SAO PAULO, Sept 26 (Reuters) - Brazilian petrochemical company Braskem BRKM5.SA has hired financial and legal advisors to evaluate options to optimize its capital structure, it said on Friday, driving its shares sharply lower.

The company, with $8.5 billion of debt in the second quarter, noted that the global petrochemical market remains in a prolonged downturn that is expected to last for several more years.

Sao Paulo-traded Braskem stock fell more than 12%, the worst performer on the Bovespa .BVSP benchmark index, which was roughly flat.

The company has been facing capital structure problems due to its debt. It ended the second quarter with $1.7 billion in cash, excluding its Mexican operations.

In early August, Braskem CEO Roberto Ramos strongly defended the company's strategy of replacing naphtha with gas as the main raw material for its operations and producing "green" chemicals derived from renewable sources such as sugarcane and corn.

At the time, Ramos said the size of Braskem's debt was not a problem because the company had no short-term maturities. He also expressed opposition to selling any assets to repay debt.

Braskem's debt has an average maturity of nine years, with 68% of it due 2030 onwards, according to company data.

Last week, ratings agency S&P downgraded the company's credit rating from "BB-" to "B+" and revised its outlook to negative.

Braskem's Mexican joint venture, Braskem Idesa, had already hired advisory firm Lazard and two other firms to help it evaluate financial alternatives.

(Reporting by Alberto Alerigi Jr.; Writing by Fernando Cardoso; Editing by Richard Chang)

((mailto: fernando.cardoso@thomsonreuters.com))