPF6 dose dependence provides a method by which to tune the rate of material degradation for various desired lifetimes.

Figure 5.

Figure 5

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Controlled depolymerization of bulk cPPA samples by photochemical SET triggering: (a–c) Photographs of a cPPA-NMAPF6 dog-bone under 375 nm radiation (0.35 W/cm2) at 5, 65, and 125 s, showing physical destruction of the polymer matrix. (d) Normalized E′ of NMAPF6-doped cPPA dog-bones during photolysis at 375 nm at varying light intensities. Samples were monitored in the absence of light for 300 s, at which point the lamp was turned on. Each curve is an average of 3 samples, and error bands represent 95% confidence. (e–i) Bulk cPPA-NMAPF6 degrading in sunlight on a sunny day in August in Champaign, IL, USA, over the span of 25 min. UV intensity during sunlight photodegradation was measured at 30 mW/cm2, and the outdoor temperature was 24.5 °C.

While photodegradable thin films have been demonstrated previously by the creative application of photoacid generators (PAGs),6,30 the thermal decomposition of PAGs precludes their use in the manufacture of monolithic photodegradable materials,31,32 which generally requires extended time at elevated temperature for melt processing. Thermally stable organic photo-oxidants present a promising tool for the manufacture of monolithic, photodegradable engineering plastics. Bulk material samples were fabricated following our previously reported procedure.20 Briefly, cPPA was solvent-blended in dichloromethane with a plasticizer (diphenyl phthalate, 40–60 phr) and a photo-oxidant (NMAPF6, 1 mol %) and drop cast in a dark enclosure to exclude ambient light. After 24 h, the blended films were pulverized, and the resultant powder was used as a feedstock for thermoforming. Type V dog-bone samples (ASTM standard D638) were fabricated by hot-pressing the cPPA-NMAPF6-DPP feedstock at 90 °C, 10 MPa for 5 min. The resultant monolithic materials were high-quality, optically transparent thermoplastics with an average storage modulus of 1.8 GPa, as measured by dynamic mechanical analysis (DMA) (Supporting Information).

To study photo-oxidative depolymerization of bulk polymers, the mechanical integrity of cPPA samples (thickness = 500 μm) was measured by DMA during exposure to UV light. Optical images of a cPPA-NMAPF6-DPP sample during UV irradiation at 0.35 W/cm2 in the DMA instrument are shown in Figure 5a–c. The bulk material completely degrades in the irradiated area, resulting in a viscous liquid composed of monomer, plasticizer, and the photo-oxidant byproducts. Unexposed and under-exposed regions remained visibly unaffected. As shown in Figure 5d the rate of material degradation is controlled by irradiation intensity. Bulk samples were monitored in the dark for a 300 s pre-exposure period to obtain a baseline stiffness, at which point the UV light source (375 nm) was turned on. Samples exposed to 0.1, 0.15, and 0.2 W/cm2 UV irradiation rapidly lost mechanical integrity, failing under tension within 600, 300, and 200 s, respectively. Samples that were not exposed to UV light maintained their original stiffness throughout the experiment. Control samples without NMAPF6 did not undergo significant change on exposure to equivalent UV irradiation (Supporting Information).

Finally, the efficacy of SET triggering for applications in environmental degradation was tested. Using the above thermoforming procedure, an I-shaped monolithic solid was manufactured (thickness = 2.0 mm). The sample was exposed to solar radiation (measured light intensity = 30 mW/cm2) and photographed at regular intervals (Figure 5e–i) on a white cardstock background. After 3 min of exposure to sunlight, the sample discolored along its surface. Within 6 min, viscous liquid had begun to pool around the sample. After 12 min of exposure, large, needlelike crystals appeared, indicating the formation of the crystalline monomer, oPA. Within 24 min of sunlight exposure, no visible polymer remained. By using the novel SET triggering mechanism, we have successfully produced a sunlight-degradable monolithic material.

