Comments below this answer to the question Which is easier to build on mars per square kilometer; greenhouse windows or photovoltaics/LEDs? discuss the possibility of using plastic instead of glass for transparent greenhouse roofs on Mars in square kilometer quantities for a contributing role in food production there.
If one is shipping then from Earth, mass is important. The densities of common transparent glass and plastic materials useful for windows are around 2.4 and 1.2 g/cm^3, so you could ship twice as many panes of plastic as glass if the dimensions were equal.
But they won't be. The completed window assembly must support some pressure differential, and insulate against an infrared radiative heat loss to space through the thin Martian atmosphere in order to keep plants at their ideal temperature for maximum growth. At night you can use opaque insulating covers, but while the windows are letting in sunlight, you're going to be radiating into space. If you don't believe me, point an IR thermometer up into a blue sky during the day, but away from the sun. It will register c-c-c-cold!
Even in the UV received on Earth at sea level, many plastics will age in direct sunlight. The UV breaks the weaker of the organic bonds in the plastic. Results are solarization, crazing, and structural weakening. In the much stronger solar UV flux on the surface of Mars, this will be accelerated.
Are there any known transparent plastic candidates for an optimized "Mars plastic" that could replace glass as 15-year greenhouse windows on Mars? Would they be strong enough so that the total shipping weight per square kilometer would be lower than that for panes of optimized "Mars glass"?
below: "Lexan window after 7 years" (on Earth, of course), from Lexan vs. Plexiglass.
below $\times$3: "This Plexiglass port is 38 years old." (on Earth, of course), from Lexan vs. Plexiglass.