The plastic in question is a commonly available material, but uses special coatings to be as reflective as a regular mirror. Here's a promo clip highlighting the mirrors' capabilities: Unbreakable.

Webb's mirrors are said to be 98% reflective — meaning they reflect 98% of incoming photons — which is about as reflective as it comes. Feinberg added that "it's also a protective gold that ...

South Australia researchers will team with industry partners to build a concentrated solar thermal plant in New South Wales featuring lightweight plastic mirrors that incorporate an aluminium-silica ...