Polynorbornene with silicon cluster pendant groups†

Nhien Q. Nguyen^a, Aracely Gonzalez^a, Holly N. Lusk^a, Lan D. Pham^a and Timothy A. Su*^ab
aDepartment of Chemistry, University of California, Riverside, CA, USA. E-mail: timothys@ucr.edu
^bMaterials Science and Engineering Program, University of California, Riverside, CA, USA

First published on 10th July 2025

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Introduction

Electrical insulators play key roles in modern electronics where they are used as dielectrics in capacitors and field-effect transistors, high-voltage insulators, and protective coatings. Their ability to build capacitive charge enables their use in energy harvesting, energy storage, and electronic devices.^1–3 As the demand for ultrathin dielectrics increases,^4–6 it is becoming increasingly critical to find new forms of insulators that are even more resistive than what is presently used.

Multicyclic oligosilane clusters, particularly the permethylated bicyclo[2.2.2]octasilane (Si[2.2.2]) motif,^7–9 are highly promising molecular materials in this regard. In 2018, the charge transport properties of Si[2.2.2] clusters with gold-binding methylthiomethyl endgroups (Fig. 1a) were interrogated in single-molecule junctions with the scanning tunneling microscopy break-junction (STM-BJ) technique.¹⁰ This study revealed that the cis-like dihedrals of the constrained oligosilane bridges invert the symmetry and energetic ordering of the frontier molecular orbitals (MOs) that are critical for charge transport. These inversions caused strong destructive σ-quantum interference (σ-DQI) to occur between the transmitting MO channels, leading to molecular junctions that are even more electrically resistive than vacuum gaps of the same length. Solomon and coworkers invoked interatomic transmission pathway calculations¹¹ to show that significant ring current reversal effects (Fig. 1a) arise from σ-DQI. Notably, Si[2.2.2] clusters were found to be far more resistive on a per-length basis than the canonical alkane and siloxane insulators that are molecular analogs of bulk scale polymeric insulators like polypropylene and silicon dioxide, and more resistive than π-conjugated DQI insulators.^12–14 These studies pointed to Si[2.2.2] clusters as potential next-generation insulators where ultrahigh resistance may be achieved through destructive σ-quantum interference effects.

Towards realizing this vision, it becomes necessary to consider how to incorporate Si[2.2.2] structures into soft materials, as molecular clusters lack the flexibility, processability, and film robustness of polymers. Indeed, cluster-polymer hybrid materials have been devised for metallic,^15–18 metal chalcogenide,¹⁹ metal-oxo,^20–22 silsesquioxane,^23–26 borane,²⁷ carborane,^28–30 phosphorous-nitrogen cages,^31–35 and other^36–38 clusters to achieve properties and functionalities that are often greater than the sum of their parts. These studies motivated us to explore whether we could similarly access Si[2.2.2] cluster-polymer hybrids (Fig. 1b). Polysilanes^39,40 and polymers with cyclosilane subunits^41–45 are known. Hybrid polynorbornenes with silane^46–53 and siloxane subunits^{25,46,54–57} are also known. But there are no examples to our knowledge of polymeric structures with multicyclic silicon cluster building blocks.

Here we enact ring-opening metathesis polymerization (ROMP) on a norbornene-terminated Si[2.2.2] monomer to access the first polymers with multicyclic silicon cluster pendant groups. Nuclear magnetic resonance (NMR) studies reveal the oligosilane clusters remain intact under ROMP conditions. UV-vis absorbance studies show that the polymer retains the optical absorbance features intrinsic to the free cluster. Meanwhile, thermogravimetric analysis studies show that Si[2.2.2] clusters become significantly more robust to thermal degradation when affixed to polynorbornene backbones compared to the free cluster molecule, a result that suggests polymer attachment\is a general strategy to increase the thermal stability of silicon clusters. The synthesis and characterization of silicon cluster-polymer hybrid materials reported herein paves a path to exploring their use as dielectric materials.

