Modulation of the Aerobic Oxidative Polymerization in Phenylazomethine Dendrimers Assembling Copper Complexes

Abstract

The aerobic oxidative polymerization of phenol derivatives can provide poly(phenylene oxide)s, which are known as engineering plastics. This oxidation can be carried out with atmospheric oxygen molecules as the oxidizing reagent in the presence of copper complexes as the catalyst; however, stoichiometric or excess amounts of bases are also generally required. By using a phenylazomethine dendrimer complexed with several equivalent amounts of copper chloride, the additive (base)-free polymerization of 2,6-difluorophenol was successful with a very small amount of the catalyst (0.7 mol% of copper for the monomer) because the dendrimer was composed of many Schiff base units, affording a base and catalyst (copper complex) condensed reaction field. The resulting polymer was nearly linear and the molecular weight was very high. When the equimolar amount of the copper complex in one dendrimer molecule was increased, the polymer obtained under this reaction condition was rather branched, resulting in a higher glass transition temperature.

Copper‐containing bimetallic complexes as efficient catalysts for aerobic oxidative coupling polymerization

“…4B). The lower constant ( a ) corresponds to the higher branching degree as described in a previous study 12. The branched PPO could also be synthesized using the dendrimer catalyst assembling high equimolar amounts of CuCl 2 .…”

“…In our previous study,12 the higher ratio of CuCl 2 and DPAG4 resulted in a higher branching degree of the product polymer because of the nucleophilic substitution to the ortho‐position. In general, polymers with a higher branching degree are more soluble than the linear polymers having the same molecular weight.…”

“…Interestingly, this dendrimer can precisely collect metal ions from the ligand unit closest to the core, followed by step‐by‐step complexation reactions to the peripheral units. We have previously reported that the dendrimer assembling copper complexes12, 13 exhibit a significantly high catalytic activity for the aerobic oxidative polymerization of phenol derivatives affording polyphenyleneoxides (PPOs)14–16 as the product. Because the dendrimer can act as the metal assembling template, this catalyst can be easily prepared by just mixing the solution of the dendrimer and copper(II) chloride.…”

“…Although this dendrimer catalyst is highly active when only one metal element (copper) was accumulated, a synergistic enhancement is expected when another metallic element was added to the system. Indeed, various metal elements including iron,17 tin,10 titanium,18 gallium,19 vanadium,19 gold,20 platinum,21 and copper12, 13 are possible for the precise metal assembly with the dendrimer. Furthermore, a bimetallic, trimetallic, or more‐metallic fine assembly was confirmed utilizing the difference in the coordination strength 19.…”

Heterogeneous Dendrimer-Based Catalysts

“…Depending simultaneously on the reaction type, dendrimer nature, and catalytic centers location, as well as on the overall weight of the stabilization or steric factors from the dendrimer ligand, the so called positive dendritic effect can be observed, appearing in the enhanced catalyst performance with the increase in the dendrimer generation [ 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 92 , 94 , 113 , 115 , 120 , 121 , 129 , 131 , 132 , 136 , 141 , 145 , 146 , 149 , 190 , 193 , 195 , 207 , 215 , 216 , 218 , 221 , 227 , 242 , 247 , 248 , 252 , 255 , 258 , 261 , 262 , 263 , 271 , 272 , 273 , 274 , 279 , 280 , 281 , …”