Covestro was spun off from Bayer in 2015, and Bayer said this latest sale marks the start of the full separation from its former unit.

Bayer, which is buying seed maker Monsanto, raised 1.8 billion euros ($2.15 billion) in January by selling a 10.4 percent stake in Covestro.

The latest sale of 28.8 million shares to institutional investors valued Covestro stock at 75.50 euros apiece, Bayer said on Friday, bringing the total gains from the sale of Covestro shares to more than 9 billion euros.

"The proceeds from the sale are higher than expected. This will be considered when determining the size of the share capital increase to finance the proposed acquisition of Monsanto," finance chief Johannes Dietsch said in a statement.

Bayer now holds 6.8 percent of Covestro shares which it acquired from Bayer Pension Trust. These will be used to repay an exchangeable bond issued in 2017 that matures in 2020, Bayer said.

BofA Merrill Lynch and J.P. Morgan acted as joint bookrunners on the placement, which took place late on Thursday.

Covestro shares closed at 76.48 euros on Thursday, while Bayer shares closed at 99.97 euros.

(Reporting by Victoria Bryan and Maria Sheahan; Editing by Edmund Blair and Amrutha Gayathri)