Dow Chemical Co., DuPont Co. and Germany's BASF, Bayer and Celanese agreed to form an online venture that will supply the $50-billion market for plastics-related materials and equipment. The unnamed company will operate as a separate entity and is expected to start Oct. 1. The venture, which will supply thermoplastic resins, molding equipment, maintenance supplies and services, initially will be worth about $50 million, the companies said. The promise of additional sales and cost savings through the Internet has been generating increasing interest among chemical makers. Worldwide revenue from electronic commerce in petrochemicals that go into everything from paints to auto parts will rise to about $713 billion a year by 2003 from about $41 billion last year, according to Forrester Research Inc., a Cambridge, Mass.-based research firm.