Tröger’s Base Network Polymers of Intrinsic Microporosity (TB-PIMs) with Tunable Pore Size for Heterogeneous Catalysis

Heterogeneous catalysis plays a pivotal role in the preparation of value-added chemicals, and it works more efficiently when combined with porous materials and supports. Because of that, a detailed assessment of porosity and pore size is essential when evaluating the performance of new heterogeneous catalysts. Herein, we report the synthesis and characterization of a series of novel microporous Tröger’s base polymers and copolymers (TB-PIMs) with tunable pore size. The basicity of TB sites is exploited to catalyze the Knoevenagel condensation of benzaldehydes and malononitrile, and the dimension of the pores can be systematically adjusted with an appropriate selection of monomers and comonomers. The tunability of the pore size provides the enhanced accessibility of the catalytic sites for substrates, which leads to a great improvement in conversions, with the best results achieving completion in only 20 min. In addition, it enables the use of large benzaldehydes, which is prevented when using polymers with very small pores, typical of conventional PIMs. The catalytic reaction is more efficient than the corresponding homogeneous counterpart and is ultimately optimized with the addition of a small amount of a solvent, which facilitates the swelling of the pores and leads to a further improvement in the performance and to a better carbon economy. Molecular dynamic modeling of the copolymers’ structures is employed to describe the swellability of flexible chains, helping the understanding of the improved performance and demonstrating the great potential of these novel materials.


General methods and equipment
Commercially available reagents and gases were used without further purification. All reactions using air/moisture sensitive reagents were performed in oven-dried or flame-dried apparatus, under a nitrogen atmosphere. TLC analysis refers to analytical thin layer chromatography, using aluminium-backed plates coated with Merck Kieselgel 60 GF254. Product spots were viewed either by the quenching of UV fluorescence, or by staining with a solution of Cerium Sulfate in aqueous H2SO4. Melting points were recorded using a Cole-Parmer Stuart™ Digital Melting Point Apparatus and are uncorrected. Low-temperature N2 (77 K) and CO2 (273 K) adsorption/desorption measurements of PIM powders were made using a Quantachrome Nova-e. Samples were degassed for 800 min at 80 °C under high vacuum prior to analysis. The data were analysed with the software provided with the instrument. NLDFT and H-K analysis were performed to calculate the pore size distribution and volume, considering a carbon equilibrium transition kernel at 273 K based on a slit-pore model; the kernel is based on a common, one centre, Lennard-Jones model.
TGAs were performed using the device Thermal Analysis SDT Q600 at a heating rate of 10 °C/min from 30 to 1000 °C. 1 H NMR spectra were recorded in the solvent stated using an Avance Bruker DPX 500 (500 MHz) instruments, with 13 C NMR spectra recorded at 125 MHz.Solid-state 13 C NMR spectra were recorded using a Bruker Avance III spectrometer equipped with a wide-bore 9.4 T magnet (Larmor frequencies of 100.9 MHz for 13 C). Samples were packed into standard zirconia rotors with 4 mm outer diameter and rotated at a magic angle spinning (MAS) rate of 12.5 kHz. Spectra were recorded with cross polarisation (CP) from 1H using a contact pulse (ramped for 1H) of 1.5 ms. High-power (ν1≈ 100 kHz) TPPM-15 decoupling of 1H was applied during acquisition to improve resolution. Signal averaging was carried out for 6144 transients with a recycle interval of 2 s. Chemical shifts are reported in ppm relative to (CH3)4Si (TMS) using the CH3 signal of L-alanine (δ = 20.5 ppm) as a secondary solid reference. SEM images were recorded with a Hitachi S-4800 field emission (~1 nm resolution).

