Axispharm offers you high purity Azide-PEG-CH2CO2H, MW 2K is a heterobifunctional poly-PEG with an azide at one end and a carboxylic acid at the other end. Azides can react with alkynes via click chemistry. Carboxyl groups can then react with amine molecules to form amide bonds.
AxisPharm offers 5000+ PEG Linkers with high purity. Different kinds of PEG Reagents may be available by custom synthesis.