Methods for preparation of propylene polymers

A method for polymerization of propylene, comprising:

• (1) performing pre-polymerization of propylene or an olefin mixture containing propylene and at least one another α-olefin comonomer in a gas phase or a liquid phase in the presence of a Ziegler-Natta catalyst at a temperature ranging from −10° C. to 50° C. and under a pressure ranging from 0.1 MPa to 10.0 MPa to obtain a propylene prepolymer, wherein the pre-polymerization multiplication is controlled within the scope ranging from 2 to 3000 g polymer/g catalyst;
• wherein the Ziegler-Natta catalyst comprises a reaction product of the following components:
• (i) at least one titanium-containing solid catalyst component;
• (ii) at least one alkyl aluminum compound;
• and
• (iii) optionally, at least one external electron donor component,
• and wherein the at least one titanium-containing solid catalyst component of the component (i) is a reaction product of contacting at least one alkoxy magnesium compound, at least one titanium compound and at least one internal electron donor compound,
• wherein
• the at least one titanium compound is selected from compounds of formula:
• Ti(OR)4-nXn, in which R is selected from C1-C14 aliphatic or aromatic hydrocarbonyl group, X is a halogen atom, n is an integer from 0 to 4 and in case of n being equal to or less than 2, the existing R groups can be same or different;
• the at least one internal electron donor compound is selected from alkyl esters of aliphatic and aromatic monocarboxylic acids, alkyl esters of aliphatic and aromatic polycarboxylic acids, aliphatic ethers, cycloaliphatic ethers and aliphatic ketones;
• and
• the at least one alkoxy magnesium compound is selected from the compounds of formula Mg(OR1)2-m(OR2)m, wherein R1 is ethyl, R2 is (2-ethyl)hexyl, and 0.001≦m≦0.5;
• (2) performing a homopolymerization of propylene or copolymerization of propylene and at least one another α-olefin comonomer in a gas phase in the presence of the propylene prepolymer as obtained in step (1) under conditions of 91-150° C. and 1-6 MPa to obtain a propylene polymer, wherein the polymerization time ranges from 0.5 h to 4 h;
• (3) continuing the homopolymerization or copolymerization of propylene in a gas phase or a liquid phase in the presence of the product as obtained in step (2) under conditions of 50-150° C. and 1-6 MPa.