Cyclic polymers from alkynes

Journal name:
Nature Chemistry
Volume:
8,
Pages:
791–796
Year published:
DOI:
doi:10.1038/nchem.2516
Received
Accepted
Published online

Abstract

Cyclic polymers have dramatically different physical properties compared with those of their equivalent linear counterparts. However, the exploration of cyclic polymers is limited because of the inherent challenges associated with their synthesis. Conjugated linear polyacetylenes are important materials for electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, gas permeability, liquid crystallinity and chain helicity. However, their cyclic analogues are unknown, and therefore the ability to examine how a cyclic topology influences their properties is currently not possible. We have solved this challenge and now report a tungsten catalyst supported by a tetraanionic pincer ligand that can rapidly polymerize alkynes to form conjugated macrocycles in high yield. The catalyst works by tethering the ends of the polymer to the metal centre to overcome the inherent entropic penalty of cyclization. Gel-permeation chromatography, dynamic and static light scattering, viscometry and chemical tests are all consistent with theoretical predictions and provide unambiguous confirmation of a cyclic topology. Access to a wide variety of new cyclic polymers is now possible by simply choosing the appropriate alkyne monomer.

At a glance

Figures

  1. Improved catalyst preparation.
    Figure 1: Improved catalyst preparation.

    Treating complex 1 with phenylacetylene leads to catalysts 2 and 3 in a 2:1 ratio and requires purification and separation procedures. Catalyst 3 exhibits a low activity and is typically discarded, which thus reduces the overall catalyst yield. An improved synthesis involves treating complex 1 with tert-butylacetylene to provide catalyst 4 exclusively and in 100% yield by simply evaporating to remove the solvent.

  2. Molecular structure of 4 with ellipsoids drawn at the 50% probability level and disordered THF atoms and lattice solvent molecule (pentane) removed for clarity.
    Figure 2: Molecular structure of 4 with ellipsoids drawn at the 50% probability level and disordered THF atoms and lattice solvent molecule (pentane) removed for clarity.

    In the solid state, complex 4 is pseudo Cs symmetric and contains a W(IV) ion in a non-standard polyhedral geometry. Complex 4 contains a tetraanionic pincer ligand that comprises two phenolates and an alkylidene connection. The phenolate O atoms span the trans positions with an O1−W1−O2 bond angle of 152.37(6)°. The coordinated THF experiences a strong trans influence from the tungsten–alkylidene, evidenced by a long W1−O3 bond of 2.328(1) Å, and is labile. For comparison, the THF ligands in 1 are labile and also have long W−O bonds (2.473(2) Å and 2.177(2) Å), with the longest being trans to the alkylidyne. The C32−C27 distance (1.312(4) Å) is significantly elongated from a typical C≡C bond length of 1.21 Å, and is better represented as a double bond and thus the resonance form of a metallacyclopropene.

  3. Proposed ring-expansion polymerization with catalyst 4 and formation of macrocyclic polyenes.
    Figure 3: Proposed ring-expansion polymerization with catalyst 4 and formation of macrocyclic polyenes.

    The catalyst is activated by dissociation of THF from the tungsten centre followed by coordination of free alkyne to give intermediate A. The coordinated alkyne then inserts into the η2-coordinated alkyne to form the metallacyclopentadiene B. Propagation via ring expansion occurs by the subsequent insertion of alkyne monomer units. Reductive elimination of the polymer chain from the metal centre yields a cyclic polyene. Coordination of free alkyne reinitiates the ring-expansion mechanism.

  4. Comparison of r.m.s. radii of linear (blue) and cyclic (orange) poly(phenylacetylene) samples reported in Table 1 in dimethylacetamide with 0.05 M LiCl over a wide range of molecular masses.
    Figure 4: Comparison of r.m.s. radii of linear (blue) and cyclic (orange) poly(phenylacetylene) samples reported in Table 1 in dimethylacetamide with 0.05 M LiCl over a wide range of molecular masses.

