Billionaire Len Blavatnik, who led the 2007 leveraged buyout that created LyondellBasell , will seek a stake of up to 15% in the reorganized petrochemical maker when it exits bankruptcy protection, a person familiar with the matter said on Wednesday.

Blavatnik's New York-based industrial holding company Access Industries has agreed to commit as much as $800 million to backstop a rights offering to take the company out of bankruptcy, along with private equity investors Apollo Management and Ares Corporate Opportunities Fund.

"Access has actively supported Lyondell during its reorganization because we believe in the company's future," Blavatnik said in a statement on Wednesday. "We have backed up that belief with our commitment to invest up to $800 million."

Apollo, which has committed to invest up to $1.5 million, according to court papers, would likely end up with a larger stake in the new Lyondell than Access, while Ares would end up with a smaller stake than Access, this person said. The person asked not to be named because the details about the equity stakes are not public.

Ares may invest up to $475 million in the rights offering, according to court papers.

Access had acquired control of Basell AF - known for producing polypropylene chemicals - in 2005. The 2007 merger with Lyondell created the third largest independent chemicals maker in the world, with 2008 revenues of $50.7 billion.

Lyondell filed for bankruptcy in January 2009 as it suddenly faced a cash crisis, under the weight of billions in debt and a decline in global demand for petrochemicals during the recession. Earlier this week the company rejected a takeover bid from India's Reliance Industries that valued the firm at $14.5 billion, and said it will seek court approval to send its own reorganization plan out to its creditors for a vote.

Lyondell hopes to exit bankruptcy later this year and plans to list new shares on the New York Stock Exchange once it emerges from court protection.