Polyethylene glycol is referred to as “PEG”. It is a high molecular weight compound formed by dehydration condensation of ethylene glycol molecules. The chemical formula is HOCH2(CH2OCH2)nCH2OH. Where n is greater than 4. The average molecular weight is 200-7000. The number after the commercial polyethylene glycol indicates the average molecular weight. For example, polyethylene glycol-400 means that the average molecular weight of the commercial polyethylene glycol is about 400.