Hence I am posting this summary to the three listservs to which I put the original query. Thanks to all who took the trouble to respond. I was mainly concerned with the ordinary transparent plastic wallets available at commercial stationers. PVC, polycarbonate, polyethylene and polypropylene are other types. It can be difficult to identify which plastic you are dealing with.
Plastics are variable.
One person commented that 'different grades of plastic contain various plasticisers and off-gas various chemicals, even the same grade from the same company can contain different ingredients from batch to batch'. Can be cheap and easy to get hold of from office stationers. However, one person commented that sealing in a plastic enclosure can also maintain a dry environment in a document conditioned to a low relative humidity 'even if relative humidity in the general atmosphere is high'. Conversely, 'if you have papers that have acclimatized in a very humid environment then you will trap moisture in the sleeve and could end up with mould growth' - although one conservator I heard about had found that melinex 'breathes sufficiently to allow damp material enclosed in a wallet to lose moisture without mould growth'. This may mean it gives off compounds as it degrades which speed up the decay of documents contained in it.