[simpsonyck]

Why a random copolmyer comprising 90% of styrene repeat units and 10% of 2-methylbut-1-en-one repeat units would behave substantially differently to polystyrene homopolymer on exposure to ultraviolet radiation.
Does anyone know why?

[jaspevacek]

At a superficial level, it is obviously because of the comonomer. Most likely it is something to do with the ketone.

The real problem that I have with answering this is that ALL of the details are missing. UV light is not a universal quantity. Different bulbs have different spectra and so the behavior of the absorbing material will be different. The degradation kinetics depend heavily on the UV wavelength - google "activation spectra" for more information on this.

[Furanone]

I would think the double bonds of the benzene rings from the polystyrene would be much more stable to UV radiation than the unstabilized double bonds from the 2-methylbut-1-en-one

[fledarmus]

There are no alkene double bonds left in either the styrene or the butene once the reaction is complete. Instead, you have pendant phenyl groups and pendant methyl ketones.

Just guessing here, but it is possible that the UV light is removing a hydrogen radical (homolytic bond dissociation) at the alpha position to the benzene ring, on the polymer chain, and that the radical is stabilized by the benzene ring. This provides a site for radical oxidation by the atmosphere, and oxidative deradation of the chain. The corresponding position on the butene section of the polymer is a quaternary carbon with no benzene ring to stabilize a radical.