    Over these molecular masses the ratio of radii of gyration at all points is in good agreement with the expected value (〈Rg2cyclic/〈Rg2linear = 0.5), which provides support that the polymers have a cyclic topology. That the ratio is maintained over a wide range of molecular masses ensures that the differences observed between the cyclic and linear samples are not just coincidental at a certain molecular mass (see Supplementary Table 4 for the raw data and ratios calculated at all the reported molecular masses).

  5. Mark–Houwink plot that compares intrinsic viscosities (η) over a range of molecular masses for the linear and cyclic polyphenylacetylene samples reported in Table 1 in THF at 35 °C.
    Figure 5: Mark–Houwink plot that compares intrinsic viscosities (η) over a range of molecular masses for the linear and cyclic polyphenylacetylene samples reported in Table 1 in THF at 35 °C.

    The ratio between the viscosities is again in good agreement for the relationship between a cyclic and linear polymer over a wide range of molecular masses.

  6. GPC traces of partially hydrogenated cyclic and linear poly(phenylacetylene) samples after ozonolysis for 30 seconds (orange) and 16 minutes (grey).
    Figure 6: GPC traces of partially hydrogenated cyclic and linear poly(phenylacetylene) samples after ozonolysis for 30 seconds (orange) and 16 minutes (grey).

    a, For the cyclic (phenylacetylene) samples it is clear that ozonolysis results in a lower elution time and higher hydrodynamic volume fractions, a result that is consistent with a cyclic topology. b, In contrast, ozonolysis of the linear sample results only in higher elution times and lower hydrodynamic volume fractions, a result expected for cutting a linear polymer into smaller sections.

  7. GPC traces that compare fully hydrogenated cyclic poly(styrene) samples (cyclic PS) with well-defined linear poly(styrene) (linear PS) standards.
    Figure 7: GPC traces that compare fully hydrogenated cyclic poly(styrene) samples (cyclic PS) with well-defined linear poly(styrene) (linear PS) standards.

    a, Overlaid GPC traces of cyclic and linear poly(styrene). Blue, cyclic poly(styrene), Mn = 31,000 Da, PDI = 1.28; orange, linear standard poly(styrene), Mn = 30,000 Da, PDI = 1.06. These samples show a significant difference in maxima, even though the absolute molecular masses are similar, which indicates significant hydrodynamic volume differences. b, Overlaid GPC trace of cyclic and linear poly(styrene). Blue, cyclic poly(styrene), Mn = 31,000 Da, PDI = 1.28; orange, linear standard poly(styrene), Mn = 20,000 Da, PDI = 1.02. To match the trace maxima a significantly lower molecular mass linear standard was required. To allow for better comparison, the cyclic sample was washed with pentane to provide a lower PDI.

Compounds

4 compounds View all compounds
  1. Trianionic pincer tungsten-alkylidyne
    Compound 1 Trianionic pincer tungsten-alkylidyne
  2. Tetraanionic pincer tungsten-tert-butyl-phenyl
    Compound 2 Tetraanionic pincer tungsten-tert-butyl-phenyl
  3. Tetraanionic pincer tungsten-phenyl-tert-butyl
    Compound 3 Tetraanionic pincer tungsten-phenyl-tert-butyl
  4. Tetraanionic pincer tungsten-tert-butyl-tert-butyl
    Compound 4 Tetraanionic pincer tungsten-tert-butyl-tert-butyl

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Author information

Affiliations

  1. Department of Chemistry, University of Florida, Center for Catalysis, PO Box 117200, Gainesville, Florida 32611, USA

    • Christopher D. Roland,
    • Hong Li,
    • Khalil A. Abboud,
    • Kenneth B. Wagener &
    • Adam S. Veige

Contributions

C.D.R. synthesized and characterized catalyst 4 and all the cyclic polyenes. C.D.R. co-wrote the paper. H.L. hydrogenated the polymers, characterized the hydrogenated the polymers and executed the ozonolysis reactions. H.L. edited the paper. K.A.A. performed single-crystal X-ray experiments and solved the solid-state structure for complex 4. K.B.W. analysed the polymer characterization data, and edited the paper. A.S.V. conceived the experiments, analysed the characterization data and co-wrote the paper.

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The authors declare no competing financial interests.

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Crystallographic information files

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    Crystallographic data for compound 4.

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