Environmental pollution is one of the problems that attracts every body's attention to itself. The main cause of this problem is disposal of plastic materials in nature which almost are not degradable. To solve the problem, various methods such as production of biodegradable polymers have been proposed. Such polymers might be produced from plant or microbial sources. Poly Lactic acid is a high strength thermoplastic polymer that its monomer can produce either synthetically or through fermentation using agricultural waste materials. In the present study producing poly lactic acid via chain extending reaction, with three different chain extenders called Hexa methylene diisocyanate, Isophoren diisocyanate and 4, 4'-diphenylmethane diisocyanat using whey permeate as the microbial culture medium was investigated. At first, lactic acid was produced through fermentation by two different homo fermentative microorganisms, which produce only the L- isomer of lactic acid. the obtained results showed that Lactobacillus casei can produce 15.47 g/L lactic acid while Lc. lactis only produce 9.72 g/L of that which lead to the selection of L . casei as the producer microorganism in this project. In the next step the produced lactic acid was purified with anion and cation exchange resins, and the identity of the product was determined with HPLC. The purity of this product was about 84.69 percent. Chain linked lactic acid polymers were synthesized through three sequential steps, in the first, purified lactic acid was dehydrated and then hydroxylated and in the third, the chain extending reaction was took place with 3 different chain extenders. They were hexa methylene diisocyanate with a linear structure, isophoren diisocyanate with a single loop in its structure and 4, 4' diphenyl methane diisocyanate with two loops in its structure. FT-IR analysis was used to monitor the formation of chain extended polymers. The FT-IR spectra confirmed the molecular structure of the polymers produced. Morphological properties of the produced polymer was investigated with SEM , and the result showed that the polymer manufactured with mediatory of 4,4' diphenyl methane diisocyanate among the other polymers has the most soften structure.Results of the molecular weight and thermal stability measurements of the manufactured polymers showed that the polymer made with 4, 4' diphenyl methane diisocyanate had the largest molecular weight and the highest thermal stability among the other polymers. According to the results, by altering the chain extender structure from having two loops to having one loop in their structures or having linear structure, these polymers specifications are lowered, consequently. As conclusion it could be said that 4, 4' diphenyl methane diisocyanate is the best chain extender for lactic acid polymerization purposes amongst