While there's been formal research done on the topic, it's going to be tough to find. I don't know what kind of library you have around you, but any book on polymer photodegradation should give you some basic info, which is what I'll try to do here. But I have to point out that its a complicated issue. I don't know what specific polymer figures are made of. I can make a guess, but well, I won't. I do know that different materials are used for different parts of figures, for example, the torso and limbs of POTF2 figures are different polymers. Same goes for vintage too, I think. I tried getting some information on this out of a few companies for a column I'd like to write, but they aren't sharing. Without that information I can't give you specific references, sorry.
Ok, quick crash course in polymer chemistry. Polymers are large molecules that are made up of many units of a smaller molecule, the monomer. And I mean large, polymers can have thousands and thousands of these units in a chain. What your figures are made of is a mixture of a polymer (or more than one) and a bunch of additives. For example there's probably a UV-absorber (I'll get back to this), plasticizer (same here), unreacted catalyst, short chains of polymer, and unreacted monomers. These aren't simple materials, and the properties of the material are governed in part by this mixture of components.
Alright...moving on to photodegradation. Most people will say keep your toys out of direct sunlight and that's totally right. In fact, that's what causes the discoloration in vintage StormTroopers and Snow Job. This discoloration is the effect of interactions of the material and UV-radiation (light, but the UV wavelength range is the problem-causing area). In both cases there, a piece of the polymer molecule is broken off of the main chain and creates a free radical. These free radicals cause all sorts of problems. Damage may be manifest through discoloration, chalking, blistering, brittleness, loss of strength, warping and cracking. What I'm saying is that there's a whole lot going on. I think that the discoloration that occurs results from a change in the electronic structure of the polymer backbone, which will change its absorbance spectrum and its color, but don't hold me to that.
What polymer chemists do is add materials to the polymer blends to prevent this. That's where the UV-absorbers come in. These chemicals gobble up the radicals that are produced, preventing them from reacting on their own. But these materials can only prolong the life of the polymers, not extend them indefinately. But if you keep the figs out of direct sunlight newer figures should last a long, long time. Polymer technologies have come a long way in 20 years, and we get to take advantage of that.
People also say not to smoke around toys... but that's just a stain, not anything chemical.
Ok... one last thing. A word about plasticizers. Actually quite a few words about plasticizers. Firstly they're materials that are added to polymers to make them more flexible and easily processible (the injection molding step for example). You know that smell when you open a McFarlane figure? That's plasticizer. New car smell? Plasticizer. In time and with exposure to heat, plasticizer will evaporate out of the material...as my car's dash will support. The loss of plasticizer causes materials to become more brittle and rigid.
Wrapping this up... you don't need to worry about your figures turning into a puddle of goo. Unless you melt them, which I don't suggest because that can kill you. There is genuine concern regarding discoloration, but if you keep your figures out of direct sunlight they'll last a long time.
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