Dear Hemant,

Quite strange since this reaction is very smooth one. Are you sure about the purity of the NCA glutamic acid? Are you working in anhydrous conditions? Do you add the PEG derivative to the NCA solution or the other way round?

Please have a look to my paper:

“Secondary Structure-Induced Micro- and Macrophase Separation in Rod-Coil Polypeptide Diblock, Triblock, and Star-Block Copolymers”  

Macromolecules 2010, 43, 1093-1100

A. Sánchez-Ferrer, R. Mezzenga*

For any further question contact me.

SALUT!

Tony

Dear Hemant,

Is your NCA pure enough (any trace of acid might remove the BOC group)? Do you work in anhydrous conditions? Do you add the polymer on the NCA solution?

Please check my paper:

“Secondary Structure-Induced Micro- and Macrophase Separation in Rod-Coil Polypeptide Diblock, Triblock, and Star-Block Copolymers”  

Macromolecules 2010, 43, 1093-1100

A. Sánchez-Ferrer, R. Mezzenga*

Let me know whether you have any doubt.

SALUT!

Tony

Hi Hemant, 

questions is, what is failing? As Tony points out correctly and as you know, NCAs react very easily with amines. Are you sure about the quality of your PEG macroinitiator? What solvent do you use? Must be water free, we typically want to have water levels at least below 30 ppm, which you must check regularly, you cannot depend on what you buy. DMF as solvent is tricky, since it can easily degrade and leave amines that screw up your polymerization. Do you dry the PEG before use. … 

Best Robert

Thank you Tony and Luxy,

The following happens when i do this reaction.

1. I am adding the NCA (dissolved in anhydrous DMF) to the BOC-NH-PEG-NH2 solution in DMF.

2. The NMR of my NCA looked pretty neat. However, even i am starting to doubt the NCA quality now. The final reaction mixture has a strange orange colour to it and when i precipitate it, the final dried product is brown and sticky... 

3. The PEG is purchased and so all i can do is trust the vendor's Certificate of analysis which claims 98.8% purity. No i do not dry the PEG before use. I store it in vacuum at -20 in a small dessicant-filled bottle.

4. The DMF is new too and is stored over moleculer sieves.

Dear hemant,

Yes, I guess that your NCA is not pure enough...

recrystallization!

SALUT!

Tony

Yes.. you are right...just to be sure i have ordered new G-glutamate benzyl ester as well.. do not want to take any chances at this stage...