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Back in the 70's I knew a bloke who experimented with plastic mirrors. I'm not sure what type of plastic, I never thought to ask. He claimed to be able to figure them and polish them but they wouldn't hold figure. The plastic was hygroscopic and expanded and contracted with changing humidity. The other issue of course is that the very softness that made them very quick and easy to figure would make them susceptable to cleaning accidents. The solution might be a protective overcoat but he couldn't pursue that idea. In hindight I wonder what you would gain in the end?
BTW, I can confirm that polypropylene is significantly hygroscopic. We sometimes determine the carbonate content of soils by weight loss after acid digestion. The samples are processed in PP centrifuge tubes and we have to ensure that the tubes have the same moisture content at each weighting (dry in oven, cool in desicator, weigh immediately after removing from desicator).
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