Dimension- and property-controlled nanoparticles (NPs) have become attractive new classes of heterogeneous catalysts for various chemical reactions, especially tandem reactions, by reducing the numbers of reaction steps and by increasing the reaction efficiencies toward targeted products. Here, we report an AuPd NP system as an active and stable catalyst to catalyze one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde, leading to the controlled polymerization and formation of polybenzoxazole (PBO). The highly pure PBO shows excellent thermal stability up to 600°C and improved chemical and mechanical stability compared with phosphoric acid-contaminated commercial PBO (Zylon, M w = 40 kDa). The reported NP-catalyzed one-pot polymerization can be easily extended to prepare various rigid organic polymers that are important for ballistic fiber, anti-flame, smart-textile, and ionic separation membrane applications.

Using nanoparticles (NPs) to catalyze chemical reactions for the formation of functional polymers with controlled polymerization is an important field of chemical research. In this article, we report an AuPd NP system as an active and stable catalyst to catalyze one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde, leading to the controlled polymerization and the formation of polybenzoxazole (PBO) (M w = 3.6 kDa). The one-pot reaction is AuPd NP size and composition dependent, and 8-nm Au 39 Pd 61 NPs are the best catalyst for the polymerization. The highly pure PBO shows excellent thermal stability up to 600°C and improved chemical and mechanical stability under challenging environmental conditions compared with commercial PBO (Zylon, M w = 40 kDa). The reported NP-catalyzed one-pot reaction to polymerization is not limited to the formation of PBO but can be extended as a general approach to rigid polymers that are important for ballistic fiber, anti-flame, and smart-textile applications.

Controlling the degree of polymerization is an essential step in obtaining polymer materials with desired macroscopic chemical and physical properties for various applications. Conventional syntheses rely on reactions among monomers under reaction conditions whereby chain transfer agents are typically needed to control polymerization degrees. These processes have been applied to produce many different kinds of polymers of commercial importance, such as Kevlar, Dacron, Kodel, Lexan, Lycra, and Zylon. Despite the common uses of these conventional methods for polymer productions, the reactions still have issues in rationally controlling the degrees and purities of polymerizations. Here, we report a new process of synthesizing highly pure polybenzoxazole (PBO) with controlled polymerization via AuPd NP-catalyzed one-pot reaction. PBO, or Zylon for the commercial product, is a subclass of polybenzoazoles. The highly aromatic nature and conjugated structure of alternating benzoxazole and phenyl rings provides the polymer with the highest yarn tensile strength (5.8 GPa), stiffness (270 GPa), and relatively low density (1.5–1.7 g cm) among all commercial synthetic polymers.These characteristics render the polymer with exceptionally superior tensile modulus and strength as well as thermal and mechanical stability, making it a promising new fiber material for applications in body armor, flame retardation, smart electronic textiles, and ionic separation membranes.However, PBOs are conventionally made by condensing diaminobenzene diol and terephthalic acid with polyphosphoric acid as both solvent and catalyst. As a result, they are inevitably contaminated with phosphoric acid (PA) units that can catalyze the hydrolysis of the benzoxazole ring upon its exposure to humid and lighted environments ( Figure S1 ), causing unexpected and fast degradation of the mechanical integrity of the polymer fibers.Our new AuPd NP-catalyzed one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde to PBO is NP size- and composition-dependent, and among the 4-, 6-, 8-, and 10-nm AuPd NPs studied, 8-nm AuPdNPs are the most efficient in catalyzing the reaction to the highest degree of polymerization (molecular weight [M] = 3.6 kDa). Compared with the Zylon (M= 40 kDa), our “lighter” PBO has not only comparable thermal stability (over 600°C) but also much improved chemical and mechanical stability against water- and organic-solvent-induced polymer degradation. Molecular mechanics (MM) and density functional theory (DFT) simulations reveal that both surface strain and surface geometry of the AuPd NPs contribute to the size-dependent polymerization. Our studies show a general approach to NP-controlled catalysis applied to polymerization.

