Side‐Chain Control of Topochemical Polymer Single Crystals with Tunable Elastic Modulus

Abstract Topochemical polymerizations hold the promise of producing high molecular weight and stereoregular single crystalline polymers by first aligning monomers before polymerization. However, monomer modifications often alter the crystal packing and result in non‐reactive polymorphs. Here, we report a systematic study on the side chain functionalization of the bis(indandione) derivative system that can be polymerized under visible light. Precisely engineered side chains help organize the monomer crystals in a one‐dimensional fashion to maintain the topochemical reactivity. By optimizing the side chain length and end group of monomers, the elastic modulus of the resulting polymer single crystals can also be greatly enhanced. Lastly, using ultrasonication, insoluble polymer single crystals can be processed into free‐standing and robust polymer thin films. This work provides new insights on the molecular design of topochemical reactions and paves the way for future applications of this fascinating family of materials.


Materials Synthesis
1,1'-dioxo-1H,1'H-[2,2'-biindene]-3,3'-diyl dipentanoate (BIT-5): [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Valeroyl chloride (0.81 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT-5 was filtered (1.18 g, 75% yield). 1  [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Hexanoyl chloride (0.95 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT-6 was filtered (1.24 g, 75% yield). 1  [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Heptanoyl chloride (1.05 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT-7 was filtered (1.32 g, 75% yield [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Octanoyl chloride (1.29 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT-8 was filtered (1.38 g, 75% yield). 1 H NMR (400 MHz, CDCl 3 ) δ 7.43 (dd, J = 7.0, 0.9 Hz, 1H), 7.37 (td, J = 7.6, 1. [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Nonanoyl chloride (1.23 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT-9 was filtered (1.46 g,
[2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. Hept-6-ynoyl chloride (1.08 g, 7.5 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was

5-methylhexanoyl chloride:
In a two-neck round-bottom flask, thionyl chloride (1.20 mL, 16.30 mmol) was added under argon atmosphere. 5-methylhexanoic acid (2.02g, 15.54 mmol) was added dropwise to the flask. The mixture was heated under reflux for 2.5h. Then the mixture was cooled to room temperature and extra thionyl chloride was removed under reduced pressure to give a colorless liquid (2.26g, 98% yield).
[2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. 5,5dimethylhexanoyl chloride (1.38 g, 8.5 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was dried via rotary evaporation and recrystallized by adding 5mL of cold methanol into the flask. Orange powder of BIT- [2,2′-bi-1H-indene]-3,3′dihydroxy-1,1′-dione (BIT-OH 2 , 1.00 g, 3.4 mmol) was added to a dry two-neck round-bottom flask, followed by addition of 40 mL anhydrous chloroform under argon atmosphere. The mixture was cooled to -15 °C in salt ice bath and 0.96g (7.6mmol) N,N-Diisopropylethylamine was added. BIT-OH 2 was dissolved, and a purple solution was formed. 5-Bromopentanoyl chloride (0.91 mL, 6.8 mmol) was added dropwise in ten minutes to the solution at -15 °C. The solution gradually turned orange during the 2 hours reaction at -15 °C and was then quenched by water. After washed with brine and dried with MgSO4, the crude product was

Single Crystal Preparation
All the BIT single crystals were prepared with a slow-evaporation method: 20mg of monomer powders were put in a 20mL vial and first dissolved in 5 mL DCM or chloroform. Then 10 mL of methanol or ethanol was added to the solution and the solution was mixed to a clear orange color. The vial was put in the fume hood with the cap open to let the solvents slowly evaporate from the vial.
After the vial was kept in the fume hood overnight, the vial is dry and needle-like (1D polymerizable crystals) or plate-like (2D nonpolymerizable crystals) were obtained.

Topochemical Polymerization Process
Monomer single crystals were first put on the weighing paper on the bench. Then the crystals were put under OLYMPUS BX3M-LEDR optical microscope accessory with distance of 1 cm.

DFT Calculations
Density Functional Theory (DFT) calculations were used to characterize the difference in lattice energies. All calculations were performed using the Vienna Ab Initio Simulation Package (VASP, version 5.4.1) [1] implemented with projector augmented wave (PAW) methods. [2] The generalized gradient approximation (GGA) by Perdew, Burke, and Ernzerhof (PBE) [3] was used as the exchangecorrelation functional with the effective-pairwise dispersion correction of Tkatchenko and Scheffler [4] applied. The plane-wave cut-off for the energy were set as 1000 eV. Initial crystal structures obtained from the experiments were relaxed with the convergence criteria of 10 -5 eV for the energy and 5×10 -3 eV•Å -1 for the gradient. The k-point mesh utilized was up to 4×3×3 in the gamma centered Monkhorst-Pack Grid generated with vaspkit package. [5] The difference in the lattice energies of crystals with the same molecular compositions were calculated by ∆ = , / − , / where , and are the energy and number of molecules in the unit cell .

Polymer Thin Film Processing
In a 50 mL beaker, PBIT polymer crystals were suspended in 20 mL chloroform. The mixture was cooled in salt ice bath and the ultrasonic processor probe was immersed into the mixture. The ultrasonic processor parameters were set as amplitude 15, 4 sec pulse and 1 sec rest. After 30-60 minutes sonication, the suspension was filtered with a vacuum filtration apparatus and a nylon membrane filter to provide free-standing polymer thin films. Each thin film is about 50 mg and 100 µm thick. Finally, PBIT polymer thin films were placed between two pieces of aluminum foils and pressed under around 5 MPa pressure at 50 ⁰C for 3 minutes. After removing the aluminum foils, smooth and robust thin films were obtained.

Polymer sample preparation for tensile stress-strain tests
Freshly prepared PBIT thin films were cut into roughly 25 mm * 5 mm strips. Then both ends of the polymer strip was glued on pieces of polyethylene terephthalate (PET) sheet with epoxy glue. The initial length for (engineering) strain calculation was measured from the distance between edges of two PET sheets. The cross-sectional area was calculated by the width of the strip multiplied by the thickness of the strip.