Conclusion

We have presented for the first time a transient polymer which undergoes rapid chain unzipping depolymerization to its constituent monomer following a single electron transfer trigger. SET triggering is achieved through multiple modes and was realized in both thermal and photochemical depolymerization of cyclic poly(phthalaldehyde). The collection of evidence presented here supports the mechanistic hypothesis of SET activation and subsequent mesolytic cleavage. This mechanism was used to tune the thermal stability of cPPA by the addition of oxidants and reductants. Additionally, photo-oxidation of cPPA using NMAPF6 was demonstrated as an effective method of depolymerization in solution and in thin films. Finally, monolithic solids composed of cPPA, diphenyl phthalate, and NMAPF6 were fabricated. These photo-oxidant-doped bulk solids were shown to exhibit desirable mechanical properties (E′ = 1.8 GPa), and to respond rapidly to applied UV light, degrading completely into the corresponding monomer. Additionally, it was shown that the monolithic cPPA materials fully degraded to monomer within 25 min of sunlight exposure.

Given the ease with which cPPA depolymerizes by frequently encountered stimuli, it is unlikely to serve as a candidate to mitigate the environmental burden of single use plastics. Nonetheless, as a bulk engineering plastic that rapidly depolymerizes via photoredox catalysis, the chemical concepts presented here may inspire the development of new transient packaging. Even in its present state the applications for cPPA are evident, such as in the manufacture of transient delivery systems, and environmental sensing applications. For example, it is conceivable that SET-triggered cPPA is well-suited for use in the manufacture of air gliding vehicles that deliver critical supplies and subsequently vanish by programmable transience, leaving no trace of the device. Most importantly, this work demonstrates a novel mode of SET-triggered transience and raises the prospect of SET as a depolymerization mechanism in a potentially broad range of polymers. Thus, SET triggering of transient polymers is viewed as a promising area for future exploration.

Methods

General

All materials were purchased from Sigma-Aldrich and used without further purification unless otherwise noted. ortho-Phthalaldehyde (oPA) was purchased from Oakwood Chemical and purified via vacuum distillation (0.1 Torr, 90 °C). Poly(tetrafluoroethylene) Petri dish liners were purchased from Welsh Fluorocarbon Inc. Ultra-high-molecular-weight polyethylene substrates were purchased from McMaster Carr. All prepared thin films were stored at −20 °C until use.

Analytical gel permeation chromatography (GPC) was performed using a Waters 1515 isocratic HPLC pump and Waters 2707 96-well autosampler, equipped with a Waters 2414 refractive index detector and 4 Waters HR Styragel column (7.8 × 300 mm, HR1, HR3, HR4, and HR5) in THF at 30 °C. The GPC system was calibrated using monodisperse polystyrene standards.

Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 TGA under a nitrogen atmosphere (90 mL/min). Dynamic TGA traces were obtained during a 5 °C/min ramp after equilibration at 40 °C. TGA samples consisted of 3–5 mg of the analyte film in a platinum pan. 1H NMR spectra were recorded on a Varian VXR 500 instrument (500 MHz) or an NMReady-60 benchtop NMR instrument (60 MHz) purchased from Nanalysis Scientific Corp. UV photo-depolymerization was performed using a custom-made 375 nm LED assembled with a 375 nm LED equipped with an AR coated aspherical condenser lens and an AR coated biconvex focusing lens (75 mm focal length). All parts were purchased from Thor Laboratories and assembled manually. A Keyence VHX-5000 series digital microscope was used to visualize photodegradation of cPPA thin films. A Canon EOS 7D camera equipped with a 100 mm macrolens from Canon was used to image photodegradation of cPPA bulk materials.

