Highly stiff propylene-ethylene block copolymer composition and a process for producing the same

In a process for producing a propylene-ethylene block copolymer characterized in that there is used a catalyst system having combined (A) a solid catalyst component containing Ti, Mg, halogen and a polyvalent carboxylic acid ester, B) an organoaluminum compound and (C) an organosilicon compound expressed by the formula R.sup.4.sub.x R.sup.5.sub.y Si(OR.sup.6).sub.z wherein R.sup.4 and R.sup.6 each represents a hydrocarbon radical, R.sup.5 represents a hydrocarbon radical or a hydrocarbon radical containing a hetereo atom, and x+y+z=4, 0.ltoreq.x.ltoreq.2, 1.ltoreq.y.ltoreq.3, 1.ltoreq.z.ltoreq.3, the molar ratio of the organoaluminum compound (B) to the organosilicon compound (C) being represented by the formula (B)/(C) is equal to 1 to 15, and the following two polymerization steps are carried out:

• a first polymerization step (I) directed mainly to propylene polymerization, wherein monomers in a ratio of ethylene/(ethylene+propylene)=0 to 5% by weight are fed in two or more polymerization vessels connected in series, to produce 60 to 95% of the total polymerized quantity, and
• a second polymerization step (II) containing a relatively large quantity of ethylene, wherein monomers in a ratio of ethylene/(ethylene+propylene)=10 to 100% by weight are fed in one or more polymerization vessels, to produce 5 to 40% by weight of the total polymerized quantity,
• the improvement wherein the process produces a highly stiff propylene-ethylene block copolymer composition characterized in that the composition is produced so that
• the maximum value of the melt flow rates (hereinafter referred to as MFR(h)) of the polymers formed in the respective vessels of the polymerization step (I) and the minimum value thereof (hereinafter referred to as MFR(1)), have a relationship of
• 0.1.ltoreq.Log(MFR(h)/MFR(1)).ltoreq.1
• and the MFR(i) of the polymers formed at the polymerization step (I), directed mainly to propylene and the MFR(ii) of the polymers formed at the polymerization step (II) containing a relatively large quantity of ethylene have a relationship of
• 3.ltoreq.Log(MFR(i)/MFR(ii)).ltoreq.7.