There's also a variant of latex (I realised this researching my reply), gutta-percha (from the tree of the same name, biological name Palaquium gutta), which was also instrumental in long-distance telecommunications as it was used to seal early undersea telegraph cables (themselves a fascinating rabbit-hole of electrical engineering). The use for cables resulted in dramatic overharvesting and a collapse of the gutta-percha supply, yet another caution in the Reliance on Markets for All Things.

Modern-day cables are armoured with polyethylene (which resists seawater rot far better than simple rubber), mylar, steel cables, aluminium, polycarbonate, and petroleum jelly.

<https://en.wikipedia.org/wiki/Gutta-percha>

(James Burke's Connections, mentioned elsewhere in this thread, also describes this use and significance of gutta-percha.)