Transparent styrol-butadiene block copolymer mixtures

1. A mixture comprising linear block copolymers made from vinylaromatic monomers and from dienes of the structure

(I) S 1 -B 1 -S 2 and

(II) S 3 -B 2 -S 4 , where

S 1 is a block made from vinylaromatic monomers with number-average molar mass M n in the range from 40,000 to 100,000 g/mol,

each of S 2 , S 3 and S 4 is a block made from vinylaromatic monomers with number-average molar mass M n in the range from 5,000 to 20,000 g/mol,

each B 1 and B 2 is a block made from dienes or a copolymer block made from dienes and from vinylaromatic monomers with number-average molar mass M n in the range from 15,000 to 40,000 g/mol, and

the molar ratio of the block copolymers (I)/(II) is in the range from 0.6 to 2.

2. A mixture as claimed in claim 1 , wherein the molar ratio of the block copolymers (I)/(II) is in the range from 0.7 to 1.5.

3. A mixture as claimed in claim 1 , wherein the ratio of vinylaromatic monomer to diene in the blocks B 1 or B 2 is in the range from 0.3 to 0.7.

4. A mixture as claimed in claim 1 , wherein each of the blocks B 1 and B 2 is a copolymer block made from dienes and from vinylaromatic monomers with random distribution.

5. A mixture as claimed in claim 1 , wherein the number-average molar mass M n of

S 1 is in the range from 45,000 to 70,000 g/mol, that of each of S 2 , S 3 , and S 4 is in the range from 8,000 to 17,000 g/mol, and that of

each of B 1 and B 2 is in the range from 18,000 to 30,000 g/mol.

6. A mixture as claimed in claim 1 , wherein the blocks S 3 and S 4 have the same number-average molar masses M n .

7. A mixture as claimed in claim 1 , wherein the number-average molar masses M n of S 2 and S 4 are identical, and the number-average molar masses of B 1 and B 2 are identical.

8. A mixture as claimed in claim 1 , which comprises from 5 to 95 percent by weight of the linear block copolyiners (I) and (II) and from 95 to 5 percent by weight of standard polystyrene or impact-modified polystyrene.

9. A mixture, obtainable by partial or complete hydrogenation of the mixture as claimed in claim 1 .

10. A mixture as claimed in claim 9 , wherein the degree of hydrogenation of the vinylaromatic monomer units is at least 90%.

11. A mixture as claimed in claim 9 , wherein, prior to the hydrogenation, and based on the entirety of the diene units, more than 30% of the copolyrnerized diene units are present in the 1,2-vinyl form.

12. A process for producing mixtures as claimed in claim 1 , which comprises preparing each of the block copolymers (I) and (II) by sequential anionic polymerization of vinylaromnatic monomers and of dienes with organo-alkali metal initiators in different batches, and then blending these with a molar ratio (I)/(II) in the range 0.6 to 2.

13. A process for preparing mixtures as claimed in claim 7 , which comprises preparing the block copolymers (I) and (II) by sequential anionic polymerization of vinylaromatic monomers and of dienes using organo-alkali metal initiators simultaneously in one reactor using two joint feeds of initiator and vinylaroniatic monomers, where the molar ratio of the amount of initiator I 1 in the first feed to the amount of initiator I 2 in the second feed is in the range from 0.6 to 2.

14. A process for preparing mixtures of ring-hydrogenated block copolyiners encompassing the steps of:

a) producing the mixture as claimed in claim 1 by preparing each of the block conolymers (I) and (II) by sequential anionic polymerization of the vinylaromatic monomers and dienes with organo-alkali metal initiators in different batches and then blending these with a molar ratio (I)/(II) in the range from 0.6 to 2.

b) termination of the polymerizations, using a protic terminator or a coupling agent,

c) hydrogenation of the resultant block copolymer mixture using a hydrogenation catalyst.

15. A process as claimed in claim 14 , wherein step a) is carried out in a cycloaliphatic hydrocarbon as solvent, and in the presence of from 0.3 to 0.5% by volume, based on the solvent, of an ether.

16. Optical media comprising the mixture claimed in claim 9 .

17. A process for preparing mixtures of ring hydrogenated block copolymers encompassing the steps of:

a) producing the mixture as claimed in claim 7 by preparing the block copolymers (I) and (II) by sequential anionic polymerization of the vinylaromatic monomers and dienes using organo-alkali metal initiators simultaneously in one reactor using two joint feeds of initiator and vinylaromatic monomers, where the molar ratio of initiator I 1 in the first feed to the amount of initiator I 2 in the second feed is in the range from 0.6 to 2,

b) tennination of the polymerizations, using a protic tenninator or a coupling agent,

c) hydrogenation of the resultant block copolymers mixture using a hydrogenation catalyst.