Warren is plunking down some more cash on another big deal: Dow Chemical Co's (NYSE: DOW) $18.8 billion cash purchase of Rohm & Haas Co. (NYSE: ROH). In this case, the contribution comes to about $3 billion in the form of a convertible preferred structure, which has a nice 8.5% coupon rate. As a result, Berkshire will become Dow's largest shareholder.