Advances in nanoparticle (NP) synthesis have motivated extensive research into defining the “tuning knobs” that can be used to control the physical and chemical properties of NPs.One of these properties is catalysis. Various NPs, especially metallic NPs, have been explored as efficient catalysts to promote chemical reactions in much milder and greener reaction conditions, which have recently been highlighted extensively in reactions such as water splitting, hydrogen evolution reaction, oxygen reduction reaction, and COreduction.Some of the noble metal NPs based on Pt, Pd, Ru, Au, and Ag are of special interest in catalyzing miscellaneous organic functionalization reactions due to their desired activity and stability.The preparation of monodisperse NPs further allows the NP catalysis to be tuned in a size-, structure-, and morphology-dependent manner, making it possible to optimize NP catalysis for chemical reactions. However, these previous studies on NP catalysis focus nearly exclusively on chemical reactions to small organics and very rarely on multi-component reactions that lead to the formation of functional materials, for example, polymers, under green chemistry reaction conditions.

Results and Discussion

2 for hydrogenation reactions. 37 Yang Y.

Xu H.

Cao D.

Zeng X.C.

Cheng D. Hydrogen production via efficient formic acid decomposition: engineering the surface structure of Pd-based alloy catalysts by design. , 38 Yu W.Y.

Mullen G.M.

Flaherty D.W.

Mullins C.B. Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces. , 39 Muzzio M.

Yu C.

Lin H.H.

Yom T.

Boga D.A.

Xi Z.

Li N.

Yin Z.Y.

Li J.R.

Dunn J.A.

et al. Reductive amination of ethyl levulinate to pyrrolidones over AuPd nanoparticles at ambient hydrogen pressure. , 40 Gu X.

Lu Z.H.

Jiang H.L.

Akita T.

Xu Q. Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic acid for chemical hydrogen storage. 41 Zhu W.

Michalsky R.

Metin O.

Lv H.

Guo S.

Wright C.J.

Sun X.

Peterson A.A.

Sun S. Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO 2 to CO. 2 was reduced in oleylamine (OAm) and oleic acid (OAc) (v/v = 50:1) at 260°C to yield AuPd NPs (see 39 Pd 61 alloy NPs, showing a very small NP size increase after the alloy formation. For simplicity of presentation, these AuPd NPs are denoted as 4-, 6-, 8-, and 10-nm NPs in this paper. The alloy structure of the NPs was characterized by high-resolution TEM (HRTEM) ( Figure 1 Preparation and Characterization of AuPd Alloy NPs Show full caption (A) TEM image of 8-nm Au NPs, scale bar: 50 nm. (B) TEM image of 8.2 ± 0.4-nm Au 39 Pd 61 NPs deposited on (C), scale bar: 50 nm. (C) HAADF-STEM image of the Au 39 Pd 61 /C and elemental mapping of NPs to show Au (red) and Pd (green) distribution within the NPs, scale bar: 10 nm. Recently, AuPd alloy NPs were studied as stable catalysts to dehydrogenate formic acid into Hfor hydrogenation reactions.These AuPd NPs were previously prepared by co-reduction methods, under which it is difficult to control both sizes and compositions of the NPs. We modified the synthesis and developed a unique seed-mediated growth-diffusion method to prepare AuPd NPs with well-controlled sizes and compositions. In our synthesis, 4-, 6-, 8-, and 10-nm Au NPs were first prepared as described.Then, in the presence of Au seeding NPs, a controlled amount of Pd(acac)was reduced in oleylamine (OAm) and oleic acid (OAc) (v/v = 50:1) at 260°C to yield AuPd NPs (see Supplemental Information ). We further deposited these AuPd NPs onto a Ketjen carbon support (C) and annealed the supported NPs, NPs/C, under forming gas at 500°C for 10 min to obtain catalytically active AuPd alloy NPs. Figure 1 shows representative transmission electron microscopy (TEM) images of 8-nm Au seeding NPs ( Figure 1 A) and 8.2 ± 0.4 nm AuPd/C ( Figure 1 B) (TEM images of other Au NPs and AuPd NPs are given in Figures S2 A–S2C and S3 A–S3C, respectively). Starting from 4-, 6-, 8-, and 10-nm Au NPs, we obtained 4.3 ± 0.2, 6.4 ± 0.3, 8.2 ± 0.4, and 10.5 ± 0.2 nm AuPdalloy NPs, showing a very small NP size increase after the alloy formation. For simplicity of presentation, these AuPd NPs are denoted as 4-, 6-, 8-, and 10-nm NPs in this paper. The alloy structure of the NPs was characterized by high-resolution TEM (HRTEM) ( Figure S4 ), high-angle annular dark field (HAADF) scanning TEM (STEM) and elemental mapping ( Figure 1 C), X-ray diffraction analysis, and X-ray photoelectron spectroscopy (XPS) ( Figures S5–S7 and Table S1 ). From these analyses, we conclude that homogeneous alloy AuPd NPs with a face-centered cubic structure are obtained.

