Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels

Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis pairs (FLPs), where latent reactivity can be utilized for the activation of small molecules. Applying FLP chemistry into polymeric frameworks transforms this chemistry into responsive and functional materials. Here, we report a versatile synthesis strategy for the preparation of macromolecular FLPs and explore its potential with the ring-opening reactions of cyclic ethers. Addition of the cyclic substrates triggered polymer network formation, where the extent of cross-linking, strength of network, and reactivity are tuned by the steric and electronic properties of the ethers. The resultant networks behave like covalently cross-linked polymers, demonstrating the versatility of FLPs to simultaneously tune both small-molecule capture and mechanical properties of materials.

and distilled over CaH2 (Acros Organics) under inert atmosphere. All solvents and the distilled reagents were degassed by freeze-pump-thaw for three cycles before use. Anhydrous pyridine, triphenylphosphine, triethylphosphine oxide and 1.4 M sec-butyllithium (sec-BuLi) in cyclohexane were purchased from Sigma-Aldrich and methanol (reagent grade) was purchased from Fisher Scientific and they were used as received.
Bis(pentafluorophenyl)borane and 4-styryl-diphenylphosphine were synthesised using the reported literature procedures. 1,2 Instrumentation 1 H, 11 B, 13 C, 19 F and 31 P nuclear magnetic resonance (NMR) data were obtained using 400 MHz Bruker AVIII (BBFO 5 mm probe), 400 MHz Bruker AVIII HD (BBO 5mm probe) and 500 MHz Bruker AVIII HD (BBO 5 mm probe) spectrometers. Young's tap NMR tubes were used for all the air-/moisture-sensitive samples. Exact masses of the small molecule mimics were determined using a Thermo Orbitrap QExactive mass spectrometer in electrospray ionisation (ESI) mode by using DCM as solvent. Molecular weights of the polymers were determined using an Agilent 1260 Infinity II Multi-Detector Gel Permeation Chromatography (GPC)/Size Exclusion Chromatography System through PLgel 5 µm columns packed with PSDVB beads.
The GPC was run with THF at a flow rate of 1.00 ml min -1 at 35 °C. Molecular weights were obtained using triple detection based on a calibration prepared with narrow dispersity polystyrene standards. Fourier transform infrared (FTIR) spectra were obtained using a Nicolet 5700 ATR-FTIR instrument with a Smart Orbit diamond single bounce crystal. The data reported are the average of 32 scans and were acquired using a resolution of 4 cm -1 at room temperature. Rheological characterisations of the prepared gels were performed using a TA Discovery Hybrid Rheometer (DHR) 2 with parallel plate geometry. Temperature of the experiments were controlled using the TA Peltier Plate smooth geometry. Top geometry was a 20 mm diameter cross-hatched tool for all the measurements. Scanning electron microscopy (SEM) images were take using a FEI Quanta 250 FEG-SEM with Oxford Instrument EDS and GATAN 3view system. All equation fittings were performed using Origin Professional 2020b and the SEM images were analysed using ImageJ.
The solution was stirred before the addition of sec-BuLi (1.4M in cyclohexane, 0.17 ml, 0.24 mmol) to initiate the polymerization which changed the colour of the solution to orange. The reaction mixture was allowed to stir for 3 hours, before being quenched with a small amount of methanol. The colourless mixture was then precipitated into methanol twice and the copolymer was recovered by filtration. The product was dried under high vacuum overnight, yielding a fine white powder (3.92 g, 90%).

SYN2. Synthesis of poly(styrene-co-cyclohexyl-bis(pentafluorophenyl)borane)
Poly(styrene-co-1,3-CHD) (1.00 g, 55.6 μmol) and bis(pentafluorophenyl)borane (0.48 g, 1.11 mmol) were mixed in DCM (5 ml) to form a cloudy solution. Upon stirring the mixture overnight at ambient temperature, the solution became colourless. DCM was removed in vacuo and the copolymer was dissolved in toluene (4 ml) prior to precipitating into hexanes (20 ml). This precipitation was repeated three times prior to filtration. The product was dried under vacuum overnight to yield a white powder (0.96 g, 65%).     Pre-weighed amounts of styrene (2.60 g, 24.9 mmol) and 4-styryl-diphenylphosphine (0.72 g, 2.5 mmol) were dissolved in toluene (9 ml). The solution was stirred before addition of sec-BuLi (1.4M in cyclohexane, 0.10 ml, 0.14 mmol) to initiate the polymerization which changed the colour of the solution to red. The reaction mixture was allowed to stir for 5 hours, before being quenched with a small amount of methanol. The colourless mixture was then precipitated twice into methanol, before filtration and drying under vacuum obtained the desired product as a fine white powder (2.8 g, 84%).

