Novel comb copolymer with long polyacrylamide backbones bearing very short polyethylene glycol (PEG) pendant chains were prepared by the free-radical copolymerisation of acrylamide (AM) and PEG acrylate macromonomers. The copolymers are effective retention aids for mechanical pulps in retaining fines and precipitated calcium carbonate (PCC). The optimum copolymer stricture had a molecular weight greater than 3 million and contained 0.5 – 1 .0 % of PEG pendant chains with 9 to 23 polyether repeat units. The retention of fines induced by copolymer/phenolic resin (PFR) dual-polymer system follows the mechanism called “complex bridging flocculation” . According to this mechanism, the copolymer molecules aggregate in the presence of PFR to form a colloidal dispersed polymer complex which hetero-flocculates with fine particles. The interpolymer complex formation were experimentally observed through precipitate isotherms measurement, dynamic viscosity measurement and fluorescent microscopy.