42 Fukumaru T.

Saegusa Y.

Fujigaya T.

Nakashima N. Fabrication of poly(p-phenylenebenzobisoxazole) film using a soluble poly(o-alkoxyphenylamide) as the precursor. 39 Pd 61 NPs are the most active catalyst for the reduction/condensation reaction ( Figure 2 Au 39 Pd 61 -Catalyzed Reaction between 1,5-Diisopropoxy-2,4-Dinitrobenzene and Benzaldehyde Show full caption (A) A new synthetic route for producing a bis-imine via the condensation of 1,5-diisopropoxy-2,4-dinitrobenzene and benzaldehyde. (B and C) The yields of bis-imine with different compositions of 8-nm AuPd (B) supported on (C). Reaction conditions: AuPd/C (2.5 mol %), 1,5-diisopropoxy-2,4-dinitrobenzene (1 mmol), benzaldehyde (2.1 mmol), NMP (3 mL), and FA (10 mmol); 80°C, 6 h. (C) The yields of bis-imine with four different sizes of Au 39 Pd 61 /C as tandem catalyst. Reaction conditions: Au 39 Pd 61 /C (2.5 mol %), 1,5-diisopropoxy-2,4-dinitrobenzene (1 mmol), benzaldehyde (2.1 mmol), NMP (3 mL), and FA (10 mmol); 80°C, 6 h. The controls achieved in preparing AuPd NPs allow us to study size- and composition-dependent catalysis for reactions leading to the formation of small subunits of PBO. In this test, we studied formic acid (FA)-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and the subsequent condensation of the 1,5-diisopropoxy-2,4-aminobenzene product with benzaldehyde to form (1E,1′E)-N,N′-(4,6-diisopropoxy-1,3-phenylene)-bis(1-phenylmethanimine) (denoted as bis-imine) in N-methylpyrrolidone (NMP) solvent ( Figure 2 A ). We attached isopropyl groups to the oxy-nitrobenzene structure to ensure that both reactants and products were soluble.The results of the AuPd-catalyzed reactions are summarized in Figures 2 B and 2C, from which we conclude that 8-nm AuPdNPs are the most active catalyst for the reduction/condensation reaction ( Figure 2 A).

39 Pd 61 /C (2.5 mol %) was used to catalyze FA-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and subsequent condensation with terephthalaldehyde in NMP to form poly(p-phenylene-(4,6-diisopropoxy-1,3-phenylene)diethanimine), denoted as pre-PBO, which was further subject to heating treatment at 300°C under a N 2 atmosphere for 6 h to remove isopropyl groups and promote ring closure for the formation of PBO ( 2 SO 4 at a scan rate of 50 mV s−1 in the potential range from 0 V to 1.7 V (versus reversible hydrogen electrode [RHE]) ( Figure 3 A New Synthetic Route for Producing PBO The 8-nm AuPd/C (2.5 mol %) was used to catalyze FA-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and subsequent condensation with terephthalaldehyde in NMP to form poly(p-phenylene-(4,6-diisopropoxy-1,3-phenylene)diethanimine), denoted as pre-PBO, which was further subject to heating treatment at 300°C under a Natmosphere for 6 h to remove isopropyl groups and promote ring closure for the formation of PBO ( Figure 3 ). From TEM ( Figure S8 ) and ICP-AES (inductively coupled plasma atomic emission spectroscopy) elemental analyses, we concluded that the NPs have no composition change before and after polymerization reaction. We further performed cyclic voltammetry (CV) studies of the AuPd catalyst to study NP redox property change, which is sensitive to the NP surface structure. We studied the catalyst CV in 0.5 M HSOat a scan rate of 50 mV sin the potential range from 0 V to 1.7 V (versus reversible hydrogen electrode [RHE]) ( Figure S9 ). We can see that the reduction peaks of the oxidized Au (at 1.12 V) and Pd (at 0.55 V) are nearly identical, indicating that there is no surface structure change of the catalyst during the polymerization process. The reduction peak area can be integrated to obtain the Au/Pd composition information, and we see no obvious surface composition change after the polymerization reaction ( Table S2 ). We also studied XPS of the alloy catalyst ( Figure S10 ). We see no obvious Au/Pd binding energy ( Figure S10 ) and atomic percentage change of the catalyst before and after the reaction ( Table S2 ). Summarizing the Au/Pd composition information we obtained from ICP-AES, CV, and XPS ( Table S2 ), we conclude that our AuPd alloy catalyst is not only active but also stable in the polymerization process.