SYN6. Synthesis of cyclohexyl-bis(pentafluorophenyl)borane
The synthesis was modified from a previous literature report. 4

SYN9. Synthesis of SM3
To a toluene (4 ml) solution containing CyB(C6F5)2 (0.21 g, 0.50 mmol) and triphenylphosphine (0.13 g, 0.50 mmol), styrene oxide (60 mg, 0.50 mmol) was added at 0 °C under inert atmosphere. The mixture was stirred at this temperature for 10 mins and then allowed to warm to RT. It was stirred for another 30 mins before being poured into hexane.
The crude product was collected by filtration and purified by re-dissolving in toluene and precipitating into hexane to give white colour powder in 72 % yield (0.28 g).

SYN10. Synthesis of SM4
To a toluene (4 ml) solution containing CyB(C6F5)2 (0.20 g, 0.47 mmol) and triphenylphosphine (0.12 g, 0.47 mmol), cyclohexene oxide (46 mg, 0.47 mmol) was added at 0 °C under inert atmosphere. The mixture was stirred at this temperature for 10 mins and then allowed to warm to RT. It was stirred for another 30 mins before being poured into hexane.
The crude product was collected by filtration and purified by re-dissolving in toluene and precipitating into hexane to give a white foam in 83% yield (0.31 g).

NMR Characterization of Product Connectivity
As X-ray quality crystals could not be obtained from these structures, NMR studies were used to correlate to preexisting structures to aid structural determination. Thanks to the works of Slootweg, Stephan and their co-workers, a library of FLP-mediated ring-opened cyclic ethers is shown in Figure S28. All the displayed zwitterions were specifically chosen as they were characterized using X-ray diffraction in their reported publications. [5][6][7][8] When an in-depth investigation of their NMR spectra was performed, a strong relationship between the coupling constants 1 JC,P and 2 JC,P (of P-RO-B groups, the carbon atoms of interest are labelled as red and green below, respectively) was found. This link is also shown in Table S3, where the carbon atom directly attached to the phosphine center has 5 to 10 times stronger coupling constants than the carbon atoms two bonds away from the P centers. This is also in agreement with the expectation that the single bond coupling would dramatically increase with the loss of phosphine lone pair. 9 Considering the strong literature precedent, the magnitudes of the coupling constants are likely diagnostic for the carbon atom bonded to P.   Figure S29). The P center attacks at the less sterically crowded carbon atom for SM2 and at the more sterically hindered carbon for SM3. As seen in Figure S29, the strongly phosphine Blue is for CH2 and red is for CH and CH3 groups.

Network Formation and Characterisation
General procedure for the preparation of polymeric FLP networks Two separate solutions of poly(Sty-co-PPh3) (0.12 g, 5.2 μmol) and poly(Sty-co-CyB(C6F5)2) (0.10 g, 5.6 μmol) were prepared in toluene (or benzene, each 0.50 ml). The ratio was adjusted to have 1:1 equivalent of LA and LB moieties based on the data provided in  Figure S30: Preparation of the polymeric FLP gels inside vials (above example using PO as the crosslinker).

Polymer network preparation for rheological characterisation
Removing polymer gels from the vials as single pieces proved challenging, and they often crumbled into many pieces. To overcome this issue, gelation reactions were performed in syringes inside a glovebox (Figure S39). A similar method was adopted to the earlier mentioned general procedure. All rheological characterisations were performed using polymer networks swollen in toluene (due to the higher boiling point compared to benzene), to prevent rapid evaporation of the sol fraction. Figure S39: Preparation of the polymeric networks inside syringes.

