Photodegradation of polystyrene in benzene solution sensitized by benzophenone was studied at 60 °C by measuring the rate of benzophenone disappearance and the change in GPC curves of polystyrene during 365-nm irradiation by a high-pressure mercury lamp. Various rate constants for the initiation step were determined by quantum yield and transient lifetime measurements. The quantum yield of benzophenone photoreduction, (_ ), is 0.0086-0.0035 for [PS] = 0-0.2 unit-M and decreases with increasing polystyrene concentration [PS], showing the influence of quenching of the benzophenone triplet by polystyrene. The photodegradation of polystyrene is supposedly initiated by successive hydrogen abstractions from benzene by triplet benzophenone and from polystyrene by the phenyl radical. The ratio p of cross-linking probability to main-chain scission probability for a polystyrene on-chain radical is 0.04-0.13 and increases with increasing benzophenone concentration. A mechanism for the degradation step of polystyrene in solution is proposed.

No abstract

ABSTRACT:Rate constants of quenching, kq, and hydrogen abstraction, k., of benzophenone triplet with oligostyrenes from dimer to hexamer as well as with cumene were determined in acetonitrile at 30oC from transient phosphorescence decay and· quantum yield for benzophenone reduction measurements. The values of kq =(4.0± 1.0) x 10 6 unit-M-1 s-1 and k. =(1.3 ±0.3) x 10 5 unit-M-1 s-1 for oligostyrenes are independent of the number of monomer unit, n. The kq for oligostyrene is three times larger than kq for cumene, its monomer model, supposingly due to a: sandwich-type encounter of benzophenone with two adjacent phenyl groups, but k. for oligostyrene is equal to k. for cumene. The chemical reaction (hydrogen abstraction) contributes in only to total deactivation of benzophenone triplet by oligostyrenes.