2 atmosphere showed that the pre-PBO has a weight loss of 25.7%, which agrees well with the calculated weight loss of 27.3% for the pre-PBO/PBO conversion (−1, respectively, which are similar to that of the commercial PBO, Zylon ( 43 Feng D.D.

Wang S.F.

Zhuang Q.X.

Guo P.Y.

Wu P.P.

Han Z.W. Joint theoretical and experimental study of the UV absorption spectra of polybenzoxazoles. 44 DiCesare N.

Belletete M.

Leclerc M.

Durocher G. Intermolecular interactions in conjugated oligothiophenes. 2. Quantum chemical calculations performed on crystalline structures of terthiophene and substituted terthiophenes. w of 2.1, 2.4, 3.6, and 3.0 kDa, respectively, as measured by gel-permeation chromatography (GPC) ( w and the NP catalyst showed no obvious composition and morphology change ( Thermal gravimetric analysis (TGA) under a Natmosphere showed that the pre-PBO has a weight loss of 25.7%, which agrees well with the calculated weight loss of 27.3% for the pre-PBO/PBO conversion ( Figure S11 ). Infrared spectra of the newly prepared PBO show characteristic benzoxazole C=N, C–N, and C–O vibration peaks at approximately 1,620, 1,360, and 1,054 cm, respectively, which are similar to that of the commercial PBO, Zylon ( Figure S12 ). UV-visible (UV-vis) absorption spectra taken in methanesulfonic acid solutions of PBO and Zylon show the nearly identical absorption and photoluminescence (PL) peaks ( Figure S13 ), indicating the highly aromatic nature and conjugated structure of alternating benzoxazole and phenyl rings within PBO and Zylon.The two split absorption peaks of 404 nm and 428 nm for PBO are induced by intermolecular interactions, consistent with that of the Zylon sample.A more interesting aspect of this reaction is that the degree of polymerization is dependent on the size of the AuPd NPs. Among 4-, 6-, 8-, and 10-nm AuPd NPs tested, pre-PBO was formed with an Mof 2.1, 2.4, 3.6, and 3.0 kDa, respectively, as measured by gel-permeation chromatography (GPC) ( Figure S14 A). The 8-nm NPs induced the highest degree of polymerization in the one-pot reaction process. The catalyst was also reusable: our preliminary tests show that after three reaction runs, the polymer prepared from each run had similar Mand the NP catalyst showed no obvious composition and morphology change ( Figures S8 and S14 B). ICP-AES measurements show that the PBO synthesized using our method is metal- and PA-free. As a comparison, Zylon contains 0.5% (by weight) of P, which means that there is one PA group for every ∼25 repeating PBO units.

34 Kanbargi N.

Hu W.G.

Lesser A.J. Degradation mechanism of poly(p-phenylene-2,6-benzobisoxazole) fibers by P-31 solid-state NMR. , 35 Froimowicz P.

Zhang K.

Ishida H. Intramolecular hydrogen bonding in benzoxazines: when structural design becomes functional. , 36 Park E.S.

Sieber J.

Guttman C.

Rice K.

Flynn K.

Watson S.

