Cyclobutene isomerisation

Fused cyclobutenes are thermally rather stable, specially those in which the second ring is five or six membered. For example the following cyclobutene does not isomerise below 380°C. 

The policyclic triene monomer undergoes metathesis polymerisation exclusively by the cyclobutene double bond under mild conditions (in toluene solution at 20 °C) to give a soluble precursor polymer. This polyacetylene precursor can be purified and characterised prior to its conversion at elevated temperature, via thermally initiated symmetry-allowed elimination (retro Diels-Alder reaction), to polyacetylene (a heat treatment of the product also results in isomerisation of the initial cis form to a more stable turns form) [150],... 

The cis,trans-2H-thiocins (4) were prepared by conrotatory opening of the cyclobutene (5), and subsequent isomerisation. ... 

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