Retinol — an oligomer of isoprene?
Biomolecules that consist of ‘isoprene oligomers’ — more typically labelled terpenes do not actually polymerise in a way that you may be used to from plastic synthesis. When generating plastics, a double bond of a monomer unit (e.g. propene) is somehow made more reactive (e.g. by the addition of a nucleophile to generate an anion or by the addition of an electrophile to give a cation) and this reactive species then continues to attack monomer units giving a saturated chain of polypropylene.

Nature chooses a different path. It is not actually isoprene ((3E)-2-methylbut-1,3-diene) that is oligomerised — the resulting reaction would be far too hard for nature to control efficiently. Instead, the actually employed monomers are isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).

 
Figure 1: The terpene biomonomers isopentenyl pyrophosphate (IPP; left or top) and dimethylallyl pyrophosphate (DMAPP; right or bottom).

As you can see, these only have one double bond but a diphosphate group attached to C4 that can be eliminated to give another. The biosynthetic pathway to couple these into oligomers begins by the removal of diphosphate from DMAPP resulting in a delocalised 3,3-dimethylallyl cation. This is captured by IPP’s terminal double bond to give a tertiary carbocation. In a final step, one of the two &#x3b1;-hydrogens in direction of former IPP’s diphosphate is eliminated to give geranyl pyrophosphate (figure 2).


Figure 2: Geranyl pyrophosphate; the dimerisation product of IPP and DMAPP.

Another addition of IPP gives farnesyl pyrophosphate which is then fed into multiple different biosynthesis pathways.

Thus, terpenes can be considered formal oligomers of isoprene but one should never think that they are generated by actual condensation of isoprene units.