Massachusetts Institute of Technology OpenCourseWare (MIT OCW) is a free and open collection of material from thousands of MIT courses, covering the entire MIT curriculum. There is no enrollment or registration required to access the courses.
All content is freely available, with no signup, and no start or end dates. Most of the content can be either viewed online or downloaded.
Many optimization modelling papers are available in the MIT OCW, including:
MIT OCW: Introduction to mathematical programming.
MIT OCW: Introduction to convex optimization.
MIT OCW: Optimization methods.
MIT OCW: Network optimization.