the biodegradable composition comprises at least poly or a biodegradable copolymer thereof and a stereospecifically regular block copolymer having the structure units of formula : ## str1 ## where r1 is a hydrocarbon group having 1 to 14 carbon atoms , optionally including double bonds , oxygen atoms or both of them ; and m and n are respectively an integer ranging from 300 to 5 , 000 , or having the structure units of formula : ## str2 ## where r 2 , r 3 , r 4 and r 5 are either a hydrogen atom or a methyl group ; and m and n are respectively an integer ranging from 300 to 5 , 000 .