Abstract: The alternating copolymerization of carbon monoxide(CO) with styrene(St) or p-ethyl styrene(ESt) was catalyzed by the transition metal Pd(Ⅱ) complexes.The polyketones of STCO and ESTCO are both regular alternating copolymers. Coplymerization kinetics show that ESt/CO system is more active than St/CO system, but the life of reactivity in ESt/CO system is shorter than that of St/CO system in the batch copolymerization processes due to drop of CO pressure.It seems that higher CO pressure is needed to match alternating coordination and chain propagation in copolymerization of ESt/CO. This is an evidence that the more active the olefin, the higher the CO pressure needed.Additionally, introducing p-ethyl group to styrene make the melting point and crystallinity of ESTCO a little lower than those of STCO and the solubility in some organic solvents is improved.