Acyclic triene metathesis

Acyclic triene metathesis (ATMET) polymerisation is a particular ADMET application useful in vegetable oils. ADMET can also be used for the polymerisation of triglycerides, offering the possibility to obtain branched polymers directly from plant oils without prior chemical modification, thereby helping to further minimise the environmental impact of snch materials. This type of polymerisation was termed ATMET with reference to monomer functionality [51]. 

Another method to create new materials from polymerized plant oils is though acyclic triene metathesis (ATMET) polymerization. In this process, hyper-branched polymers are created beginning with a triglyceride that contains three alkene bonds. These hyperbranched polymers can display different physical attributes, such as lower viscosity and increased solubihty, when compared to their linear analogs. 

To obtain a branched polymer, acyclic triene metathesis (ATMET) can be used. As shown in Fig. 3.10, self-metathesis of a polyunsaturated TAG (the triene ie, triolein in the figure) would yield a hyperbranched polymer devoid of cross-links (Biermann et al., 2010). Of note, through employment of methyl acrylate as chain stopper, the resultant polymer will possess polymerizable carboxylic acid methyl ester end groups that can undergo further polymerization. 

FIGURE 3.10 Acyclic triene metathesis (ATMET) of triolein, using methyl acrylate as chain stopper. Adapted from Biermann, U., Metzger, J.O., Meier, M.A.R., 2010. Acyclic triene metathesis oligo- and polymerization of high oleic sunflower oil. Macromol. Chem. Phys. 211, 854-862. 

Recently, Li and co-workers reported on the synthesis of a 4 -branched exomethylene carbocyclic nucleoside analog as potential mimic of olefmic carbocyclic nucleosides. Treatment of acyclic triene 58 with 10 mol % of 4 in refluxing dichloromethane gave the corresponding carbocycle in 74% yield. It is surprising that isomerisation of the exocyclic double bond, which has been known to occur ruthenium-based metathesis catalysts, did not occur under the reaction conditions reported. Carbocycle 59 was then readily converted to the corresponding exomethylene carbocyclic nucleoside analog in just a few additional steps. 

The metathesis of acyclic alkadienes and polyenes may follow an inter- or intramolecular pathway. The intramolecular metathesis of an a,tfi-diene yields ethylene and a cyclic alkene, while the intermolecular reaction results primarily in the formation of ethylene and a symmetric triene (eq. (2)). The loss of a small molecule like ethylene serves to drive the equilibrium to the product side. 

The cross-metathesis of ethene with higher olefins has been termed ethenolysis (Bradshaw 1967), and provides a useful means of reducing the extent of substitution of an olefin feedstock and of converting cyclic olefins into linear dienes, trienes, etc. Some examples for acyclic olefins are shown in Table 9.1. The reactions are best done at 50 bar in order to drive the reaction to the right and to minimize self-metathesis of the substrate. 

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