Journal of Molecular Catalysis A: Chemical 254 (2006) 165–173 In situ NMR investigations into the ADMET-reaction of 1,4-diheptyloxy-2,5-divinylbenzene via a Schrock-type molybdenum alkylidene complex Ralf M. Peetz a,1 , Volker Sinnwell b , Emma Thorn-Cs´ anyi a,∗ a Institute of Technical and Macromolecular Chemistry, University of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany b Institute for Organic Chemistry, University of Hamburg, Martin-Luther-King-Pl. 6, 20146 Hamburg, Germany Available online 5 May 2006 Dedicated to Richard R. Schrock Abstract The acyclic diene metathesis (ADMET) polycondensation of 1,4-diheptyloxy-2,5-divinylbenzene (DHepODVB) with the Schrock-type alkyli- dene complex Mo(NAr Me 2 )(CHCMe 2 Ph)[OCMe(CF 3 ) 2 ] 2 was investigated by means of in situ 1 H/ 13 C NMR spectroscopy. Efforts were made to gain insight into the reaction mechanism and explain the relatively high reaction temperatures (>60 ◦ C instead of room temperature in the case of the diheptylsubstituted analog) necessary to achieve useful reaction rates. Different reaction charges were investigated in dependence on reaction time and ratio catalyst/monomer. At least four novel alkylidene species were detected of which three species could be assigned to structures (two of which are binuclear, presumably due to the nearly stoichiometric ratios catalyst/monomer). These three structures show a Mo-coordination from the oxygen of a heptyloxy side chain from the substrate resulting in stabilized intermediates. These intermediates act as low energy “traps” that make the higher reaction temperatures necessary. The stabilization also seems to effect decomposition of active species, substantially slowing it down. Even after 24 days, alkylidene signals could be detected without noticeable reduction in relative amount. © 2006 Elsevier B.V. All rights reserved. Keywords: Schrock-type catalysts; Olefin metathesis; ADMET; Molybdenum alkylidene complex; Reaction mechanism 1. Introduction Olefin metathesis has become a valuable catalysis tool in the synthesis of polymers and otherwise hardly accessible lower molecular weight organic compounds [1–6]. We reported on the acyclic diene metathesis (ADMET) polycondensation of ring-substituted p-divinylbenzenes [7–11]. The products yielded are soluble mono- or 2,5-disubstituted p-phenylene vinylene (PV) oligomers, with all trans configuration and defect-free microstructure. The oligomers are suitable for use in highly advanced optical and electronic applications (e.g., OLEDs) [12]. The objective of this contribution is to provide an insight into the mechanism of the ADMET polycondensation reaction of ∗ Corresponding author. Tel.: +49 40 42838 3166; fax: +49 40 42838 6008. E-mail address: thorn@uni-hamburg.de (E. Thorn-Cs´ anyi). 1 Present address: Center for Engineered Polymeric Materials (CePM), City University of New York/College of Staten Island, 2800 Victory Boulevard, Staten Island, NY 10314, United States. 1,4-diheptyloxy-2,5-divinylbenzene (DHepODVB) illustrated in Scheme 1. The products are polydisperse oligomers of 2,5-diheptyloxy- p-phenylene vinylene (DHepO-PV). These oligomer mixtures are readily separated into fractions of single oligomers [10]. The monodisperse oligomers are valuable materials for optoelectronical applications as well as model substances for the corresponding polymer. The catalyst used is the Schrock-type alkylidene complex Mo(NAr Me 2 )(CHCMe 2 Ph) [OCMe(CF 3 ) 2 ] 2 ([Mo] = CH(CH 3 ) 2 Ph, A) depicted in Scheme 2. Based on investigations in our group regarding struc- ture/reactivity relationships of several Schrock-type tungsten and molybdenum alkylidene complexes [13,14], this molybde- num complex has been shown to be the most active catalyst for the ADMET of substituted divinylbenzenes. The initiation step and first catalytic cycle for the ADMET condensation of DHepODVB with the alkylidene complex are sketched in Scheme 3. 1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2006.01.069