Experimental

Materials and characterization methods

Unless otherwise specified, all chemicals were used as purchased without further purification. Solvents used for recrystallization, column chromatography and polymer workup were reagent grade and used as received. Reaction solvents toluene, tetrahydrofuran (THF), dichloromethane (DCM) were dried and degassed on a J. C. Meyer solvent dispensing system following the manufacturer's recommendations for solvent preparation and dispensation.

Synthesis of 3

An oven-dried 250 mL Schlenk flask with a stir bar was charged with 1 (3.92 g, 7.11 mmol, 1.0 equiv.), potassium tert-butoxide (0.79 g, 7.11 mmol, 1.0 equiv.), 18-crown-6 (1.88 g, 7.11 mmol, 1.0 equiv.) and dissolved in dry toluene (60 mL). The reaction was stirred at room temperature overnight under nitrogen. The reaction mixture was cooled to −78 °C with a dry ice-acetone bath, upon which 1-bromo-4-(t-butyldimethylsilyloxy)butane (1.90 g, 7.11 mmol, 1.0 equiv.) was added dropwise into the mixture. After addition, the dry ice bath was removed, and the reaction was stirred at room temperature for 2 h. The solution changed from orange to colorless as it slowly warmed to room temperature. The reaction mixture was quenched with deionized water (15 mL). The aqueous layer was extracted with anhydrous diethyl ether (3 × 15 mL) and the combined organic layers were brine-washed, dried over sodium sulfate, then concentrated in vacuo yielding the crude material as an off-white semi solid (4.87 g). The crude material was used without further purification, as higher yields were obtained over a two-step sequence this way.

Synthesis of 4

Synthesis of 4 was modified from Khan et al.:⁵⁸ an oven-dried 100 mL RBF with a stir bar was charged with 3 (1.5 g, 2.25 mmol, 1.0 equiv.) and dissolved in dry THF (10 mL). Acetyl chloride (0.801 mL, 11.27 mmol, 5.0 equiv.) and methanol (0.456 mL, 11.27 mmol, 5.0 equiv.) were added at 0 °C over an ice bath. The ice bath was removed after addition of reagents, and the reaction was stirred at room temperature under nitrogen for 1 hour. The reaction mixture was quenched with deionized water (5 mL). The aqueous layer was extracted with anhydrous diethyl ether (3 × 15 mL) and the combined organic layers were brine-washed, dried over sodium sulfate, then concentrated in vacuo. The product was purified via silica gel chromatography by using 10% ethyl acetate in hexanes as eluent to yield 4 as a white solid (0.48 g, 40% yield).

Synthesis of 5

An oven-dried 100 mL Schlenk flask with a stir bar was charged with exo-5-norbornene carboxylic acid (0.17 g, 1.20 mmol, 1.0 equiv.), N,N′-dicyclohexylcarbodiimide (DCC, 0.38 g, 1.80 mmol, 1.5 equiv.), 4-dimethylaminopyridine (4-DMAP, 0.015 g, 0.12 mmol, 0.1 equiv.), 4 (0.73 g, 1.32 mmol, 1.1 equiv.) and dissolved in dry DCM (15 mL). The byproduct formed was filtered over a fritted funnel after the reaction completed. Aqueous workup was carried out with NaHCO₃ (10 mL). The aqueous layer was extracted with DCM (3 × 15 mL) and the combined organic layers were brine-washed, dried over magnesium sulfate, then concentrated in vacuo. The product was purified via silica gel chromatography by using 30% DCM in hexanes as eluent to yield 5 as a white solid (0.63 g, 78% yield).

