The behaviour of photoproteeting of copolymer of styrene-2，2，6，6-tetramethyl-4-piperidinyl methacrylate (PDS) and bis-(2

2，6，6-tetramethyl-4-piperidinyl) sebaeate (Tinuvin-770) for polypropylene have been studied by determination of mechanical pro-perties，molecular weight

ESR and IR of polypropylene．The results showed that there are some analogies and some differences from each other in their behaviour of photopro-teetion，i．e．either PDS or Tinuvin．770 are stabilized by mechanism of stable free radi-eal(>N—O') and also can graft on polypropylene．Their dlfferences of photoproteetion are attributed to forming macromolecule of PDS to improve its heat remstance．extrac- tion resistance and compatibility of PDS with polypropylene．Furthermore，there are about 20 piperidine funetional groups on each PDS molecular chain，that has a slightiy-crosqlinkinng effect．As a result nonvolatile carbonyls are accumulated faster than in the polypropylene containing Tinuvin-770

and strength retention is better and molecu-lar weight decraesing is slower than that of polypropylene confining Tinuvin-770．The measurement of photo-oxidation process of polypropylene showed that the variation of concentration of stable free radical(>-ON') with the increasing of photo-oxidation time is relatively small，especially in later periOd while there is a sharp peak value in the curve for Tinuvin-770 at earlier period．PDS is much more resistant to photolysis so that PDS possesses high photoproteeting effectiveness．