Method for making polycarbonate

1. A method of making polycarbonate, comprising:

melt-polymerizing an aromatic dihydroxy compound and a carbonic acid diester in a presence of a polymerization catalyst in a reactor system producing a byproduct stream, wherein the polymerization catalyst comprises a quaternary phosphonium compound; and

purifying the byproduct stream to separate the carbonic acid diester, wherein the separated carbonic acid diester has a phosphorous concentration of less than or equal to about 30 parts per million.

2. The method of claim 1 , wherein the dihydroxy compound is a bisphenol, wherein the carbonic acid diester is diphenyl carbonate, and wherein the quaternary phosphonium compound is tetrabutylphosphonium acetate.

3. The method of claim 1 , wherein the reactor system comprises a first reactor and a second reactor disposed downstream of the first reactor, wherein the first reactor and the second reactor are each operated at about 150° C. to about 400° C., and wherein a second reactor temperature is greater than a first reactor temperature.

4. The method of claim 3 , wherein the first reactor comprises a first reactor overhead stream in fluid communication with a scrubber.

5. The method of claim 3 , wherein the reactor system further comprises a third reactor disposed downstream of the second reactor and a fourth reactor disposed downstream of the third reactor, wherein the second reactor and the third reactor are each operated at about 250° C. to about 350° C., and wherein a third reactor temperature is greater than a second reactor temperature and a fourth reactor temperature is greater than the third reactor temperature.

6. The method of claim 1 , wherein the purifying the byproduct stream from the reactor system further comprises passing the byproduct stream through a purification system comprising a series of purifying columns to separate diphenyl carbonate.

7. The method of claim 6 , wherein the byproduct stream comprises phenol, bisphenol A, diphenyl carbonate, and quaternary phosphonium compound decomposition products.

8. The method of claim 7 , wherein the purification system comprises a first purifying column, a second purifying column disposed downstream of and in fluid communication with the first purifying column, and a third purifying column disposed downstream of and in fluid communication with the second purifying column.

9. The method of claim 8 , wherein the first purifying column is operated at a first purifying column temperature of about 100° C. to about 220° C., a first purifying column pressure of about 90 mbar to about 150 mbar, and a first purifying column reflux ratio of about 0.5 to about 5.

10. The method of claim 9 , wherein the second purifying column is operated at a second purifying column temperature of about 70° C. to about 220° C., a second purifying column pressure of about 20 mbar to about 100 mbar, and a second purifying column reflux ratio of about 2 to about 20.

11. The method of claim 8 , wherein a first top stream of the first purifying column comprises greater than or equal to about 99 wt. % phenol and less than or equal to about 3 ppm phosphorous, wherein the weight percent of phenol is based upon a net weight of the phenol separated in the first purifying column.

12. The method of claim 11 , wherein a second top stream of the second purifying column comprises greater than or equal to 99 wt. % phenol and less than or equal to about 3 ppm phosphorous, wherein the weight percent of phenol is based upon a net weight of the phenol separated in the second purifying column.

13. The method of claim 8 , wherein a third top stream of the third purifying column comprises greater than or equal to 99 wt. % diphenyl carbonate and less than or equal to about 30 ppm phosphorus, wherein the weight percent of diphenyl carbonate is based upon a net weight of diphenyl carbonate separated in the third purifying column.

14. The method of claim 12 , wherein the third purifying column is operated at a third purifying column temperature of about 140° C. to about 240° C., a third purifying column pressure of about 2 mbar to about 60 mbar, and a third purifying column reflux ratio of about 0.1 to about 2.

15. The method of claim 8 , wherein the purification system comprises a purge point disposed between the second purifying column and the third purifying column.

16. A method for making polycarbonate, comprising:

melt-polymerizing bisphenol A and diphenyl carbonate in the presence of tetrabutylphosphonium acetate and sodium hydroxide in a multistage reactor comprising a first reactor, a second reactor disposed downstream of the second reactor, a third reactor disposed downstream of the second reactor, and a fourth reactor disposed downstream of the third reactor, wherein the first reactor comprises a first reactor byproduct stream, wherein the second reactor comprises a second reactor byproduct stream, wherein the third reactor comprises a third reactor byproduct stream, and wherein the fourth reactor comprises a fourth reactor byproduct stream;

separating phenol from the first reactor byproduct stream using a scrubber disposed in fluid communication with the first reactor;

separating phenol from the second, third, and fourth byproduct streams using a first distillation column, wherein the first distillation column produces a first top stream and a first bottom stream;

separating phenol from the first bottom stream using a second distillation column, wherein the second distillation column produces a second top stream and a second bottom stream; and

separating diphenyl carbonate from the second bottom stream using a third distillation column, wherein the third distillation column produces a third top stream and a third bottom stream, wherein the diphenyl carbonate separated comprises a phosphorous concentration of less than or equal to about 30 parts per million, based upon a total net weight of the carbonic acid diester separated in the third distillation column.

17. The method of claim 16 , further comprising recycling the diphenyl carbonate to the multistage reactor.

18. The method of claim 16 , wherein the third top stream comprises greater than or equal to 99 wt. % diphenyl carbonate, wherein the weight percent of diphenyl carbonate is based upon a net weight of diphenyl carbonate separated in the third distillation column.

19. The method of claim 16 , wherein the third distillation column is operated at a third distillation column temperature of about 140° C. to about 240° C., a third distillation column pressure of about 2 mbar to about 60 mbar, and a third distillation column reflux ratio of about 0.1 to about 2.

20. The method of claim 19 , wherein the first distillation column is operated at a first distillation column temperature of about 100° C. to about 220° C., a first distillation column pressure of about 90 mbar to about 150 mbar, and a first distillation reflux ratio of about 0.5 to about 5; and wherein the second distillation column is operated at a second distillation column temperature of about 70° C. to about 220° C., a second distillation column pressure of about 20 mbar to about 100 mbar, and a second distillation column reflux ratio of about 2 to about 20.