Porous organic polymers (POPs) are composed of amorphously linked bonds with high surface areas. Urea based porous organic polymer (UPOP) was synthesized by urea condensation reaction. Porous nature of UPOP was identified by Brunauer-Emmett-Teller (BET) analysis. To evaluate the possibility as heterogeneous catalysts, Pd was loaded onto UPOP, namely Pd@UPOP. XPS analysis and element mapping data confirmed that Pd was supported on Pd@UPOP, then Suzuki-Miyaura coupling reaction was performed with various aryl bromides. The optimized conditions for the catalytic activity of Pd@UPOP are using mixture of (H₂O:EtOH 3:1) (v/v) as a solvent and K₂CO₃ as a base. In this condition, reactions reached to 99% yields with Pd only 0.1 mol% under 1 h. Also, in recycling experiments, yields were maintained above 90% during five runs.

다공성 고분자(porous organic polymer, POPs)는 높은 표면적을 가지는 무정형의 고분자이다. 본 연구에서는 우레아 축합반응으로 UPOP라는 다공성 고분자를 합성하였다. UPOP는 Brunauer-Emmett-Teller(BET) 분석을 통해 다공성을 확인하였고, 불균일 촉매 활용성을 보기 위하여 팔라듐을 담지하여 Pd@UPOP를 합성하였다. Pd@UPOP는 XPS 분석을 통해 Pd 담지를 확인하였고, Suzuki-Miyaura 커플링 반응을 이용하여 촉매활성을 측정하였다. Pd@UPOP의 촉매활성은 물과 ethanol이 3:1이고 K₂CO₃를 사용하였을 때 가장 높았다. 특히 이 조건에서 Pd@UPOP를 촉매로 사용하여 공기 중에서 반응시켰을 때 Pd 함량은 0.1 mol%였음에도 불구하고 99% 이상의 수율을 보였다. 또한 5번 재사용하였을 때도 90%의 수율을 유지하였다.

Keywords: porous organic polymer, polyurea, Suzuki-Miyaura cross-coupling, catalyst, recycle

As organic chemistry has become more sophisticated, catalysts are required to have more functions for reaction rate control, reduction of activation energy of reaction, and selective reaction progress. Therefore, developing an optimized catalyst for the reaction is always an important issue. Although homogeneous catalysts are usually highly active and selective, these catalysts are hard to separate from the reaction mixture and difficult to recycle for multiple runs. In these senses, heterogeneous catalysts are often preferred. As prerequisites of the heterogeneous catalyst, the structure should be stable, and the transition metal should be well supported. The nitrogenrich polymers are considered to fulfill these prerequisites under which the catalyst can be supported.¹
Porous organic polymers (POPs) are noticeable materials with special properties such as surface permanent porosity and a large surface area. In addition, POPs have been recently used in gas storage,^2-10 sensing,^11,12 light emitting,^4,11,13-15 and heterogeneous catalysts^16-20 because of their ability to control pore size and tune functional groups by easy chemical reactions. Especially in recent years, significantly advances have been made to use POPs as heterogeneous phase catalysts because many reactions use expensive noble metals as catalysts. When noble metals are used as direct catalysts, noble metals may be aggregated so reaction efficiency is lowered. In addition, depending on the reaction, a ligand that changes the state of electrons in a noble metal is required. In order to use the large surface area of POPs as catalyst supports, attempts also have been directed to prevent the aggregation of precious metals and to manipulate the structure to change the electronic state of the noble metals.^17,18 In this regard, nitrogen-rich polymers could be good candidates since they have a high affinity for metals.
A urea group, which has been reported long ago, have many advantages since it can be formed by simple condensation reaction under the mild condition without byproduct molecules. ²¹ Urea-based polymers can be synthesized with relatively inexpensive starting materials and they have high affinity with various solvents.²¹ Moreover, since the structure contains a large amount of N and O, many sites that can absorb the transition metal are expected to be useful as heterogeneous catalysts.
Suzuki-Miyaura cross-coupling reaction is one of the most important reactions for building up C-C bonds. Since the C-C construction is so useful, coupling reaction is widely in various fields. This reaction is used in agricultural chemistry, electrochemistry, pharmaceutical, natural products, and so on. Most Suzuki-Miyaura cross-coupling reactions are performed using palladium catalysts and the catalyst performance is largely dependent on the ligand of palladium. Especially, in the earlier days, the palladium loading performance of a phosphine ligand was known to be excellent, and the electronic properties of the phosphine ligand could be easily changed by controlling the functional groups, therefore it has been widely used for the Suzuki-Miyaura reaction.^22-24 However, phosphorus ligands are less preferred due to their cost and toxicity. Recently, nitrogenbased ligands have attracted more attention. Not only nitrogenbased ligands can be synthesized with relatively economic reagents but also steric and electronic properties of nitrogenbased ligands are easily tunable by Schiff bases,^25-28 hydrazones, ^29,30 amines,³¹ etc.
In this study, we prepared POP containing urea group namely, UPOP by one-step condensation polymerization of 1,4-phenylene diisocyanate with melamine, respectively. We evaluated pore structure, structure properties, thermal properties of UPOP by SEM, TEM, BET, FTIR, NMR, TGA, and XPS. The UPOP was immobilized with palladium for catalytic performance for Suzuki-Miyaura cross-coupling reaction. We optimized the reaction conditions and tested the recycling capability of the as-prepared catalysts.