Polymerization of beta-butyrolactone (BBL) and beta-valerolactone (BVL) using the zinc alkoxide initiator (BDI-1)ZnO(i)()Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] proceeds very rapidly under mild conditions to produce poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV), respectively. For the monomer-to-initiator ratio 200:1, PHB number-average molecular weights (M(n)) are proportional to conversion throughout the reaction and polydispersity indices (PDIs) are narrow, consistent with a living polymerization. Higher monomer-to-initiator ratios can be used to achieve high molecular weight PHB (M(n) > 100 000). End-group analysis verifies that the polymerization of BBL follows a coordination-insertion mechanism, where complexes of the form (BDI-1)ZnOCH(Me)CH(2)CO(2)R are the active species. Variable temperature NMR experiments show that (BDI-1)ZnO(i)()Pr is monomeric in benzene-d(6) solution. In contrast, (BDI-2)ZnO(i)()Pr [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] is a poor initiator at room temperature because it prefers to form a bis-mu-isopropoxide dimer in solution. According to kinetic studies, propagation by (BDI-1)ZnO(i)()Pr is first order in both monomer as well as (BDI-1)ZnO(i)()Pr concentration. These results lead us to propose a monometallic active species. Several results suggest that elimination side reactions are slowly catalyzed by zinc alkoxides, leading to degradation of the polymer.

Die Natur als Vorbild: Die Bildung spezifischer Wasserstoffbrücken, eine wesentliche Grundlage für Prozesse in lebenden Organismen, lässt sich im Grenzgebiet von supramolekularer Chemie und homogener Katalyse gezielt einsetzen, so bei der Synthese neuartiger Elastomere (siehe schematische Darstellung), deren Eigenschaften auf starken intermolekularen Wasserstoffbrücken beruhen.

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Many natural and synthetic polymers derive their unique functions and precise structures from hydrogen bonding. The double-helix conformation of DNA and the secondary structures of many proteins, both vital to the central mechanisms of living organisms, depend strongly upon the formation of specific hydrogen bonds. The amazing tensile strength and elasticity observed in spider silk results from strong intra-and intermolecular hydrogen bonds that stabilize the formation of helices and b-sheets. [1] Hydrogen bonding between polymer chains also dictates the mechanical properties of many synthetic polymers: the high tensile strength of aromatic polyamides [2] and the elastomeric properties of polyurethane block copolymers [3] arise from intermolecular hydrogen bonding.