Synthesis of Cyclic Poly(phthalaldehyde)

Cyclic poly(phthalaldehyde) (cPPA) was prepared via the cationic polymerization of purified oPA using a Lewis acid catalyst, BF3-EtO2, according to a known procedure.14 All reactions were run in anhydrous dichloromethane at −78 °C. Briefly, 40 g of ortho-phthalaldehyde (300 mmol) was dissolved in 200 mL of anhydrous dichloromethane (1.5 M). The solution was first cooled down to −78 °C for 2 min upon which 0.8 mL of BF3-EtO2 (6.5 mmol) was added to the reaction. The solution was stirred using a mechanical stirrer for 2 h, at which point it had become highly viscous. The reaction was then quenched using 2 mL of pyridine and was stirred for an additional 2 h. Finally, the reaction mixture was precipitated in 4 L of methanol. The precipitated polymer was dried via vacuum filtration for 1 h and subsequently dried on hi-vac overnight to afford 36.5 g (92%) of a white solid. The polymer was stored at −20 °C until use.

1H NMR (CD2Cl2, 500 MHz): δ 5.75–7.75 (br, 6 H). Mn = 250 kDa, Đ = 1.67.

Synthesis of N-Methylacridinium Iodide

N-Methylacridinium iodide (NMAI) was synthesized according to the literature procedure.33 Briefly, a 100 mL oven-dried round-bottom flask was charged with a stir bar. A 5.0 g portion of acridine (27.6 mmol) was dissolved in 20 mL of DMF and was heated to 35 °C for 10 min. A 3.4 mL portion of iodomethane (55 mmol) was added into the reaction flask. The reaction flask was heated to 50 °C overnight under dry nitrogen. A 100 mL portion of diethyl ether was added to the reaction mixture, and the precipitated solid was then isolated via vacuum filtration. The compound was dried on hi-vac overnight to yield 6.6 g of material (74%) and was used without further purification.

1H NMR (CD2Cl2, 500 MHz): δ 10.02 (s, 0.95 H), 8.67 (d, 2 H), 8.61 (dd, 1.96 H), 8.47 (ddd, 1.99 H), 8.02 (ddd, 1.97 H), 5.00 (s, 3.16 H).

Synthesis of N-Methylacridinium Hexafluorophosphate

A 1.09 g portion of NMAI (3.3 mmol) was dissolved in 120 mL of water, and a 20 mL aqueous potassium hexafluorophosphate (1.22 g, 6.6 mmol) solution was added. A yellow precipitate formed immediately. The solution was stirred for 30 min, and the precipitate was isolated via vacuum filtration. The precipitate was washed with 3 × 50 mL of water and was then dried under vacuum overnight. The dried product was purified via recrystallization in methanol, forming small needlelike crystals (253 mg, 28.6% yield). The resulting compound was dried under vacuum and stored away from light.

1H NMR (CD2Cl2, 500 MHz): δ 9.77 (s, 0.98 H), 8.52 (ddd, 3.95 H), 8.45 (m, 1.99 H), 8.02 (ddd, 1.95 H), 4.87 (s, 3.13 H).

Solvent Casting cPPA Thin Films

Freestanding pristine cPPA thin films were prepared according to the literature procedure.20 Briefly, cPPA (100 mg) was dissolved in HPLC grade dichloromethane (3 mL). The solution was then cast into a 50 mm diameter PTFE-lined Petri dish and allowed to dry for 24 h in a light-free cardboard enclosure with solvent-saturated atmosphere. Films used in reductant and oxidant stability tests were doped with 0.5, 1.0, 1.5, and 2.0 mg of 1,3,5-trimethoxybenzene (TMB) or p-chloranil (pCA) before casting. Films used in trapping experiments were doped with 2.0 mg of TEMPO or TMB before casting.

Photodegradable thin films were prepared by dissolving 300 mg of cPPA in 5 mL of dichloromethane along with a known amount of NMAPF6 (0, 3.8, 7.7, and 15.5 mg for 0, 0.5, 1.0, and 2.0 mol % NMAPF6 loading, respectively). cPPA-NMAPF6 solutions were cast into 50 mm diameter PTFE-lined dishes as above.

cPPA Trapping Experiments

Small sections of cPPA thin films were cut and weighed out (∼5 mg). The films were placed into the bottom of scintillation vials. The vials were then immersed in an oil bath that was kept at 100 °C. The vials were removed at 5 min intervals, and the repolymerization was quenched using an ice bath. The vial contents were dissolved in 0.25 mL of THF and analyzed by GPC. The GPC traces were normalized by the initial mass of the film, and conversion was determined via the ratio of the normalized area of high-molecular-weight peaks (retention times between 20 and 35 min) of samples subjected to thermolysis and a control sample that was not subjected to thermolysis.