Synthesis of monomers 1,3,5-Tris(aminophenyl)benzene 1
1,3.5-tribromobenzene (2.0 g, 1 equiv., 6.4 mmol) and 4-aminophenylboronic pinacolate (4.48 g, 3.2 equiv., 20.5 mmol) were dissolved in a mixture of THF: toluene (100 mL, 50:50 v/v), followed by addition of NaOH (3.84 g, 15 equiv., 318 mmol). The resulting mixture was degassed for 15 min by a flow of nitrogen, and Pd(PPh3)2Cl2 (0.35 g, 0.08 equiv., 0.5 mmol) was added. The resulting mixture was degassed again for 10 min by a flow of nitrogen and was heated to 90 ºC for 20 h under nitrogen atmosphere. The reaction mixture was cooled to room temperature and the solvents were removed under reduced pressure. The remaining crude product was solubilized in hot ethyl acetate and the mixture was hot filtrated over celite, which was washed with hot ethyl acetate many times. The solvent was removed under reduced pressure and the obtained solid was washed with hot methanol and filtrated to yield a pale-yellow powder (1.3 g, 58% yield

Synthesis of Polymers and co-polymers General procedure
The polymers were prepared following literature procedure with some modifications. 6 The appropriated monomer (1 equiv) was reacted with dimethoxymethane (8 equiv) in DCM (approximately 10 mL), followed by dropwise addition of trifluoroacetic acid, TFA (37 equiv). The reaction was left to stir at room temperature for approximately 16 h and was crashed out in ammonia/ice and stirred overnight. The product was then filtered, washed with plenty of water, and refluxed in acetone, THF, DCM and methanol, before being dried in a vacuum oven at 85 0 C for 20 h. The co-polymers were prepared in the same way, using the combination of two monomers: A (1 equiv) and B (1.5 equiv) (Table S1).

General catalysis test:
Malononitrile: benzaldehyde (3:1) solvent free A glass vial was charged with a mixture of benzaldehyde (15 mmol) and malononitrile (5 mmol), then the catalyst was added (1 mol%), and the reaction mixture was stirred at room temperature for 2 h. Fractions (10 microliters) were removed each 10 min and analysed by 1 H NMR.
Malononitrile: benzaldehyde (3:1) solvent A glass vial was charged with a mixture of benzaldehyde (15 mmol), malononitrile (5 mmol) and 2 mL of solvent (ethanol or DCM). Then, the catalyst (1 mol%) was added and the reaction mixture was stirred at room temperature for 2 h. Fractions (10 microliters) were removed each 10 min and analysed by 1 H NMR.
Malononitrile: benzaldehyde (1:1) solvent A glass vial was charged with a mixture of benzaldehyde (5 mmol), malononitrile (5 mmol) and 2 mL of solvent (ethanol or DCM). Then, the catalyst (1mol%) was added and the reaction mixture was stirred in at room temperature for 2 h. Fractions (10 microliters) were removed each 10 min and analysed by 1 H NMR. Table ESI 1. Combination of the different monomers that form TB-polymers and co-polymers. a calculated from CO2 adsorption at 273K b the first decomposition temperature refers to the retro Diels-Alder step that we used to assess the correct stoichiometry of the two co-monomers (see also Figure ESI

Recyclability test
A glass vial was charged with a mixture of 4-tert-butylbenzaldehyde (25 mmol), malononitrile (25 mmol) and 10 mL of ethanol. Then, TAPBext(A1)-PIM (1mol%) was added and the reaction mixture was stirred at room temperature for 3 h. After this period, the reaction was analysed by NMR and the catalyst was recovered from the reaction by simple filtration, refluxed in different solvents (acetone, DCM and methanol), dried in a vacuum oven (at 100 0 C for 20 h) and reused. This procedure was repeated for more six consecutive cycles.
The tBu-derivative was separated and weighed, to be sure that the yield matched with the conversion seen by 1 H NMR. To ensure that the structure of the polymer has not changed during the recycling tests, we occasionally repeated the physical characterisation (BET and FT-IR), finding that the physical properties were not affected. widely utilised to explore PIMs) 9-11 with forcefield assigned charges. Both Electrostatic terms and van der Waals terms were calculated with atombased summation method, cubic spline truncation method, 12.5 Å cutoff distance, 1 Å spline width and 0.5 Å Buffer width. Dynamics parameters were 1 fs time step, Nose Thermostat 12 (Q ratio 0.01) and Berendsen barostat 13 (0.1 ps decay constant) for NPT ensembles.