Polymerization of 5 to PNB-Si[2.2.2]

Each polymerization was carried out with the same general protocol. Synthesis of entry 3 (M:I = 2000:1) from Table 1 is provided here as an example. 5 (100 mg, 0.149 mmol, 2000 equiv.) was weighed out into a 20 mL vial equipped with a stir bar, then transferred into the glovebox and diluted with 4 mL DCM. A stock solution of Grubbs-2 catalyst (5.0 mg (weighed out with an ultramicrobalance) in 0.625 mL DCM) was also prepared inside the glovebox. 7.90 µL of this stock solution (0.0745 µmol, 1.0 equiv.) was added to a solution of 5 at room temperature and allowed to stir for 1 hour. 1.0 mL of ethyl vinyl ether was used to quench the polymerization. Excess ethyl vinyl ether was removed in vacuo. The crude polymer was dissolved in 2 mL of anhydrous DCM and added dropwise into 40 mL of stirring methanol, forming a white precipitate. The resulting polymer was isolated from the filtrate filtration with a 0.45 µm membrane filter, then dried in a vacuum oven at 75 °C to yield the polymer (0.0821 g, 82% yield). The ¹H, ¹³C, and ²⁹Si-DEPT NMR spectra are provided in Fig. 2 as well as the ESI.†

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Polymerization of exo-n-butyl 5-norbornene-2-carboxylate to PNB-C

exo-n-Butyl 5-norbornene-2-carboxylate was synthesized according to Gumbley et al.⁵⁹ The polymer was synthesized with a 500:1 M:I ratio. exo-n-Butyl 5-norbornene-2-carboxylate (165 mg, 0.849 mmol, 500 equiv.) was weighed out into a 20 mL vial equipped with a stir bar, then transferred into the glovebox and diluted with 4 mL DCM. A stock solution of Grubbs-2 catalyst (5.0 mg in 0.625 mL DCM, weighed out with a microbalance) was also prepared inside the glovebox. 0.180 mL of this stock solution (1.70 µmol, 1.0 equiv.) was added to a solution of 7 at room temperature and allowed to stir for 1 hour. 1.0 mL of ethyl vinyl ether was used to quench the polymerization. Excess ethyl vinyl ether was removed in vacuo. The crude polymer was dissolved in 2 mL of anhydrous DCM and added dropwise into 40 mL of stirring methanol, forming floating white precipitate. The resulting polymer isolated from the filtrate filtration with a 0.45 µm membrane filter, then dried in a vacuum over at 75 °C to yield the polymer (0.089 g, 90% yield). The ¹H and ¹³C NMR spectra matched the previous report in Gumbley et al.⁵⁹

Results and discussion

Several approaches exist for the synthesis of polysilanes,^39,42 but many bear reactivity challenges that render them incompatible with cyclic and multicyclic oligosilane monomers. Under Wurtz coupling conditions, alkali metal reduction of permethylated cyclooligosilanes may initiate ring-contraction and redistribution processes.⁶⁰ Anionic polymerization conditions are unsuitable, as even weak alkoxide anions can attack and cleave Si–Si bonds or ring-open cyclosilanes.^8,61,62 Promisingly, Klausen and Kawakami have shown Si–Si σ-bonds remain intact under metathesis polymerization conditions.^46,63,64 This prior work suggests that ROMP could be a viable approach for polymers containing silicon cluster building blocks, which inspired us to target polynorbornene backbones with Si[2.2.2] cluster pendants.

