I have been trying to get LPG with consistent molecular weight (Mn). In this regard, I have set the theoretical Mn = 30 kDa and ran a couple of anionic polymerization using the same monomer batch, initiator (tetrabutylammonium azide), and activator (TIBA) under inert conditions. I have maintained moisture-free and inert conditions (using the Schlenk line) throughout the synthesis process. Every time, I got polymers with different molecular weights (Mn). For example, I got 18.3 kDa polymers from one polymerization, while from another polymerization, I got 27 kDa polymers. In both cases, PDI was around 1.0.

Now my concern is, why I am getting polymers with different Mn although I am targeting the same Mn and using identical batches of monomers and other components to get the polymers? Are there parameters or conditions that I need to consider to get the LPG with consistent Mn?

Also, is there any explanation for such a discrepancy in the Mn of synthesized polymers?

Any sort of ideas and advice will be very helpful. Thanks in advance.