First Controlled Cyclopolymerization of Diethyl Dipropargylmalonate by MoCl5−n-Bu4Sn−EtOH−Quinuclidine and MoOCl4−n-Bu4Sn−EtOH−Quinuclidine To Give Highly Regular Polyenes Consisting Exclusively of 1,2-(Cyclopent-1-enylene)−Vinylene Units

Diethyl dipropargylmalonate (DEDPM) was cyclopolymerized by MoCl5−n-Bu4Sn−EtOH−quinuclidine (1:1:5:1) and MoOCl4−n-Bu4Sn−EtOH−quinuclidine (1:1:2:1), respectively, to produce poly(DEDPM) exclusively based on 1,2-(cyclopent-1-enylene)vinylene units (EtOH = ethanol). The exact ratio of five- to six-membered rings was determined by quantitative 13C NMR measurements. The initiator efficiency of MoOCl4−n-Bu4Sn−EtOH−quinuclidine (1:1:2:1) was as high as 91%, the highest value ever reported for such systems, whereas the efficiency for the corresponding MoCl5-based initiator was ≤67%. The absorption maximum λmax for poly(DEDPM) was 587 nm. A maximum effective conjugation length (N eff) of 49 was estimated therefrom. Molecular weights and polydispersity indices (PDIs) of the polymers were determined in CHCl3 by size exclusion chromatography (SEC) vs polystyrene (PS). Additionally, light-scattering (LS) data were collected at λ = 690 nm in order to calculate absolute molecular weights. Polydispersities ≤1.9 were obtained. A plot of number of monomers (N) added vs molecular weights as determined by LS shows a linear dependence for both initiators. Multistage polymerizations of DEDPM indicated for both initiator systems that the catalytic species were active for at least 6 h in the presence of monomer yet do not fulfill the criteria of a truly living polymerization. The 1H NMR spectrum of the in situ prepared MoCl5-based initiator showed two triplet carbene resonances at δ = 11.27 and 11.02 ppm, indicating the presence of more than one butylidene species. Accordingly, the 13C NMR spectrum of the MoCl5-based initiator showed carbene resonances at δ = 276.0, 275.9, 272.4, 271.3, 262.7, and 261.6 ppm. Identical triplet resonances were observed for the MoOCl4-based initiator at δ = 11.27 and 11.07. The corresponding 13C NMR spectrum revealed carbene resonances at δ = 276.1, 262.7, and 261.6 ppm.