Saturated oligosilane clusters are sterically bulky; we thus designed our monomer so an alkane tether separates the large cluster from the reactive norbornene site. We first prepared the known bis-trimethylsilyl-terminated Si[2.2.2] cluster 1 on multigram scale over four steps from commercial reagents, from which we could generate the nucleophilic cluster silanide 2 via trimethylsilyl cleavage via potassium tert-butoxide (Scheme 1).⁸ As the compatibility of oligosilyl anions with organic functional groups is not well-charted relative to carbanions, we first screened approaches for installing alkyl-linked norbornenes onto silanide 2. As we show in Scheme S1,† cluster silanide nucleophiles are incompatible with norbornene double bonds as well as organic azides. Scheme 1 shows our most fruitful approach, where we access the novel norbornene monomer 5 with a bicyclo[2.2.2]octasilane pendant group in three steps from 1 (see ESI† for more details). We first generate 2 from the potassium tert-butoxide-mediated cleavage of a terminal trimethylsilyl group in 1. We then add 1-bromo-4-(t-butyldimethylsilyloxy)butane in the same pot to access silyl ether 3. We remove the tert-butyldimethylsilyl protecting group via acetyl chloride to give alcohol 4.⁵⁸ Steglich esterification of exo-5-norbornenecarboxylic acid with 4 furnishes monomer 5 in a 31% overall yield from 1.

Next, we used Grubbs G2 catalyst for the ring-opening metathesis polymerization of 5 in dichloromethane to give PNB-Si[2.2.2]. Conversion was determined to be complete within an hour, after which the reaction was quenched with ethyl vinyl ether and the polymer precipitated in methanol. ¹H NMR analysis shows characteristic signal broadening in the polymer compared to the monomer, accompanied by disappearance of the endocyclic ethenyl proton resonances at ∼6.2 ppm (Fig. 2a) and emergence of the exocyclic ethenyl proton resonances between 5.1–5.5 ppm. The inset of Fig. 2a supports that the Si[2.2.2] cage remains intact with this polymerization approach, as we find the same 2:2:1 integration ratio of cluster sila-methyl proton resonances in both the monomer and polymer ¹H NMR spectra. Fig. 2b plots the ²⁹Si-DEPT NMR spectra of 5 and PNB-Si[2.2.2] and shows there is essentially no change in ²⁹Si peak position between monomer and polymer, further supporting that the Si cluster remains intact after ROMP. Table 1 depicts how the monomer:initiator ratio may be controlled to tune polymer size. Larger monomer equivalences allow us to obtain polymers of increasing molecular weight (M_n = 902–1289 kDa) while maintaining narrow polydispersity indices (PDI = 1.54–1.68). While the percent yields are roughly the same for entries 1 and 3 (82–86% yield), it is slightly lower for entry 2 (64% yield), which we speculate arises from experimental variance in the synthesis and workup steps.

With PNB-Si[2.2.2] in hand, we sought to address two questions: (1) How do Si cluster pendants impact the physical properties of polynorbornene backbones? (2) How does the behavior of Si[2.2.2] clusters deviate between its polymer-bound and free molecule forms?

To address these questions, we evaluated the structural, thermal, and optical properties of PNB-Si[2.2.2] against those of free molecule Si[2.2.2] clusters 1 and 5, as well as a polynorborene control polymer PNB-C (Fig. 3)⁵⁹ with n-butyl ester sidechains that is structurally similar to PNB-Si[2.2.2] but lacks the Si[2.2.2] pendant. We synthesized PNB-C using a similar ROMP approach as for PNB-Si[2.2.2] (Table S1†).