Rheological characterisation
All tests were performed under air inside a fume-hood, after gels were removed from the inert atmosphere. Oscillatory frequency sweep tests were performed at 0.1% strain for the OX-linked polymer network and at 1% strain for the others. Strain levels were kept low to ensure the materials remained in their linear viscoelastic regions (LVERs) throughout the frequency sweeps. Frequency range was increased from 0.1 rad s -1 to 100 rad s -1 . Amplitude sweep tests were performed at 10 rad s -1 and the amplitude was increased from 0.01% to 400%. Both experiments were performed at 20 °C. Temperature ramp experiments were also performed at 10 rad s -1 while keeping the material in LVER. The heating rate was adjusted to be 0.05 °C s -1 and the samples were heated from 0 °C to 40 °C. Further increases in temperatures were avoided to prevent the evaporation of toluene from the swollen gels. Creep experiments were performed with an applied stress of 200 Pa over 300 seconds. This was followed by a recovery period of 600 seconds. All the experiments were repeated at least three times and the reported data are the averages of these repeats.  Figure S41: G' and G'' values of a covalently crosslinked polymer network is expected to be independent of frequency and any dependency is often attributed to an 'imperfect' network structure. Dependency factor/index (x) was calculated using by fitting the G' data in Figure   S40 and fitting it into the power law relationship G' ~ ω x , where x = 0 for a perfect polymer network.
reacted to form crosslinks. According to the affine network model, the rubbery plateau (G') of a polymer network is directly related to its crosslink density at a constant temperature -G ∝ vkT, where v is the crosslink density. From here, it can be stated that the crosslink density is the highest with N1 which has a G' value of 10500 Pa (Table S4). This also corresponds to the highest number of occupied B/P sites. The number of linked sites then decreases four-fold with N2. This trend also continues with the N3 and N4. Overall, the number of unreacted P/B units increases as the crosslink density decreases.      Figure S43 and Figure S44. Ratio of flow strain (G'=G'') to yield strain were also calculated and it was found that N1 and N2 have a greater tendency to brittle fracturing (~2) than N3 (~2.5) and N4 (~4).  Obtained creep curves were fitted using the Burgers model ( Figures S33 and S34, Equation   S2) to quantitatively analyze the data. 11 The highest elastic and viscous components were calculated for N1 (Table S4), confirming the strong resistance of the macromolecular chains to slipping and stretching in this polymer network. Figure S47: Illustration of the four-parameter Kelvin-Voigt model, also known as the Burgers model. [11][12][13] In this model, compliance, J, is used instead of strain, which is defined as strain divided by applied stress, as a stress-independent parameter. G and η are the elastic and viscous components, respectively.   Figure S50. The same degradation experiment was also repeated with N2. 39 mg N2 xerogel was weighted and transferred into a vial under inert atmosphere at ambient temperature. It was then added into 4 ml of 1 M BCl3 in DCM and the changes in the xerogel were observed, as shown in Figure   S49 and Figure S50.

GPC Characterization
The supernatant during the degradation experiments was collected, vacuumed down and the resultant solid was characterized by both GPC and NMR spectroscopy. GPC traces shown below indicated the polymer gel had degraded into polymeric species that have comparable molecular weights compared to the parent polymers.

Small Molecule Studies
To better understand the degradation process, similar studies were performed with SM1 and SM2 and BCl3 as outlined below.  The results show that a tetracoordinate phosphine center is maintained when considering 31 P NMR spectra, however both the 11 B and 19 F NMR spectra indicate substantial environment changes. The initial boron resonances (1.77 (SM1) and 1.38 ppm (SM2)) disappeared, along with generation of another boron species. One principal peak can be seen at 6 ppm, which is believed to be the tetra-coordinated BCl3, suggesting displacement of the incumbent 1, which is then subject to other degradation processes. The protons arising from the opened ether moieties are also shifted further downfield, reinforcing this theory since a more de-shielded environment could occur as a result of BCl3 displacing 1.
Exposure to Pyridine 0.1 g partially swollen N1 was transferred to a vial under air and submerged in pyridine at ambient temperature. The polymer gel was left undisturbed for 3 weeks. At the end of this period, the network structure was completely broken down and the polymers had dissolved in pyridine.  Figure S54: Degradation of N1 in pyridine.
In addition, the swelling ratio of N2 using pyridine was also measured to investigate the degradation process: 36 mg N2 xerogel was weighted and transferred into a vial under inert atmosphere at ambient temperature. It was then added into 4 ml anhydrous pyridine and the changes in the swelling ratio of the xerogel was recorded.

Changes in Xerogel Mass During Degradation
When the xerogel was solvated by pyridine, an increase in the weight can initially be observed.
After this initial swelling, the mass starts to decrease as a result of degradation. Degradation of the xerogels with pyridine was slow when compared to BCl3. For the first 7200 min (5 days) the swelling ratio of the gels only decreased to 357%. However, when this pyridine pre-soaked gel was shortly exposed to air, it degraded quickly within ~30 mins. As a control experiment, toluene immersed gel with exposure to air was also performed and no degradation was observed, indicating both pyridine and moisture are key factors of degradation.

GPC Characterization
The supernatant of the pyridine degradation experiments were collected and vacuumed down. The residue was characterised again using GPC.    NMR spectra for the small molecule studies performed by mixing SM2 with 1 equivalent pyridine. The NMR spectra obtained 24 hours after mixing the starting materials.
In both cases, small molecule studies showed the formation of C6F5H species in the 19 F NMR spectra, this is commonly attributed to protodeborylation reactions. Given that the degradation of the xerogel N2 sped up upon exposure to moisture, protodeborylation is understood to participate in the degradation of the cyclic ether-linked poly(FLP) gels in pyridine.