Holmes G. Methodology for detecting residual phosphoric acid in polybenzoxazole fibers. 2 atmosphere ( w = 3.6 kDa) displays an onset decomposition temperature at 600°C, whereas Zylon (M w = 40 kDa) has it at 650°C. After immersion in water or DMSO under ambient conditions for 1 month, the onset decomposition temperature for Zylon and PBO were comparable (610°C and 600°C, respectively; 5 ) and 20% (T 20 ) mass loss temperatures. Zylon displayed a significant depression of T 5 (587°C) and T 20 (689°C) than the PBO (T 5 /T 20 at 635°C/693°C). To confirm that use of PA in the synthesis can lead to the fast hydrolysis of PBO, we immersed both Zylon and our PBO samples in boiling 0.5% PA aqueous solution for 5 days and then measured their thermal and mechanical properties ( w Zylon film was stronger prior to environmental challenges ( Figure 4 The Thermal Stability and Mechanical Properties of the as-Prepared PBO Show full caption (A) TGA data for Zylon and PBO after immersing in H 2 O or DMSO at ambient temperature for 1 month. (B) TGA data for Zylon and PBO after immersing in boiling water and boiling 0.5% PA aqueous solution for 5 days. (C) Tensile strength of Zylon and PBO as a function of time at a rate of 0.1 mm min−1 before and after immersing in boiling water and boiling 0.5% PA aqueous solution for 5 days. A large concern of PBO-based materials has been its long-term thermochemical and mechanical stability, which has been attributed to accelerated hydrolysis due to residual phosphorus contaminants from the PA used in their synthesis.As the PBO generated through our method is PA-free, it allows us to study for the first time the intrinsic stability properties of this PBO in different environmental conditions. We first performed the TGA of our PBO and the commercial PBO Zylon under a Natmosphere ( Figure 4 A ). Our pristine PBO (M= 3.6 kDa) displays an onset decomposition temperature at 600°C, whereas Zylon (M= 40 kDa) has it at 650°C. After immersion in water or DMSO under ambient conditions for 1 month, the onset decomposition temperature for Zylon and PBO were comparable (610°C and 600°C, respectively; Figure 4 A). After the samples were immersed in boiling water for 5 days, the onset decomposition temperature of Zylon was reduced to 500°C, while the PBO was still at ∼600°C ( Figure 4 B). The difference in PBO and Zylon thermal stability was also observed in 5% (T) and 20% (T) mass loss temperatures. Zylon displayed a significant depression of T(587°C) and T(689°C) than the PBO (T/Tat 635°C/693°C). To confirm that use of PA in the synthesis can lead to the fast hydrolysis of PBO, we immersed both Zylon and our PBO samples in boiling 0.5% PA aqueous solution for 5 days and then measured their thermal and mechanical properties ( Figures 4 B and 4C). The onset decomposition temperature of Zylon drops even further to ∼450°C, while that of the PBO is at ∼550°C. This PA-induced hydrolysis study supports the notion that the presence of PA can accelerate PBO hydrolysis, and our PA-free PBO made from the one-pot catalytic reaction is more stable against this hydrolysis degradation than the PA-contaminated Zylon. Tensile stress measurements on 7.6-μm-thick PBO and 10.5-μm-thick Zylon films ( Figure S15 ) revealed that the higher MZylon film was stronger prior to environmental challenges ( Figure 4 C). After immersing the samples in boiling water or boiling 0.5% PA aqueous solution for 5 days, Zylon was subject to a more significant drop in mechanical strength to 15.1 MPa than the PBO film to 21.4 MPa ( Figure 4 C). Our studies demonstrate that the highly pure PBO, even at a significantly lower degree of polymerization than Zylon, can display improved thermal and mechanical stability after accelerated hydrolysis conditions.

−1. A similar trend is observed for the hydrogenation of 1,5-diisopropoxy-2,4-dinitrobenzene ( 45 Yoo J.S.

Zhao Z.J.

Norskov J.K.

Studt F. Effect of boron modifications of palladium catalysts for the production of hydrogen from formic acid. 46 Zhou X.W.

Johnson R.A.

Wadley H.N.G. Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers. 39 Pd 61 NP (111) surface ( 39 Pd 61 NPs is 1.37%, 1.08%, 0.86%, and 0.66%, respectively ( 2 hydrogenation reaction step, migration of surface H* toward O–NO is the only endothermic reaction step ( 47 Ruban A.