The trends of Mn vs conversion for samples doped with TMB and with TEMPO were compared against those of pristine cPPA films run at the same time using an F-test. The trends were not found to be statistically different at the 0.05 significance level.

Solution State Photolysis of cPPA

A solution of cPPA (20 mg/mL) in dichloromethane was transferred to a 1 cm UV quartz cell, and a stir bar was added. A separate solution containing cPPA (20 mg/mL) and NMAPF6 (0.35 mM) was similarly prepared. Both solutions were stirred and irradiated with a 375 nm LED for 4 min. Before and after photolysis, the crude reaction mixtures were analyzed by 1H NMR. After photolysis, the solvent was evaporated via a rotary evaporator, and the residue was dissolved in THF and analyzed via gel permeation chromatography (GPC). In the absence of NMAPF6, no significant change was observed in either 1H NMR or GPC. In the presence of NMAF6, both 1H NMR and GPC indicate complete conversion of cPPA to oPA over the course of 4 min.

Thin Film Photolysis of cPPA

Pristine cPPA and NMAPF6-doped cPPA films were cut into small square sections (0.5 cm × 0.5 cm) and were placed into scintillation vials. A 375 nm UV LED was positioned above the square samples at the focal point of the light source. The films were then irradiated for a given time (0–10 min). Following UV exposure, the irradiated films were dissolved in dichloromethane-d2 and characterized using 1H NMR (60 MHz, CD2Cl2). The conversion of polymer to monomer was monitored by comparing the ratio of the oPA aldehyde resonance peak (10.5 pm) and the phenyl resonance peaks of both oPA and cPPA (ca. 8–6 ppm). The percent of cPPA in each film was calculated according to the following equations:

graphic file with name oc9b01237_m001.jpg 1
graphic file with name oc9b01237_m002.jpg 2
graphic file with name oc9b01237_m003.jpg 3

Here, I10.5 is the integral of the resonance at 10.5 ppm, corresponding to the aldehyde proton of oPA; I8–6 is the integral of the broad resonance in the 8–6 ppm region, which corresponds to all six proton resonances of cPPA and the four aryl protons of oPA. To account for the incomplete relaxation of the aldehydic protons of oPA, a correction factor (Cf) of 1.26 was introduced into the calculation. Equation 1 shows the general calculation for the percent of cPPA in a sample. In eq 2, the relative concentration of cPPA is calculated by subtracting the integral contribution in the 6–8 ppm region from oPA (2 times I10.5, with a correction factor), normalized by the proton count (6). Equation 3 calculates the relative concentration of oPA by correcting I10.5 and normalizing by the proton count (2).

Ambient Light cPPA Thin Film Degradation

Pristine cPPA and NMAPF6-doped cPPA films were cut into small square sections (0.5 cm × 0.5 cm) which were placed into scintillation vials. The samples were exposed to ambient light by placing them inside of a fume hood (ambient light intensity = 6 μW/cm2) and leaving them for a given amount of time (0–7 days). Following ambient light exposure, the irradiated films were dissolved in dichloromethane and characterized using 1H NMR. The conversion of polymer to monomer was monitored by 1H NMR using the method described in the Thin Film Photolysis of cPPA section above.

