Cycloaddition Four-Center Polymerization

The Diels Alder reaction, involving the [4 + 2]-cycloaddition of an unsaturated group (di-enophile) to a 1,3-diene, has been studied for the synthesis of ladder polymers, such as the reaction of 2-vinyl-l,3-butadiene with benzoquinone [Bailey, 1972] (Eq. 2-247). Related polymerizations are those utilizing the [2 + 2]-cycloaddition reaction [Dilling, 1983]. While [4 + 2]-cycloaddition reactions are thermally induced, [2 + 2]-cycloaddition reactions are 

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Triplet states occur in the four-center polymerization of distyryl pyra-zines. The polyreaction represents a (27t + 2 r) cycloaddition ... 

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