[betasheet]

I am using DCC/DMAP coupling for heptaethylene glycol mono methyl ether with N-Boc-1,4-diaminobutane.
After deprotection I am trying to form urea with an isocyanate but it fails.

I have tried the same reaction with tetra ethylene glycol mono methyl ether and urea formation works smoothly.

Do you have any suggestion?

P.S. I am using 4M HCl in dioxane (commercial) for and dry DCM.

[Doc Oc]

Just a quick correction, if you connect an amine to an alcohol with a carbonyl group, that's a carbamate, not a urea.

As far as your specific issue, unless I'm missing something, you don't have a carbonyl group anywhere to form this linkage.  You also should not be using acidic conditions because your amine will protonate and not react.