Yuval, my company makes PEGylation reagents, and I have a fair amount of PEGylation experience, although not with carboxyl-modified PS beads. For specific protocols, it is hard to do better than Bioconjugate Chemistry, 3rd edition, by Greg T. Hermanson. Greg is a widely recognized authority on bioconjugation, and his book is the standard reference on the subject of bioconjugation. I strongly recommend it to you. The specific reference is:

Greg T. Hermason, Bioconjugate Chemistry, 3rd ed. New York: Academic Press, 2013.

There is an entire chapter (Chapter 14) in the 3rd edition devoted to conjugating different compounds to microparticles and nanoparticles. This includes different ways to the conjugate m-PEG amine to carboxyl-modified polystyrene beads. Again, I strongly recommend that you acquire a copy of Greg's book and follow his protocols.

If you follow Greg's procedure and it doesn't work for you, I'll be happy to help you troubleshoot it. Greg's procedures, though, are carefully written, well-researched, and reasonably detailed. They usually work as written.

Thanks so much Robert, sounds exactly what I need!

Dear Yuval, you can also see Imaging, biodistribution and in vitro study of smart 99m Tc-PAMAM G4 dendrimer as novel nano-complex Imaging, biodistribution and in vitro study of smart 99m Tc-PAMAM G4 dendrimer as novel nano-complex, article and its references in pegylation part.