SHANGHAI Petrochemical is destined to be among the first batch of mainland companies to list in Hongkong, and it is an appropriate choice for symbolic as well as financial reasons. The giant works is a symbol to both the power and flaws of central planning. Twenty five years ago the site in Jinshan County was thinly populated and its residents had to break their backs scraping an income much lower than that earned by workers in nearby Shanghai. But in 1972 Chairman Mao Zedong and Premier Zhou Enlai resolved to build the country's biggest and best chemical works in Jinshan county, and tens of thousands of people moved in to a development which would not only end up as one of the 10 largest industrial plants in China, but have model schools, hospitals, housing and other welfare facilities. The first process was switched on in 1977. This integrated model for development may have proved attractive to workers, who noted that Jinshan literally means ''gold county'', but has caused a serious headache in the flotation process. No Hongkong investor wants shares in a hospital. The works has had to be split up, and it looks likely that the split is being made three ways. The local authorities will get the welfare and housing, the state will keep the research and some other industrial facilities, and the core industrial works will be turned into a company and floated. This in turn creates a problem of finding a source of funds for these costly facilities. Most estimates say the company will raise around $2.5 billion from Hongkong, with a smaller listing of shares for mainland investors in Shanghai. The giant plant now employs 60,000 workers and covers 14 square kilometres. It is the largest plant in China for ethylene, the raw material for plastic. More importantly, it is a major earner of foreign exchange, which it will need to pay dividends to its Hongkong shareholders, with 1990 overseas earnings passing US$45 million.