Acyclic diene metathesis (ADMET) polymerization of allyl undec-10-enoate and some related esters
Graphical abstract
Allyl undec-10-enoate undergoes ADMET to give a polymer: typical DPs in the range 41–87. This is a rare example of an allyl ester polymerizing. Substituted allyl ester groups in a polymer and in macrocycles also undergo olefin metathesis. It is suggested that allyl undec-10-enoate may polymerize via ring-closing metathesis and entropically driven ring-opening polymerization.
Introduction
Condensation polymers and the corresponding monomers and macrocycles can generally be interconverted by a series of closely related reactions, where the nature of the major reaction product(s) depends greatly on the concentrations of the reactants [1], [2], [3].
The reactions are summarized in Scheme 1 for olefin-containing reactants that are interconverted via olefin metathesis [4], [5], [6]. In connection with our interest in the potential applications of such reactions [2], [7], in particular the preparation of combinatorial libraries of either macrocycles [8] and/or polymers [9], we sought to polymerize a range of ω-alkenyl undec-10-enoates by acyclic diene metathesis (ADMET) [10] using the commercially available Grubbs “first generation” catalyst 1. Our aim was to determine the minimum number of methylene groups that can be present in the O-alkenyl group for the polymerization to be successful using this catalyst. We wished to use catalyst 1 rather than the Grubbs “second generation” catalyst 2 because the latter can cause carbon–carbon double bond migration [11], [12] and this could clearly cause problems in combinatorial studies.
Various ω-alkenyl esters have been polymerized successfully by ADMET before using Schrock’s catalysts 3 or 4 [13], [14], but attempts to polymerize allyl esters have met with little success [14]. For example, diallyl terephthalate (5) did not polymerize when treated with Schrock’s catalyst 3 [14]. The failure of allyl esters to polymerize has been attributed to a “negative neighbouring group effect” in which the ester carbonyl group binds to the metal centre as part of a six-membered ring, see formula 6, and in so doing deactivates the catalyst [14], [15], [16]. Success at polymerizing carbonate-containing α,ω-dienes using a molybdenum-based catalyst depends similarly on the number of methylene spacers between the carbonyl groups and the vinyl groups [17]. So too does the polymerization of ether-containing α,ω-dienes using a tungsten-based catalyst even though the ether oxygen atom is a weaker Lewis acid than an ester carbonyl oxygen atom [18]. A macrocycle containing an olefinic moiety flanked by two –CH2O– has however been found to undergo ED-ROMP successfully [19].
It should be noted that using metathesis to form olefinic linkages between the repeat units of a polymer is much more demanding than, for example, a RCM reaction. The latter might be deemed a success if the yield is 75%, but a similar efficiency in an ADMET polymerization would give a product with an DP of only 4, i.e. only very small oligomers [20]. To obtain a polymer with a DP of, say, 50 will require the formation of 49 linkages with an average yield for each one of 98% [20]. Thus, the fact that catalyst 1 has been used successfully, sometimes with only modest yields however, to metathesize, for example, allyl esters [21], [22], [23], [24], [25], allyl ethers [23], [24], [26], [27] and allyl alcohol [11], often in cross metathesis [21], [22] or RCM reactions [24], [25], [26] and for end-capping polymers [21], [22], does not necessarily mean analogous reactions can be used successfully for polymer synthesis.
In this paper we report that several ω-alkenyl undec-10-enoates undergo ADMET polymerization successfully using Grubbs’ “first generation” catalyst 1 including, surprisingly, allyl undec-10-enoate (7). Several other allyl esters failed to polymerize but they did oligomerize. However, substituted allyl ester moieties in polymers and in macrocycles react successfully using catalyst 1 and/or Grubbs’ “second generation” catalyst 2.
Section snippets
Results and discussion
Initially, to establish our polymerization procedure, we polymerized deca-1,9-diene (8) [28], [29]. The neat diene 8 was first stirred with 1 mol% of catalyst 1 at 20 °C under an atmosphere of dry argon. The mixture effervesced vigorously as ethene was evolved. After 18 h, to remove any remaining ethene, the solid product was dissolved in chloroform and the solution evaporated to dryness. The residue was then dissolved in dichloromethane and retreated at 20 °C with fresh catalyst 1 for 18 h. During
Conclusions
When several diallyl esters were subjected to ADMET using Grubbs “first generation” catalyst 1 only oligomerization occurred (DPs < 7), but with allyl hex-5-enoate (21) the product had a DP of 14, and with allyl undec-10-enoate (7) products 18 had DPs up to 87. We suggest that with the diallyl esters an intermediate analogous to 6 is formed and that this is sufficiently stable to suppress polymerization. A similar effect has also been observed when α,ω-divinyl amides are treated with catalyst 1
Experimental
Experimental details are as given previously [34]. Catalyst 1 was purchased from Strem Chemicals UK and catalyst 2 from Aldrich Chemicals. Both catalysts were used as received. Reaction solvents were dried and distilled immediately before use. 13C NMR spectra were obtained on a Unity 500 MHz instrument with samples in CDCl3. For integrations a “slow pulse” sequence was used (pulse delay 30 s; >200 repetitions)
Acknowledgements
We thank the EPSRC (A.J.H.) and the British Council (S.D.K.) for financial support.
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