Structure of high density poly ethylene (HDPE) was changed after Plasma-induced graft polymerization of methyl methacrylate (MMA) monomer onto HDPE. Continuous polymerization of monomer wetted HDPE pellets was carried out sequentially via in-line plasma pretreatment followed by reactive extrusion. It was influenced by plasma pretreatment times between 15 s and 120 s on degree of grafting and molecular structure of the extrudate. Maximum degree of grafting was limited to 6% after 30 s plasma pretreatment. The addition of dicumyl peroxide (DCP) increased the degree of grafting to 10% after 15 s of plasma treatment. Gel formation to levels in the range 3-5% was detected, increasing with plasma pretreatment time. The formation of long chain branched (LCB) architecture, of the extruded PMMA-gHDPE, was confirmed by rheological test in oscillatory shear, which results in increases in low frequency storage and loss moduli and melt viscosity than that of neat HDPE, especially after DCP addition. Finally, formation of a fully crosslinked structure was observed after annealing at 210 degrees C for 1 has evidenced by a change in rheological properties at lowest frequencies to solid-like behaviour. Such extensive cross linking could be useful in producing high strength adhesive joints between HDPE and other materials. (C) 2017 Elsevier B.V. All rights reserved.