Polymerite is a plastic that can be cured to give it the strength of steel. It can be formed using injection molding or additive manufacturing. After it is formed, it is cured by exposing it to gamma rays, either directly or by bombarding it with positrons to selectively stimulate matter-antimatter annihilation similar to a PET scan.
There are different grades of polymerite that can be used for different applications. The four most well-known are pink, orange, green, and blue. Polymerite is naturally fluorescent and translucent. It can be painted, but is often left bare to achieve a singular aesthetic.
Even after curing, polymerite is biodegradable, albeit under very specific conditions that do not occur naturally.
This is my interpretation of the plasteel found in other sci-fi. I first heard the term used in Dune, but I don't recall reading an explanation of how it's manufactured or what its benefits are.