SAO PAULO, June 12 (Reuters) -

Shares of Brazilian petrochemical firm Braskem soared on Monday after it confirmed a bid from Unipar Carbocloro for a controlling stake in the company at a premium of more than 40% over its previous closing price.

Unipar, a leading producer of chlorine and soda in South America, offered conglomerate Novonor 36.50 reais per share for a 34.37% stake Braskem, the company said, a move that could end up settling a long-lasting muddle over Braskem's ownership.

Braskem shares jumped as much as 13%, making it the top gainer on Brazil's Bovespa stock index, which was near flat. Shares in Unipar, which is backed by billionaire Luiz Barsi, rose 3.9%.

Novonor, formerly known as Odebrecht, is Braskem's main shareholder alongside state-run oil firm Petrobras, but has been long looking to sell its controlling stake as part of a broader restructuring.

Proceeds of a potential sale would pay the conglomerate's creditors, mainly large banks, which would still need to greenlight any deal as Novonor's stake in Braskem is pledged as collateral.

"Our sentiment on the sale of Braskem to a private player has improved, and we believe Unipar is seeking to align as many interests as possible for a successful operation," BTG Pactual analysts said in a note to clients.

Trigono Capital's chief investment officer, Werner Roger, who holds a position in Unipar, said the deal would be probably good for the company, especially if it is able to sell some Braskem assets in Mexico and the United States.

He raised some doubts, however, including the view of Brazilian antitrust watchdog CADE, as Unipar would hold nearly 100% of the local chlor-alkali market, and how Unipar would obtain funding.

Unipar's offer represents a premium of 43% over Braskem's closing price on Friday, competing with a previous cash-and-debenture 47.00-real-per-share bid from U.S. asset manager Apollo and the Abu Dhabi National Oil Company (ADNOC).

BTG analysts said the Unipar bid was more appealing because it appeared to be entirely in cash, estimating it to be 22% higher than the Apollo/ADNOC offer, but noted it still implied a big debt haircut for creditors.

Newspaper Valor Economico reported Unipar's offer was worth 10 billion reais ($2.05 billion), while BTG estimated that creditors still need to receive around 14 billion-15 billion reais from Novonor.

Under the proposed transaction, Novonor would retain a minority stake of 4% in Braskem. Unipar said it would still hold talks with Petrobras over its stake. ($1 = 4.8748 reais) (Reporting by Gabriel Araujo; Editing by Steven Grattan and Leslie Adler)