Self-organising amphiphilic oligothiophene block co-polymers

A new route to oligothiophene-PEO-block-co-polymers has been developed, in which well-defined a-oligothiophene blocks (from bithiophene to sexithiophene) alternate with poly(ethylene oxide) blocks. These materials show high solubility in common organic solvents. Sexithiophene derivatives with short PEO chains attached at the a-positions have been synthesised as model systems for the above sexithiophene polymer. PEO side-chains as short as pentaethylene glycol had a good solubilising effect on the otherwise insoluble sexithiophene. All compounds prepared exhibited surface activity, as shown by monolayer experiments on a Langmuir trough. Sexithiophene model compounds with short PEO side-chains could be compressed to form a two-dimensional condensed phase. X-ray diffraction of melt cast films of the oligothiophene polymers showed that a minimum oligothiophene block-length of five thiophenes (pentathiophene) was necessary to fully disturb PEO crystallisation. The sexithiophene polymer formed regions of crystalline/aggregated sexithiophene in the bulk. UV/Vis and fluorescence studies in solution indicated that the oligothiophene segments were molecularly dissolved in good solvents like chloroform. Aggregation of the oligothiophenes occurred in THF/water mixtures, which was consistent with observed shifts of the UV absorption maxima towards the blue and quenching of the fluorescence. An oligothiophene length of four thiophenes (quaterthiophene) was necessary to observe this aggregation phenomenon. A sexithiophene derivative with chiral pentaethylene glycol side chains showed a large circular dichroism effect in aqueous solution and no effect in chloroform solution, indicating the formation of a chiral superstructure upon aggregation

Synthesis of high molecular weight poly(p-benzamide)s

The polymerization of aromatic para-amino acid ester derivatives was studied using model compounds. Mechanistic and kinetic experiments led to the discovery of some side reactions. Finally, high molecular weight poly(p-benzamide)s were synthesized and characterized. The use of highly reactive pentafluorophenol ester lead to polymers up to molecular weights of around 50 000 Da. Poly(benzamides) carrying both N-alkyl or N-benzyl groups on the amine could be polymerized to high molecular weight

Functional end groups in living ring-opening metathesis polymerization

Over the last two decades many synthetic methods have been reported to selectively introduce a number of different functional groups at the chain end of a living ring- opening metathesis polymer. In this personal account, I would like to focus on a few such methods developed in my research group over the last several years and how these led to the discovery of catalytic living ring-opening metathesis polymerization, a ring-opening metathesis polymerization method controlled by the polymer end groups

End Capping Ring-Opening Olefin Metathesis Polymerization Polymers with Vinyl Lactones

The selective placement of a functional group at the chain end of a ring-opening metathesis polymer using ruthenium carbene initiators has been a significant limitation. Here we demonstrate a highly effective and facile end-capping technique for ROMP with living ruthenium carbene chain ends using single-turnover olefin metathesis substrates. Vinylene carbonate and 3H-furanone are introduced as functionalization and termination agents for the ruthenium-initiated ring-opening metathesis polymerization. This leads directly to the formation of functional polymer end groups without further chemical transformation steps. Aldehyde and carboxylic acid end groups can be introduced by this new method which involves the decomposition of acyl carbenes to ruthenium carbides. The high degrees of chain-end functionality obtained are supported by ^1H NMR spectroscopy, MALDI-ToF mass spectrometry, and end-group derivatization

Transient anions of cis- and trans-cyclooctene studied by electron-impact spectroscopy

The effect which deformation of the double bond in trans-cyclooctene (TCO), compared to cis-cyclooctene (CCO), has on its negative ion – and indirectly on the π* virtual orbital – was studied by electron-impact spectroscopy. Differential elastic and vibrational excitation cross sections were measured at a scattering angle of θ = 135°. The vertical attachment energy (VAE) derived from the vibrational excitation spectra is 1.87 eV in TCO, only 0.09 eV lower than in the unstrained CCO, 1.96 eV. The substantial deformation of the C[double bond, length as m-dash]C bond in TCO thus stabilizes its transient negative ion by a surprisingly small amount and this effect is ascribed in part to the Pauli (steric) destabilization of the TCO π* orbital by the alkyl chain facing the π* lobes. An interesting effect is observed in the elastic cross section which is about 45% larger for TCO at low energies ([similar]0.4 eV), despite the similar geometrical size of the two molecules. Ramsauer–Townsend minima are observed in the elastic cross section at 0.13 and 0.12 eV for CCO and TCO, respectively. Implications of the findings on enhancement of the dienophile capacity of TCO are discussed

