Roger Krakoff, a five-year venture partner at Boston-based Sigma Partners, has left the firm to form his own fund, according to a report on Dow Jones VentureWire. After planning the new firm for about six months, Krakoff left Sigma on July 1. A 10-year venture capitalist, Krakoff is currently raising money for his new IT-focused firm, which will also be based in Boston. None of Krakoff’s ventures with Sigma have yet reached an exit, the report said.