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The kinetics of the initiation and propagation of the ring-opening metathesis polymerization of exo, exo-5,6-bis(methoxycarbonyl)-7-oxabicyclo[2.2.1]hept-2-ene catalyzed by Grubbs' catalyst (Cl 2 (PCy 3 ) 2 RuACHPh) were measured by ultravioletvisible and 1 H NMR spectroscopy, respectively. Activation parameters for these processes were also determined. Although the ratio of the rate constant of initiation to the rate constant of propagation was determined to be less than 1 for this system, this polymerization showed many of the characteristics of a living system, including low polydispersities. EXPERIMENTALAll manipulations were carried out under an inert nitrogen atmosphere with standard Schlenk and

Ectopic pregnancy in a cesarean scar is an important diagnosis to consider in a woman who has had a history of cesarean delivery and whose early ultrasonography shows a thin, lower uterine segment or a low implantation site. If the diagnosis is not clear on initial ultrasound examination, the patient should be followed up with serial ultrasound examinations. Once recognized, patients with this complication may be treated either surgically or medically as indicated by the clinical situation.