Phillips Petroleum Co. is quietly seeking to raise up to $1.5 billion in the private placement market in what is believed to be the second-largest offering of its kind ever.

Phillips is raising the money in connection with the company's proposed financial restructuring, which would increase its debt by several billion dollars. Under the terms of the financial restructuring, which has not yet been approved by stockholders, Phillips would replace about one-third of the company's common stock with several billion dollars of debt.

Under the plan, about 32 million common shares would be sold by the company to a new employe stock ownership plan for an estimated $1.6 billion. Analysts said yesterday the "hush-hush" private placement could be used to finance the employe stock ownership plan.

The recapitalization plan was devised late last month in connection with Phillips' repurchase of stock held by Mesa Partners, an investor group led by T. Boone Pickens Jr., which agreed to end its threatened takeover of the company. The Pickens group earned a profit estimated at $89 million by selling its Phillips stock back to the company in a separate transaction that did not require stockholder approval.

The $1.5 billion private placement is being handled by Morgan Stanley & Co. and First Boston Corp., according to Corporate Financing Week, the Wall Street newsletter that reported the unusual financing earlier this week. Beth Selby, managing editor of the newsletter, said the offering was the second-largest ever. Selby said the largest private placement occurred about 10 years ago when Standard Oil of Ohio raised slightly more money.

There are several reasons why Phillips may have chosen to raise the money privately, rather than attempting a public offering. The private placement could be cheaper, or quicker, or easier, because it requires less disclosure than a public offering. Alternatively, it is possible the private placement was the only way to raise the sum at comparable rates. Phillips securities are expected to be severely downgraded because of the increased leverage, or level of debt, that would be on the company's balance sheet if stockholders approve the restructuring.

Officials of Phillips, First Boston and Morgan Stanley refused to comment on the offering. Selby said Morgan Stanley and First Boston were trying to keep the deal quiet and to complete the placement as quickly as possible.

Potential investors, some of whom reportedly are considering purchasing several hundred million dollars each, were telephoned, rather than visited in person, as is customary in a deal this large.

Institutional investors told Corporate Financing Week they view Phillips' credit rating as low as Baa3, well below its present rating.

The private placement reportedly includes notes that range in maturity from three to 10 years, ranging from about 115 basis points over Treasury bills on the three years to 250 basis points over Treasuries for the 10-year notes (100 basis points equals 1 percentage point).

The private placement is for a minimum of $750 million and a maximum of $1.5 billion, depending on how enthusiastically it is received by institutional investors.