Polyethylene glycol (PEG) is an important reagent in ligation reactions, find out why.
Greg Lohman PhD:
Polyethylene glycol or PEG, is included in ligation reactions to increase the reaction rate and the overall yield. PEG does this by being a macromolecular crowding agent, which ends up increasing the effective concentration of both the ligase and the DNA substrate in the tube.
You're gonna wanna use a PEG containing buffer, such as NEB's quick ligation kit. Whenever you need a very fast reaction time, or have some very difficult ligate substrates. It's important to remember that if you're going to transform by electroporation you will need to clean up the reaction mixture to remove any PEG before you do that.