I hope that this supplementary note will help in your research project. It presents an easy procedure for the preparation of  thiol-terminated poly(ethylene glycol) so it will be a dithiol with EG repeating units in between something like

HS...EG-EG-EG-EG-EG...SH with a proper number of EG so as there is an acceptable molecular weight of PEG.

http://www.rsc.org/suppdata/py/c3/c3py01474f/c3py01474f.pdf

Dear all,

it is commercially available from sigma-aldrich for exemple.

http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=108187113

Professor J. Milton Harris has many papers and review on the end group chemistry of PEG. Regards

www.nanochemazone.com