Fabrication of cPPA Monoliths

cPPA Feedstock Preparation

A 5.0 g portion of cPPA and diphenyl phthalate (2.0 or 3.0 g for dog-bone feedstock and I-shaped monolith feedstock, respectively) was dissolved in HPLC grade dichloromethane (40 mL). To prepare the photo-oxidant-doped feedstock, 127 mg (1 mol % with respect to polymer repeat unit) of NMAPF6 was added to the solution. Solutions were then tape cast onto an ultra-high-molecular-weight polyethylene substrate in the dark. Film thickness (200 μm) was set with a high-precision film applicator. Films were left in a dichloromethane-saturated environment for 24 h. This process generated a freestanding film which was then pulverized into a powder feedstock using a coffee grinder.

cPPA Monolith Preparation

A 300 mg portion of cPPA feedstock prepared above (with or without added NMAPF6) was placed into an aluminum dog-bone mold (ASTM Standard D638 Type V). The filled mold was then preheated at 90 °C for 5 min. Samples were then pressed at 10 MPa and 90 °C for 5 min. The mold was cooled down (10 °C/min) to room temperature, and the dog-bones (500 μm thick) were removed from the mold. The same process was used to produce the I-shaped monoliths using 1.5 g of cPPA feedstock.

Storage Modulus Determination

Dynamic mechanical analysis (DMA) was performed using a TA Instruments RSA III. Both pristine cPPA and NMAPF6-doped cPPA samples were loaded onto the DMA using thin film grips provided by TA Instruments. The gauge length was set to 8 mm. Oscillatory load was applied at 10 Hz and 0.1% strain amplitude at 20 °C. The storage modulus of three samples was measured for both pristine cPPA samples and NMAPF6-doped samples. The average storage moduli of pristine and NMAPF6-doped samples were 1.54 ± 0.10 and 1.78 ± 0.11 GPa, respectively.

Dynamic Mechanical Analysis of cPPA during Photodegradation

Pristine cPPA and NMAPF6-doped cPPA specimens were loaded onto the DMA, and the gauge length was set to 25 mm. Oscillatory loading was applied at 10 Hz and 0.1% strain amplitude at 20 °C. The samples were first characterized in the dark for a 300 s pre-exposure period after which the UV light source (0.1, 0.15, or 0.2 W/cm2) was turned on. Specimens were tested until they failed in tension. Degradation was monitored optically with a Canon EOS 7D camera equipped with a 100 mm macrolens.

Sunlight Depolymerization of cPPA

I-shaped cPPA monoliths prepared above were placed on a glass Petri dish and placed outside on a sunny day, September sixth, 2019 in Urbana, Illinois. Photographs of the specimens were taken at 5 s intervals for 25 min. The UV intensity during sunlight photodegradation was measured to be 30 mW/cm2, and the outdoor temperature was 24.5−30.5 °C. The NMAPF-doped cPPA sample showed complete degradation after 24 min of exposure whereas the pristine cPPA sample showed no visual signs of degradation.

Acknowledgments

This manuscript is dedicated to the late Prof. Scott White, a constant mentor, inspiration, and friend. The authors gratefully acknowledge the support of the National Science Foundation through a Phase I Center for Chemical Innovation Grant (CHE-1740597). E.M.L. thanks the Arnold and Mabel Beckman Foundation for financial support. The authors thank Dorothy Loudermilk for assistance with graphics. The authors also thank the Beckman Institute for Advanced Science and Technology for the facilities and support to conduct this research.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.9b01237.

  • Representative UV–vis and 1H NMR spectra; representative thermogravimetric data; and additional control experiment data (PDF)

  • Video S1: sunlight-triggered depolymerization of doped cPPA monolith with an undoped control (MP4)

  • Video S2: UV-triggered depolymerization of a doped cPPA dog-bone (MP4)

Author Contributions

⊥ A.M.F. and O.D. contributed equally. J.S.M. and N.R.S. directed this research. J.S.M. and A.M.F. conceived the idea. O.D., E.M.L., D.G.I., B.S., and A.M.F. performed the experiments. All authors participated in writing the manuscript.

The authors declare no competing financial interest.

Supplementary Material

oc9b01237_si_001.pdf (1.2MB, pdf)
oc9b01237_si_002.mp4 (48.5MB, mp4)
oc9b01237_si_003.mp4 (34.9MB, mp4)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

oc9b01237_si_001.pdf (1.2MB, pdf)
oc9b01237_si_002.mp4 (48.5MB, mp4)
oc9b01237_si_003.mp4 (34.9MB, mp4)

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