First, we find that inclusion of the rigid oligosilane cluster along the polynorbornene backbone solidifies the polymer. Whereas PNB-C is a colorless gel, PNB-Si[2.2.2] is a white solid. We characterize PNB-Si[2.2.2] powders as structurally amorphous based on its powder X-ray diffraction pattern (Fig. 4). Comparison against a polycrystalline powder of 1 shows that the sharp peaks that occur between 2θ = 8°–14° are lost in the PXRD spectrum of amorphous PNB-Si[2.2.2] that shows broad, undefined peaks. This difference suggests it is more challenging for the pendant Si[2.2.2] clusters to access well-defined long-range ordering interactions when bound to the polynorbornene backbone compared to the free cluster molecule. Next, we explored the thermal stability of PNB-Si[2.2.2] compared to PNB-C via thermogravimetric analysis (TGA). The TGA curve in Fig. 5 shows the onset of thermal decomposition (T_d) in PNB-Si[2.2.2] occurs at 340 °C with 5% weight loss, which is 44 °C lower than that of PNB-C (T_d = 384 °C). The reduced thermal stability of PNB-Si[2.2.2] compared to PNB-C suggests the bicyclo[2.2.2]octasilane clusters are the first substructures to degrade within the polymer. This T_d value is close to those of polysilanes with main chain Si–Si σ-bonds where values between 210 °C to 320 °C are commonly reported.^39,44,65,66 Notably, Fig. 5 shows that monomer 5 (T_d = 260 °C) decomposes 80 °C lower in temperature than PNB-Si[2.2.2], which indicates that the thermal stability of Si[2.2.2] increases significantly when it is bound to the organic polymer backbone rather than in its free form. We conducted DSC measurements from −30 °C to 320 °C under nitrogen and found a weak glass transition (T_g) feature for PNB-Si[2.2.2] at 116 °C (Fig. S1 and S2†), which is close in value to previous reports of polynorborenes with pendant disilane (T_g ∼ 120 °C)⁴⁶ and trimethylsilane (T_g ∼ 113 °C)⁶⁷ groups.

Finally, solution-state UV-vis spectra of monomer 5, PNB-Si[2.2.2], and PNB-C in cyclohexane at room temperature show that both 5 and PNB-Si[2.2.2] exhibit a strong absorbance feature at 233 nm (Fig. 6), which is consistent with the energies of σ-π*-like optical transitions previously reported for Si[2.2.2] clusters.^68,69 The similar λ_max position between 5 and PNB-Si[2.2.2] indicates the Si clusters remain electronically isolated as pendants along the polynorbornene backbone and remains a wide band gap polymer (Fig. 6).

Conclusion

In this work, we outline a general approach to attach norbornene termini to saturated oligosilane clusters. The norbornene functionalization approach revealed for Si[2.2.2] in Scheme 1 should similarly apply to the many multicyclic oligosilane clusters with trimethylsilyl endgroups that exist and can be readily converted to cluster silanides similar to 2.⁷⁰ We thus anticipate that a much broader scope of polynorbornenes with diverse silicon cluster pendants can be realized with the ROMP approach we show here, some of which may be promising future candidates as Si-rich preceramic polymers for hypersonic applications.⁷¹

Additionally, our TGA studies show that covalently tethering silicon clusters to organic backbones significantly increases its T_d compared to the free cluster, suggesting that polymer attachment may be a generalizable strategy for improving the thermal stability of functional silicon cluster materials. The improved thermal stability of PNB-Si[2.2.2] may find relevance for high temperature (>300 °C) dielectric applications such as capacitive energy storage in geothermal, electric aircraft, and space exploration contexts.^72–74 Finally, our UV-vis studies reveal that the characteristic optical features from the discrete cluster are retained in the cluster-polymer hybrid. It is the same set of molecular orbitals that give rise to the optical transitions in Si[2.2.2] that also underlie the strong σ-DQI observed when measuring charge transport across Si[2.2.2]-based single-molecule junctions. This commonality suggests the electrical insulation properties we observe in single molecules should also extend to the cluster-polymer hybrid, and may give rise to new forms of polymeric insulators that operate from destructive quantum interference effects. With PNB-Si[2.2.2] in hand, we are now eager to explore the extent to which destructive σ-quantum interference, a concept first discovered in molecular scale electronics, extends to the design of dielectric materials.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work was supported by UC Riverside startup funds. Mass spectrometry characterization was supported by the Air Force Office of Scientific Research under award number FA9550-23-1-0192. A. G. was supported by the UCR Chancellor's Research Fellowship. We thank Mr Ricardo De Luna (UC San Diego) for gel permeation chromatography characterization, Mr Fernando Alfaro and Mr Imex Aguirre Cardenas (UC Riverside) for small molecule mass spectrometry characterization, and Prof. William Neary (UC Riverside) for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5py00465a

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