Hammer B.

Stoltze P.

Skriver H.L.

Norskov J.K. Surface electronic structure and reactivity of transition and noble metals. Figure 5 Strain Distributions on the Au 39 Pd 61 NPs and Free Energy Diagrams of the Three Reaction Steps Show full caption (A) Atomistic model of Au 39 Pd 61 NP. (B) Color-coded strain distribution on the (111) facets of AuPd NPs with different diameters. (C) Average surface compression on the NPs as a function of their diameter. (D) Free energy diagram for the FA dehydrogenation on the (111) surface of the NP under zero strain (black) and 2% compression (red). (E) Free energy diagram for the hydrogenation of 1,5-diisopropoxy-2,4-dinitrobenzene on the NP under zero (black) and 2% compression (red). (F) Free energy diagram for the condensation of 1,5-diisopropoxy-2,4-diaminobenzene and benzaldehyde on the NP under zero (black) and 2% compression (red). The adsorption geometries are shown in the insets: yellow, cyan, gray, blue, red, and white spheres represent Au, Pd, C, N, O, and H atoms, respectively. To understand why the catalytic formation of PBO in the one-pot reaction depends on the size of AuPd NPs, we analyzed the model reaction of FA-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and the amine condensation with benzaldehyde ( Figure 2 A) in three reaction steps: FA dehydrogenation, reduction of the nitro groups, and condensation of the diamine with aldehyde. From the NP size-dependent dehydrogenation of FA ( Figure S16 A), we can see that the 4-nm NP catalyst provides the highest initial turnover frequency (TOF) value. As the size increases, the activity drops and the TOF decreases from 223 to 170 h. A similar trend is observed for the hydrogenation of 1,5-diisopropoxy-2,4-dinitrobenzene ( Figure S16 B). However, for the condensation of two equivalents of benzaldehyde with 1,5-diisopropoxy-2,4-diaminobenzene, larger NPs (8 and 10 nm) are more efficient, and 8-nm NPs are the best catalyst for the reaction ( Figure S16 C), which is consistent with what we observed in Figure 2 C and in the PBO synthesis. We should clarify here that the presence of AuPd NPs in the reaction solution is essential for the condensation reaction step ( Table S3 ). Without AuPd NPs as the catalyst, the condensation reaction could not proceed smoothly and could only produce the imine product in <35% yield. FA dehydrogenation on metal surfaces has been studied by DFT,which indicates that H* binds too strongly on pure Pd (111) but too weakly on pure Au (111). Using combined classical MMand DFT simulations, we can elucidate the observed size-dependent polymerization on the AuPd NPs. From the strain distribution on the AuPdNP (111) surface ( Figure 5 A ), we find that the smaller NPs exhibit a higher degree of compression ( Figure 5 B), and the average surface compression on 4-, 6-, 8-, and 10-nm AuPdNPs is 1.37%, 1.08%, 0.86%, and 0.66%, respectively ( Figure 5 C). The free energy diagrams are calculated to estimate the overpotential for each reaction step ( Figures 5 D–5F). We find that on an AuPd slab without the compressive strain, the metal surface binds H* too strongly, resulting in a high overpotential of 0.62 V; on an AuPd surface with 2% compression, the overpotential is lowered to 0.5 V. Since 4-nm NPs have the largest surface compression of 1.37%, they should be the most active catalyst for FA dehydrogenation. In the -NOhydrogenation reaction step, migration of surface H* toward O–NO is the only endothermic reaction step ( Figure 5 E); the surface compression decreases the migration energy barrier and promotes the hydrogenation reaction. Thus, the smaller the NPs (4 nm in the study), the higher the activity. In the condensation reaction step, the formation of the C–N bond is the rate-determining step, which is also enhanced by the surface compression thanks to weakened adsorption of the amino and carbonyl groups ( Figure 5 F). On the other hand, the condensation reaction involves two large molecules (amine and aldehyde), requiring larger NPs for the reaction ( Figure S17 ). Otherwise, the reactants are too close to the undercoordinated edge sites, lowering the reaction activity ( Figure S18 ).By taking both strain and geometric factors into consideration, we conclude that 8-nm NPs are the most active for catalyzing the condensation and further polymerization reactions.