Polymeric box creation
Since the focus of the modelling is not the polymerisation reaction itself, the process of polymer creation is hasten starting from a semi-reacted To enhance the formation of a highly networked polymer, some small seeds structures are created to grow the polymer from. The seed is formed by a central trifunctional monomer, surrounded by a first "shell" of bifunctional monomers and a second "shell" of trifunctional monomers. In the seed core, each functional group is reacted, part from the six outermost groups of the terminal monomers.
Using the seed as the starting point of chain growth, an iterative process is run. At each step the polymer chain (i.e. the seed for the first iteration or the previous step output for successive iterations) is arranged in an amorphous box together with a defined number of free monomers at a density of 1 g cm -3 , the details are illustrated in Table ESI 6. The Amorphous Cell algorithm locate all the polymeric chains in the box, growing them one segment at the time, by Monte Carlo moves (the Monte Carlo algorithm probability is calculated with respect to Flory's RIS theory). 14 A series of short NVT dynamics (with fixed amount of substance (N), volume (V) and temperature (T)) are performed and after each dynamic the mutual spatial arrangement of the free monomers and the chain's free functional groups are investigated. If a free monomer and a chain reactive group get sufficiently close to each other, and with a suitable orientation, a Tröger's base link between the two is created. If no bond is created after 4 iterations the system is heated up to 500 K for 10 ps and then cooled at 300 K for 0.1 ps. During the growth process, the free monomers are free to link to the existing chain with one or more of their reactive sites, therefore it can happen that two outermost reactive groups of the chain get linked by a bridging free monomer. At the end of each step, all monomers not linked to the main polymer chain are deleted. In the third step of the process, the amorphous box is filled with both bi-and tri-functional monomers to promote the formation of side chains, while in the last step 20 bivalent monomers are inserted.

Steps outputs
Step 0 Step 1 Step 2 Step 3 Step 4 Core →  Table ESI 7. The slight changes in the structures reflect the good randomness of the polymer creation process.

Amorphous Cell
The chain is placed in an box with some free monomers Crosslinking code A series of short NVT are performed until some free monomer get linked to the chain

Chain extration
The grown chain is extracted to be used as input for next step

Hydrated boxes creation
In order to study how the polymer structure changes with respect to its degree of swelling, considering both solvent mixtures, polymeric box at different density are packed with the same solution at density 1 g cm -3 .
The solvent mixture used is Ethanol, 4-tert-butylbenzaldehyde and Malononitrile in the ratio 6.8: 1:1 and 0:3:1, the same used for the experimental tests.
Two hydrated samples box are realised by firstly creating an amorphous box containing only the polymer at a given density, then the void space of the box is isolated tracing an isosurface on the van der Waal radii of the chain atoms. Finally, the void space inside the isosurface is packed with the mixture molecules at the desired ratio and density 1 g cm -3 . This value is chosen because preliminary tests performed on the mixture 6.8:1:1 alone show that its density is slightly below 1 g cm -3 .
Each concentration is equivalent to the various degree of swelling when the solvent is inserted. The higher value is chosen from preliminary test A 300 ps NVT dynamics was performed to each structure at 323 K in order to promote molecules motion before the long equilibration step of 2 ns NPT dynamics at 298 K and 1 bar. At the end of the process, some structural properties of the boxes are evaluated.

Structural characterization
The molecular conformation behaviour of polymer chains is analysed by calculating the radius of gyration, (Rg), which gives a sense of the size of the polymer coil and is defined as: where and . . represent the position vector of the ith atom and the center of mass of the polymer chain, respectively. The radius of gyration is also the quantity that is experimentally accessed.