STUDIES ON IMMOBILIZED POLYMER-BOUND IMIDAZOLE-COPPER(II) COMPLEXES AS CATALYSTS .3. IMMOBILIZATION OF COPPER(II) COMPLEXES OF POLY(STYRENE-CO-N-VINYLIMIDAZOLE) BY GRAFTING ON SILICA AND THEIR CATALYSIS OF OXIDATIVE COUPLING OF 2,6-DISUBSTITUTED PHENOLS

Copper(II) complexes of poly(styrene-co-N-vinylimidazole) (Cu(II) -PS - Im) were immobilized on modified silica by radical graft copolymerization of N-vinylimidazole and styrene and characterized by VIS and EPR spectrometry. In this graft copolymerization, the same reactivity ratios were found as obtained from nongraft copolymerization initiated by AIBN; i.e., r1 = 0.07 and r2 = 7.0 for N-vinylimidazole and styrene, respectively. In a batch process under comparable conditions, these Cu(II) - PS - Im complexes grafted on silica are catalytically more active for oxidative coupling of 2,6-dimethylphenol in toluene/methanol (13/2 v/v) than those immobilized on silica by quaternization or adsorption. However, they are three times less active than nonimmobilized Cu(II) - PS - Im or their low molar mass analogue for ligand/Cu = 4, which is primarily attributed to a nonoptimum Cu(II) complex formation. Decreasing reaction rates for still higher ligand/Cu ratios probably result from the high viscosities of the silica suspensions. The specificity for the formation of poly(2,6-dimethyl-1,4-phenylene oxide) is not influenced by the grafting of the catalyst for ligand/Cu = 4. This silica-grafted Cu(II) - PS - Im demonstrates a stable conversion under suitable conditions for at least 150 h in the oxidative coupling of 2,6-di-tert-butylphenol in a continuously stirred tank reactor.