With a stroke of a pen, Occidental Petroleum probably will become the sixth largest producer of chemicals in the U.S., up from 17th a year ago. This is a far cry from the situation just a few years ago, when the company was wondering if it should sell its chemical businesses. With the Los Angeles-based oil and natural gas producer's decision last month to acquire Cain Chemical, Occidental will add about $1.5 billion to its annual sales, bringing its total chemical sales to the $4.5 billion level. In addition, the acquisition will add a number of new products and a captive supply of ethylene for the company's large polyvinyl chloride (PVC) resin business. Although it may have come as a surprise that Cain Chemical was available for takeover, it should not surprise anyone that Occidental is the buyer. Occidental is a company that has built its chemical operations on acquisitions. It had built its PVC ...