Updated from 1:07 p.m. ET
Shares of Nova Chemicals (NCX) were up $3.87, or 289%, to $5.21 Monday on news that International Petroleum Investment Co. of Abu Dhabi will buy the Canadian company for $6 a share in a $2.3 billion deal.Nova is one of North America's top plastics and chemicals companies and gets 45% of its revenue in the U.S., 35% in Canada, and 20% in Europe and elsewhere. "The opportunity to join IPIC comes at a good time for Nova Chemicals and will enable us to offer both stability and long-term growth to many of our stakeholders," said Jeff Lipton, Nova CEO. "IPIC is well financed and has a track record of working successfully with companies like ours." Rosedale-Minn.-based EnteroMedics ( ETRM) shares rose 45 cents, or 39.3% to $1.59 on news Friday of a $16 million private placement. Healthspring ( HS) shares plunged $4.83, or 32.6% to $9.98 after receiving a downgrade Monday by BMO Capital Markets from market perform to underperform. Humana ( HUM) fell $9.71, or 25% to $30.83 on concerns that Medicare health care providers would be hurt after the federal government proposed a much lower-than-expected payment-rate hike for 2010.