Ruthenium-catalyzed ring-opening metathesis polymerization of norbornene initiated by O-2

Katayama and coworkers used TpRuCl(=C=CHPh) (PPh3) [Tp = hydridotris(pyrazo lyl)borate] as a catalyst precursor for ring-opening metathesis polymerizat ion(ROMP) of norbornene(NBE). As Cp'RuCl .(PPh3)(2) [Cp' = (un)substituted cyclopentadienyl] compounds also can reacted with HC=CPh to form vinylidene ruthenium complexes which might be active for the ROMP of NBE, we examined the activity of CpRuCl(PPh3)(2) and HC=CPh system for the polymerization of NBE. But the activity of this system was poor. Lately, O-2 was used as cocatalyst instead of HC=CPh and found to form in s itu. an efficient catalyst system for the ROMP of NBE. The catalytic activi ty of another Ru complex [(CH3OCH2CH2Cp)RuCl(PPh3)(2)] was also examined in this article. The influence of different monomer/catalyst, O-2/catalyst an d polymerization temperature is studied. The polymerization conversion is a function of O-2/catalyst ratio, when the ratio is between 1 to 3, the poly merization conversion is high, when the ratio is higher or lower, the polym erization conversion is decreased. The ratio of monomer/catalyst has a litt le effect on the conversion, and the conversion can slightly be improved by elongating the polymerization time to 72 hour. The higher polymerization t emperature is beneficial to the polymerization conversion. This implies tha t the catalytic centers are stable at higher temperature. The trans-content of the PNBE is dependent on O-2/catalyst molar ratio, polymerization tempe rature and the nature of the catalyst. Three typical polymer samples were characterized by H-1 MMR (CDCl3). There were two signals at delta = 9 similar to 10 in the H-1 NMR spectrum, these peaks may be the absorption of protons on aldehyde group formed in the poly merization process. According to our experiment and the work of Ivin and co workers, we suggest a possible mechanism of polymerization of NBE catalyzed by Cp'RuCl(PPh3)(2). First the ruthenium complex and oxygen react to form a ruthenium oxygen complex, then the complex reacts with NBE to form oxorut heniumcyclobutane, and this oxorutheniumcyclobutane is converted to rutheni umcarbene which can initiate ROMP of NBE.