Polymerization catalyst for polyester, method for producing it and process for producing polyester using it

1. A polymerization catalyst for the preparation of a polyester, comprising:

combined catalyst components comprising at least metal elements (1) and (2);

wherein:

metal element (1) is at least one metal element selected from the group consisting of metal elements of Group 4B of the Periodic Table;

metal element (2) is at least one metal element selected from the group consisting of metal elements of Group 2A of the Periodic Table, aluminum, manganese, iron, cobalt, zinc, gallium and germanium;

when a total amount of metal atoms derived from metal element (1) is represented by t (mol/catalyst kg), and a total amount of metal atoms derived from metal element (2) is represented by m (mol/catalyst kg), a value of m/t is given by: 0.50<m/t<3.50;

the catalyst is a liquid; and

the liquid exhibits a turbidity of at most 20% in a light path length of 10 mm.

2. The polymerization catalyst for a polyester according to claim 1 , which has a pH of at most 7.

3. The polymerization catalyst for a polyester according to claim 1 , which further comprises the following component (3):

(3) an organic solvent and/or water.

4. The polymerization catalyst for a polyester according to claim 3 , wherein component (3) is an organic solvent and water.

5. The polymerization catalyst for a polyester according to claim 3 , wherein the organic solvent for component (3) is an alcohol.

6. The polymerization catalyst for a polyester according to claim 1 , which further contains component (4) which is at least one acid selected from the group consisting of organic acids and inorganic acids.

7. The polymerization catalyst for a polyester according to claim 6 , wherein the acid of component (4) is an organic acid.

8. The polymerization catalyst for a polyester according to claim 7 , wherein the organic acid of component (4) is an aliphatic carboxylic acid.

9. The polymerization catalyst for a polyester according to claim 1 , wherein metal element (1) is titanium.

10. The polymerization catalyst for a polyester according to claim 1 , wherein metal element (2) is at least one member selected from the group consisting of metal elements of Group 2A of the Periodic Table, iron and zinc.

11. The polymerization catalyst for a polyester according to claim 10 , wherein metal element (2) is magnesium.

12. A method for producing the polymerization catalyst for a polyester as defined in claim 1 , which comprises:

stirring a compound of metal element (1), a compound of metal element (2), and components (3) and (4) identified below at a temperature ranging from 0 to 200° C. for at least 5 minutes:

(3) an organic solvent and/or water,

(4) at least one acid selected from the group consisting of organic acids and inorganic acids.

13. The method for producing the polymerization catalyst for a polyester according to claim 12 , wherein the compound containing metal element (1) is at least one compound selected from the group consisting of tetra-n-propyl titanate, tetra-i-propyl titanate, tetra-n-butyl titanate, tetra-n-butyl titanate tetramer, tetra-t-butyl titanate, acetyl-tri-i-propyl titanate, titanium acetate, titanium oxalate and titanium chloride.

14. A polymerization catalyst for a polyester obtained by the method as defined in claim 12 .

15. A process for producing a polyester, comprising:

conducting an esterification reaction and a polycondensation reaction of a dicarboxylic acid component comprising terephthalic acid as the main component and a diol component comprising ethylene glycol as the main component in the presence of the polymerization catalyst for a polyester as defined in claim 1 .