First Controlled Cyclopolymerization of Diethyl Dipropargylmalonate by MoCl 5 − n -Bu 4 Sn−EtOH−Quinuclidine and MoOCl 4 − n -Bu 4 Sn−EtOH−Quinuclidine To Give Highly Regular Polyenes Consisting Exclusively of 1,2-(Cyclopent-1-enylene)−Vinylene Units

Diethyl dipropargylmalonate (DEDPM) was cyclopolymerized by MoCl 5 − n -Bu 4 Sn−EtOH−quinuclidine (1:1:5:1) and MoOCl 4 − n -Bu 4 Sn−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 13 C NMR measurements. The initiator efficiency of MoOCl 4 − n -Bu 4 Sn−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 MoCl 5 -based initiator was ≤67%. The absorption maximum λ max 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 CHCl 3 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 1 H NMR spectrum of the in situ prepared MoCl 5 -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 13 C NMR spectrum of the MoCl 5 -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 MoOCl 4 -based initiator at δ = 11.27 and 11.07. The corresponding 13 C NMR spectrum revealed carbene resonances at δ = 276.1, 262.7, and 261.6 ppm.