Functional metathesis catalyst through ring closing enyne metathesis: one pot protocol for living heterotelechelic polymers

Enyne ring closing metathesis has been used to synthesize functional group carrying metathesis catalysts from a commercial (Ru-benzylidene) Grubbs’ catalysts. The new Grubbs-type ruthenium carbene was used to synthesize living heterotelechelic ROMP polymers without any intermediate purification. Olefin metathesis with a mono substituted alkyne followed by ring closing metathesis with an allylic ether provided efficient access to new functional group carrying metathesis catalysts. Different functional benzylidene and alkylidene derivatives have been investigated in the synthesis of heterotelechelic polymers in one pot

Heterotelechelic polymers by ring‐opening metathesis and regioselective chain transfer

Heterotelechelic polymers were synthesized by a kinetic telechelic ring-opening metathesis polymerization method relying on the regioselective cross-metathesis of the propagating Grubbs’ first-generation catalyst with cinnamyl alcohol derivatives. This procedure allowed the synthesis of hetero-bis-end-functional polymers in a one- pot setup. The molecular weight of the polymers could be controlled by varying the ratio between cinnamyl alcohol derivatives and monomer. The end functional groups can be changed using different aromatically substituted cinnamyl alcohol derivatives. Different monomers were investigated and the presence of the functional groups was shown by NMR spectroscopy and MALDI-ToF mass spectrometry. Labeling experiments with dyes were conducted to demonstrate the orthogonal addressability of both chain ends of the heterotelechelic polymers obtained

Degradable precision polynorbornenes via ring-opening metathesis polymerization

In an attempt to introduce monomer sequence control in a growing polynorbornene via ring-opening metathesis polymerization, we employ dioxepins to efficiently determine the location of the monomers on the macromolecule backbone. Owing to the acid-labile acetal group, dioxepins allow scission of the polymer at the point of the dioxepin insertion and thus provide an indirect way to determine the monomer location. Additionally, dioxepins are used as spacers in the synthesis of multiblock polynorbornenes that are readily cleavable to afford the individual polynorbornene blocks

Branched polymers via ROMP of termimers

Today's olefin metathesis catalysts show high reactivity, selectivity, and functional group tolerance and allow the design of new syntheses of precisely functionalized polymers. Here the synthesis of a new end-capping reagent is investigated allowing the introduction of a highly reactive activated ester end-group at the polymer chain end as well as its prefunctionalization to directly introduce functional moieties. The versatility of this new end-capping reagent is demonstrated by utilizing it to synthesize a so-called termimer (a monomer with termination capabilities). Copolymerization of a norbornene derivative with the termimer leads to hyperbranched ring-opening metathesis polymerization polymers as proven by gel permeation chromatography and MALDI-ToF-(matrix-assisted laser desorption/ionization time of flight) mass spectrometry

Synthesis of telechelic poly(p-benzamide)s

Well-defined telechelic poly(benzamide)s were synthesized by chain-growth polycondensation of phenyl-4-amino benzoate and pentafluorophenyl-4-amino benzoate derivatives with a bifunctional initiator in the presence of LiTMP as base. The polymerization was carried out at −70 °C to prevent self-initiated polymerization. To confirm the control over molecular weight, different defined molecular weight polymers were synthesized and analyzed by GPC. Taking advantage of the labile ester end groups of these poly(benzamide)s, we carried out postpolymerization modifications to introduce different end functional groups such as alkyne, amine, alcohol, alkyl halide, and olefin suitable for different types of postpolymerization reactions. Successful end group modification was confirmed by 1H NMR spectroscopy and isotopically resolved MALDI-ToF mass spectrometry