At first you did not tell us  what was your source of PLA and molecular weight? Thus I am giving the simplest answer that ring opening polymerization from lactides (dimers of lactic acid) is giving higher Mw. From polycondensation of lactic acid the molecular weight is much lower.

So long

Helena

Helena is completely right with her answer. In order to obtain high Mw PLA, you should use ROP from lactides.

cheers,

Miguel

Thank  you for your suggestion mam. Actually I am preparing PLA using lactide by ROP technique. I wanted to do the polymerization at atm pressure. So difficult to get higher molecular weight. ( getting around 30-40K Da Mw)

So you are looking for some chain extender of molecular weight for  just obtained PLA by ROP?.

Am I right?

Helena

You won't get this from any place from any catalogue. I suggest that you synthesize it yourself. It is easy and then purify it, which is also not that difficult.

In the following paper, high molecular weight PLA was prepared by ring opening polymerization using a chain extender (which is hexamethylene diisocyanate). My regards.

http://www.hindawi.com/journals/ijps/2013/315917/