Lyondell Chemical Company and Millennium Chemicals Inc. announced March 29 that their boards of directors have approved, and the companies have executed, a definitive agreement for a stock-for-stock business combination of the companies. As a result, Millennium, the second-largest producer of TiO2 in the world, will become a wholly owned subsidiary of Lyondell.      Based on recent trading, the transaction is valued at approximately $2.3 billion, including approximately $1.3 billion of Millen...