Carbon-13 Nuclear Magnetic Resonance (NMR) technique is used to study the microst-ructure of a series of chlorinated atactic polypropylene. Results show that the chlorination occure mainly in the way of mono-substitution up to a chlorine content of 52.84% by weight. The mean relative reactivity of three kinds of protons towards chlorination is R(CH)>R (CH2) >R(CH3)

and the chlorine atoms are distributed homogeneously in the molecular chain. It is shown that the chlorination of polypropylene could be influenced by three struc-ture factors: C-H bond energy

stability of the macromolecular free radical and configuration of the molecular chain.