MOSCOW, NOV. 19 -- American industrialist Armand Hammer, in collaboration with Soviet oil producers, a Japanese firm and an Italian company, today announced plans for a $6 billion chemical complex, designed to be the biggest joint business venture combining Soviet and western interests.

Hammer, who flew here last week to clinch the deal, billed the project as a combination of "the great natural resources of the Soviet Union with western technology." The complex will be owned and operated by Hammer's Occidental Petroleum Corp., Montedison SpA of Italy, Marubeni Corp. of Japan and the Soviet oil ministry.

Some Moscow-based western businessmen see the new complex as a spark for further business contacts between the Soviet Union and the United States, which have so far lagged other improvements in relations between the two countries.

In the past year, the Kremlin has sought to expand foreign business investment in the Soviet Union, but many U.S. executives, skeptical about the profitability of joint ventures, have adopted a wait-and-see attitude. So far, the Soviets have signed a handful of joint ventures and are negotiating about 100 others.

The plant, which will process natural gas liquids and sulphur produced near the Caspian Sea, will take up to three years to build.

Eventually, it is projected to produce a million tons of polyethelene and polypropelene annually, which would make it the world's largest producer of those substances. It will also produce up to one million tons of commercial grade sulphur and other plastic materials, Hammer said.

Hammer today declined to say how much the firm will earn, but noted that "the Soviets understand that capitalists do not operate in a country without making a profit."

In accordance with Soviet regulations, the Soviets will own 51 percent of the business and the western companies 49 percent. The partners have determined that 50 percent of the company's output will be exported and the remainder sold in the Soviet Union, Hammer said.

The 89-year-old Hammer was acquainted with Vladimir Lenin, the founder of Soviet communism, and has more than 60 years of experience in dealing with the Soviet Union.